openssh/openssh-9.6p1-gssapi-keyex.patch

diff --color -ruNp a/auth2.c b/auth2.c
--- a/auth2.c	2024-09-16 11:45:56.858133241 +0200
+++ b/auth2.c	2024-09-16 11:46:34.688939755 +0200
@@ -71,6 +71,7 @@ extern Authmethod method_passwd;
 extern Authmethod method_kbdint;
 extern Authmethod method_hostbased;
 #ifdef GSSAPI
+extern Authmethod method_gsskeyex;
 extern Authmethod method_gssapi;
 #endif
 
@@ -78,6 +79,7 @@ Authmethod *authmethods[] = {
 	&method_none,
 	&method_pubkey,
 #ifdef GSSAPI
+	&method_gsskeyex,
 	&method_gssapi,
 #endif
 	&method_passwd,
diff --color -ruNp a/auth2-gss.c b/auth2-gss.c
--- a/auth2-gss.c	2024-09-16 11:45:56.858133241 +0200
+++ b/auth2-gss.c	2024-09-16 11:46:34.689939776 +0200
@@ -51,6 +51,7 @@
 #define SSH_GSSAPI_MAX_MECHS	2048
 
 extern ServerOptions options;
+extern struct authmethod_cfg methodcfg_gsskeyex;
 extern struct authmethod_cfg methodcfg_gssapi;
 
 static int input_gssapi_token(int type, u_int32_t plen, struct ssh *ssh);
@@ -59,6 +60,48 @@ static int input_gssapi_exchange_complet
 static int input_gssapi_errtok(int, u_int32_t, struct ssh *);
 
 /*
+ * The 'gssapi_keyex' userauth mechanism.
+ */
+static int
+userauth_gsskeyex(struct ssh *ssh, const char *method)
+{
+	Authctxt *authctxt = ssh->authctxt;
+	int r, authenticated = 0;
+	struct sshbuf *b = NULL;
+	gss_buffer_desc mic, gssbuf;
+	u_char *p;
+	size_t len;
+
+	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal_fr(r, "parsing");
+
+	if ((b = sshbuf_new()) == NULL)
+		fatal_f("sshbuf_new failed");
+
+	mic.value = p;
+	mic.length = len;
+
+	ssh_gssapi_buildmic(b, authctxt->user, authctxt->service,
+	    "gssapi-keyex", ssh->kex->session_id);
+
+	if ((gssbuf.value = sshbuf_mutable_ptr(b)) == NULL)
+		fatal_f("sshbuf_mutable_ptr failed");
+	gssbuf.length = sshbuf_len(b);
+
+	/* gss_kex_context is NULL with privsep, so we can't check it here */
+	if (!GSS_ERROR(mm_ssh_gssapi_checkmic(gss_kex_context,
+	    &gssbuf, &mic)))
+		authenticated = mm_ssh_gssapi_userok(authctxt->user,
+		    authctxt->pw, 1);
+
+	sshbuf_free(b);
+	free(mic.value);
+
+	return (authenticated);
+}
+
+/*
  * We only support those mechanisms that we know about (ie ones that we know
  * how to check local user kuserok and the like)
  */
@@ -267,7 +310,7 @@ input_gssapi_exchange_complete(int type,
 	if ((r = sshpkt_get_end(ssh)) != 0)
 		fatal_fr(r, "parse packet");
 
-	authenticated = mm_ssh_gssapi_userok(authctxt->user);
+ 	authenticated = mm_ssh_gssapi_userok(authctxt->user, authctxt->pw, 1);
 
 	authctxt->postponed = 0;
 	ssh_dispatch_set(ssh, SSH2_MSG_USERAUTH_GSSAPI_TOKEN, NULL);
@@ -315,7 +358,7 @@ input_gssapi_mic(int type, u_int32_t ple
 	gssbuf.length = sshbuf_len(b);
 
 	if (!GSS_ERROR(mm_ssh_gssapi_checkmic(gssctxt, &gssbuf, &mic)))
-		authenticated = mm_ssh_gssapi_userok(authctxt->user);
+		authenticated = mm_ssh_gssapi_userok(authctxt->user, authctxt->pw, 0);
 	else
 		logit("GSSAPI MIC check failed");
 
@@ -333,6 +376,11 @@ input_gssapi_mic(int type, u_int32_t ple
 	return 0;
 }
 
+Authmethod method_gsskeyex = {
+	&methodcfg_gsskeyex,
+	userauth_gsskeyex,
+};
+
 Authmethod method_gssapi = {
 	&methodcfg_gssapi,
 	userauth_gssapi,
diff --color -ruNp a/auth2-methods.c b/auth2-methods.c
--- a/auth2-methods.c	2024-07-01 06:36:28.000000000 +0200
+++ b/auth2-methods.c	2024-09-16 11:46:34.689939776 +0200
@@ -50,6 +50,11 @@ struct authmethod_cfg methodcfg_pubkey =
 	&options.pubkey_authentication
 };
 #ifdef GSSAPI
+struct authmethod_cfg methodcfg_gsskeyex = {
+	"gssapi-keyex",
+	NULL,
+	&options.gss_authentication
+};
 struct authmethod_cfg methodcfg_gssapi = {
 	"gssapi-with-mic",
 	NULL,
@@ -76,6 +81,7 @@ static struct authmethod_cfg *authmethod
 	&methodcfg_none,
 	&methodcfg_pubkey,
 #ifdef GSSAPI
+	&methodcfg_gsskeyex,
 	&methodcfg_gssapi,
 #endif
 	&methodcfg_passwd,
diff --color -ruNp a/auth.c b/auth.c
--- a/auth.c	2024-07-01 06:36:28.000000000 +0200
+++ b/auth.c	2024-09-16 11:46:34.690939798 +0200
@@ -356,7 +356,8 @@ auth_root_allowed(struct ssh *ssh, const
 	case PERMIT_NO_PASSWD:
 		if (strcmp(method, "publickey") == 0 ||
 		    strcmp(method, "hostbased") == 0 ||
-		    strcmp(method, "gssapi-with-mic") == 0)
+		    strcmp(method, "gssapi-with-mic") == 0 ||
+		    strcmp(method, "gssapi-keyex") == 0)
 			return 1;
 		break;
 	case PERMIT_FORCED_ONLY:
diff --color -ruNp a/canohost.c b/canohost.c
--- a/canohost.c	2024-07-01 06:36:28.000000000 +0200
+++ b/canohost.c	2024-09-16 11:46:34.690939798 +0200
@@ -35,6 +35,99 @@
 #include "canohost.h"
 #include "misc.h"
 
+/*
+ * Returns the remote DNS hostname as a string. The returned string must not
+ * be freed. NB. this will usually trigger a DNS query the first time it is
+ * called.
+ * This function does additional checks on the hostname to mitigate some
+ * attacks on legacy rhosts-style authentication.
+ * XXX is RhostsRSAAuthentication vulnerable to these?
+ * XXX Can we remove these checks? (or if not, remove RhostsRSAAuthentication?)
+ */
+
+char *
+remote_hostname(struct ssh *ssh)
+{
+	struct sockaddr_storage from;
+	socklen_t fromlen;
+	struct addrinfo hints, *ai, *aitop;
+	char name[NI_MAXHOST], ntop2[NI_MAXHOST];
+	const char *ntop = ssh_remote_ipaddr(ssh);
+
+	/* Get IP address of client. */
+	fromlen = sizeof(from);
+	memset(&from, 0, sizeof(from));
+	if (getpeername(ssh_packet_get_connection_in(ssh),
+	    (struct sockaddr *)&from, &fromlen) == -1) {
+		debug("getpeername failed: %.100s", strerror(errno));
+		return xstrdup(ntop);
+	}
+
+	ipv64_normalise_mapped(&from, &fromlen);
+	if (from.ss_family == AF_INET6)
+		fromlen = sizeof(struct sockaddr_in6);
+
+	debug3("Trying to reverse map address %.100s.", ntop);
+	/* Map the IP address to a host name. */
+	if (getnameinfo((struct sockaddr *)&from, fromlen, name, sizeof(name),
+	    NULL, 0, NI_NAMEREQD) != 0) {
+		/* Host name not found.  Use ip address. */
+		return xstrdup(ntop);
+	}
+
+	/*
+	 * if reverse lookup result looks like a numeric hostname,
+	 * someone is trying to trick us by PTR record like following:
+	 *	1.1.1.10.in-addr.arpa.	IN PTR	2.3.4.5
+	 */
+	memset(&hints, 0, sizeof(hints));
+	hints.ai_socktype = SOCK_DGRAM;	/*dummy*/
+	hints.ai_flags = AI_NUMERICHOST;
+	if (getaddrinfo(name, NULL, &hints, &ai) == 0) {
+		logit("Nasty PTR record \"%s\" is set up for %s, ignoring",
+		    name, ntop);
+		freeaddrinfo(ai);
+		return xstrdup(ntop);
+	}
+
+	/* Names are stored in lowercase. */
+	lowercase(name);
+
+	/*
+	 * Map it back to an IP address and check that the given
+	 * address actually is an address of this host.  This is
+	 * necessary because anyone with access to a name server can
+	 * define arbitrary names for an IP address. Mapping from
+	 * name to IP address can be trusted better (but can still be
+	 * fooled if the intruder has access to the name server of
+	 * the domain).
+	 */
+	memset(&hints, 0, sizeof(hints));
+	hints.ai_family = from.ss_family;
+	hints.ai_socktype = SOCK_STREAM;
+	if (getaddrinfo(name, NULL, &hints, &aitop) != 0) {
+		logit("reverse mapping checking getaddrinfo for %.700s "
+		    "[%s] failed.", name, ntop);
+		return xstrdup(ntop);
+	}
+	/* Look for the address from the list of addresses. */
+	for (ai = aitop; ai; ai = ai->ai_next) {
+		if (getnameinfo(ai->ai_addr, ai->ai_addrlen, ntop2,
+		    sizeof(ntop2), NULL, 0, NI_NUMERICHOST) == 0 &&
+		    (strcmp(ntop, ntop2) == 0))
+				break;
+	}
+	freeaddrinfo(aitop);
+	/* If we reached the end of the list, the address was not there. */
+	if (ai == NULL) {
+		/* Address not found for the host name. */
+		logit("Address %.100s maps to %.600s, but this does not "
+		    "map back to the address.", ntop, name);
+		return xstrdup(ntop);
+	}
+	return xstrdup(name);
+}
+
 void
 ipv64_normalise_mapped(struct sockaddr_storage *addr, socklen_t *len)
 {
diff --color -ruNp a/canohost.h b/canohost.h
--- a/canohost.h	2024-07-01 06:36:28.000000000 +0200
+++ b/canohost.h	2024-09-16 11:46:34.690939798 +0200
@@ -15,6 +15,9 @@
 #ifndef _CANOHOST_H
 #define _CANOHOST_H
 
+struct ssh;
+
+char		*remote_hostname(struct ssh *);
 char		*get_peer_ipaddr(int);
 int		 get_peer_port(int);
 char		*get_local_ipaddr(int);
diff --color -ruNp a/clientloop.c b/clientloop.c
--- a/clientloop.c	2024-07-01 06:36:28.000000000 +0200
+++ b/clientloop.c	2024-09-16 11:46:34.690939798 +0200
@@ -115,6 +115,10 @@
 #include "ssherr.h"
 #include "hostfile.h"
 
+#ifdef GSSAPI
+#include "ssh-gss.h"
+#endif
+
 /* Permitted RSA signature algorithms for UpdateHostkeys proofs */
 #define HOSTKEY_PROOF_RSA_ALGS	"rsa-sha2-512,rsa-sha2-256"
 
@@ -1590,6 +1594,14 @@ client_loop(struct ssh *ssh, int have_pt
 		/* Do channel operations. */
 		channel_after_poll(ssh, pfd, npfd_active);
 
+#ifdef GSSAPI
+			if (options.gss_renewal_rekey &&
+			    ssh_gssapi_credentials_updated(NULL)) {
+				debug("credentials updated - forcing rekey");
+				need_rekeying = 1;
+			}
+#endif
+
 		/* Buffer input from the connection.  */
 		if (conn_in_ready)
 			client_process_net_input(ssh);
diff --color -ruNp a/configure.ac b/configure.ac
--- a/configure.ac	2024-09-16 11:45:56.870133497 +0200
+++ b/configure.ac	2024-09-16 11:46:34.691939819 +0200
@@ -774,6 +774,30 @@ int main(void) { if (NSVersionOfRunTimeL
 	    [Use tunnel device compatibility to OpenBSD])
 	AC_DEFINE([SSH_TUN_PREPEND_AF], [1],
 	    [Prepend the address family to IP tunnel traffic])
+	AC_MSG_CHECKING([if we have the Security Authorization Session API])
+	AC_TRY_COMPILE([#include <Security/AuthSession.h>],
+		[SessionCreate(0, 0);],
+		[ac_cv_use_security_session_api="yes"
+		 AC_DEFINE([USE_SECURITY_SESSION_API], [1],
+			[platform has the Security Authorization Session API])
+		 LIBS="$LIBS -framework Security"
+		 AC_MSG_RESULT([yes])],
+		[ac_cv_use_security_session_api="no"
+		 AC_MSG_RESULT([no])])
+	AC_MSG_CHECKING([if we have an in-memory credentials cache])
+	AC_TRY_COMPILE(
+		[#include <Kerberos/Kerberos.h>],
+		[cc_context_t c;
+		 (void) cc_initialize (&c, 0, NULL, NULL);],
+		[AC_DEFINE([USE_CCAPI], [1],
+			[platform uses an in-memory credentials cache])
+		 LIBS="$LIBS -framework Security"
+		 AC_MSG_RESULT([yes])
+		 if test "x$ac_cv_use_security_session_api" = "xno"; then
+			AC_MSG_ERROR([*** Need a security framework to use the credentials cache API ***])
+		fi],
+		[AC_MSG_RESULT([no])]
+	)
 	m4_pattern_allow([AU_IPv])
 	AC_CHECK_DECL([AU_IPv4], [],
 	    AC_DEFINE([AU_IPv4], [0], [System only supports IPv4 audit records])
diff --color -ruNp a/gss-genr.c b/gss-genr.c
--- a/gss-genr.c	2024-07-01 06:36:28.000000000 +0200
+++ b/gss-genr.c	2024-09-16 11:46:34.708940181 +0200
@@ -42,9 +42,33 @@
 #include "sshbuf.h"
 #include "log.h"
 #include "ssh2.h"
+#include "cipher.h"
+#include "sshkey.h"
+#include "kex.h"
+#include "digest.h"
+#include "packet.h"
 
 #include "ssh-gss.h"
 
+typedef struct {
+	char *encoded;
+	gss_OID oid;
+} ssh_gss_kex_mapping;
+
+/*
+ * XXX - It would be nice to find a more elegant way of handling the
+ * XXX   passing of the key exchange context to the userauth routines
+ */
+
+Gssctxt *gss_kex_context = NULL;
+
+static ssh_gss_kex_mapping *gss_enc2oid = NULL;
+
+int
+ssh_gssapi_oid_table_ok(void) {
+	return (gss_enc2oid != NULL);
+}
+
 /* sshbuf_get for gss_buffer_desc */
 int
 ssh_gssapi_get_buffer_desc(struct sshbuf *b, gss_buffer_desc *g)
@@ -60,6 +84,159 @@ ssh_gssapi_get_buffer_desc(struct sshbuf
 	return 0;
 }
 
+/* sshpkt_get of gss_buffer_desc */
+int
+ssh_gssapi_sshpkt_get_buffer_desc(struct ssh *ssh, gss_buffer_desc *g)
+{
+	int r;
+	u_char *p;
+	size_t len;
+
+	if ((r = sshpkt_get_string(ssh, &p, &len)) != 0)
+		return r;
+	g->value = p;
+	g->length = len;
+	return 0;
+}
+
+/*
+ * Return a list of the gss-group1-sha1 mechanisms supported by this program
+ *
+ * We test mechanisms to ensure that we can use them, to avoid starting
+ * a key exchange with a bad mechanism
+ */
+
+char *
+ssh_gssapi_client_mechanisms(const char *host, const char *client,
+    const char *kex) {
+	gss_OID_set gss_supported = NULL;
+	OM_uint32 min_status;
+
+	if (GSS_ERROR(gss_indicate_mechs(&min_status, &gss_supported)))
+		return NULL;
+
+	return ssh_gssapi_kex_mechs(gss_supported, ssh_gssapi_check_mechanism,
+	    host, client, kex);
+}
+
+char *
+ssh_gssapi_kex_mechs(gss_OID_set gss_supported, ssh_gssapi_check_fn *check,
+    const char *host, const char *client, const char *kex) {
+	struct sshbuf *buf = NULL;
+	size_t i;
+	int r = SSH_ERR_ALLOC_FAIL;
+	int oidpos, enclen;
+	char *mechs, *encoded;
+	u_char digest[SSH_DIGEST_MAX_LENGTH];
+	char deroid[2];
+	struct ssh_digest_ctx *md = NULL;
+	char *s, *cp, *p;
+
+	if (gss_enc2oid != NULL) {
+		for (i = 0; gss_enc2oid[i].encoded != NULL; i++)
+			free(gss_enc2oid[i].encoded);
+		free(gss_enc2oid);
+	}
+
+	gss_enc2oid = xmalloc(sizeof(ssh_gss_kex_mapping) *
+	    (gss_supported->count + 1));
+
+	if ((buf = sshbuf_new()) == NULL)
+		fatal_f("sshbuf_new failed");
+
+	oidpos = 0;
+	s = cp = xstrdup(kex);
+	for (i = 0; i < gss_supported->count; i++) {
+		if (gss_supported->elements[i].length < 128 &&
+		    (*check)(NULL, &(gss_supported->elements[i]), host, client)) {
+
+			deroid[0] = SSH_GSS_OIDTYPE;
+			deroid[1] = gss_supported->elements[i].length;
+
+			if ((md = ssh_digest_start(SSH_DIGEST_MD5)) == NULL ||
+			    (r = ssh_digest_update(md, deroid, 2)) != 0 ||
+			    (r = ssh_digest_update(md,
+			        gss_supported->elements[i].elements,
+			        gss_supported->elements[i].length)) != 0 ||
+			    (r = ssh_digest_final(md, digest, sizeof(digest))) != 0)
+				fatal_fr(r, "digest failed");
+			ssh_digest_free(md);
+			md = NULL;
+
+			encoded = xmalloc(ssh_digest_bytes(SSH_DIGEST_MD5)
+			    * 2);
+			enclen = __b64_ntop(digest,
+			    ssh_digest_bytes(SSH_DIGEST_MD5), encoded,
+			    ssh_digest_bytes(SSH_DIGEST_MD5) * 2);
+
+			cp = strncpy(s, kex, strlen(kex));
+			for ((p = strsep(&cp, ",")); p && *p != '\0';
+				(p = strsep(&cp, ","))) {
+				if (sshbuf_len(buf) != 0 &&
+				    (r = sshbuf_put_u8(buf, ',')) != 0)
+					fatal_fr(r, "sshbuf_put_u8 error");
+				if ((r = sshbuf_put(buf, p, strlen(p))) != 0 ||
+				    (r = sshbuf_put(buf, encoded, enclen)) != 0)
+					fatal_fr(r, "sshbuf_put error");
+			}
+
+			gss_enc2oid[oidpos].oid = &(gss_supported->elements[i]);
+			gss_enc2oid[oidpos].encoded = encoded;
+			oidpos++;
+		}
+	}
+	free(s);
+	gss_enc2oid[oidpos].oid = NULL;
+	gss_enc2oid[oidpos].encoded = NULL;
+
+	if ((mechs = sshbuf_dup_string(buf)) == NULL)
+		fatal_f("sshbuf_dup_string failed");
+
+	sshbuf_free(buf);
+
+	if (strlen(mechs) == 0) {
+		free(mechs);
+		mechs = NULL;
+	}
+
+	return (mechs);
+}
+
+gss_OID
+ssh_gssapi_id_kex(Gssctxt *ctx, char *name, int kex_type) {
+	int i = 0;
+
+#define SKIP_KEX_NAME(type) \
+	case type: \
+		if (strlen(name) < sizeof(type##_ID)) \
+			return GSS_C_NO_OID; \
+		name += sizeof(type##_ID) - 1; \
+		break;
+
+	switch (kex_type) {
+	SKIP_KEX_NAME(KEX_GSS_GRP1_SHA1)
+	SKIP_KEX_NAME(KEX_GSS_GRP14_SHA1)
+	SKIP_KEX_NAME(KEX_GSS_GRP14_SHA256)
+	SKIP_KEX_NAME(KEX_GSS_GRP16_SHA512)
+	SKIP_KEX_NAME(KEX_GSS_GEX_SHA1)
+	SKIP_KEX_NAME(KEX_GSS_NISTP256_SHA256)
+	SKIP_KEX_NAME(KEX_GSS_C25519_SHA256)
+	default:
+		return GSS_C_NO_OID;
+	}
+
+#undef SKIP_KEX_NAME
+
+	while (gss_enc2oid[i].encoded != NULL &&
+	    strcmp(name, gss_enc2oid[i].encoded) != 0)
+		i++;
+
+	if (gss_enc2oid[i].oid != NULL && ctx != NULL)
+		ssh_gssapi_set_oid(ctx, gss_enc2oid[i].oid);
+
+	return gss_enc2oid[i].oid;
+}
+
 /* Check that the OID in a data stream matches that in the context */
 int
 ssh_gssapi_check_oid(Gssctxt *ctx, void *data, size_t len)
@@ -168,6 +345,7 @@ ssh_gssapi_build_ctx(Gssctxt **ctx)
 	(*ctx)->creds = GSS_C_NO_CREDENTIAL;
 	(*ctx)->client = GSS_C_NO_NAME;
 	(*ctx)->client_creds = GSS_C_NO_CREDENTIAL;
+	(*ctx)->first = 1;
 }
 
 /* Delete our context, providing it has been built correctly */
@@ -193,6 +371,12 @@ ssh_gssapi_delete_ctx(Gssctxt **ctx)
 		gss_release_name(&ms, &(*ctx)->client);
 	if ((*ctx)->client_creds != GSS_C_NO_CREDENTIAL)
 		gss_release_cred(&ms, &(*ctx)->client_creds);
+	sshbuf_free((*ctx)->shared_secret);
+	sshbuf_free((*ctx)->server_pubkey);
+	sshbuf_free((*ctx)->server_host_key_blob);
+	sshbuf_free((*ctx)->server_blob);
+	explicit_bzero((*ctx)->hash, sizeof((*ctx)->hash));
+	BN_clear_free((*ctx)->dh_client_pub);
 
 	free(*ctx);
 	*ctx = NULL;
@@ -216,7 +400,7 @@ ssh_gssapi_init_ctx(Gssctxt *ctx, int de
 	}
 
 	ctx->major = gss_init_sec_context(&ctx->minor,
-	    GSS_C_NO_CREDENTIAL, &ctx->context, ctx->name, ctx->oid,
+	    ctx->client_creds, &ctx->context, ctx->name, ctx->oid,
 	    GSS_C_MUTUAL_FLAG | GSS_C_INTEG_FLAG | deleg_flag,
 	    0, NULL, recv_tok, NULL, send_tok, flags, NULL);
 
@@ -246,8 +430,42 @@ ssh_gssapi_import_name(Gssctxt *ctx, con
 }
 
 OM_uint32
+ssh_gssapi_client_identity(Gssctxt *ctx, const char *name)
+{
+	gss_buffer_desc gssbuf;
+	gss_name_t gssname;
+	OM_uint32 status;
+	gss_OID_set oidset;
+
+	gssbuf.value = (void *) name;
+	gssbuf.length = strlen(gssbuf.value);
+
+	gss_create_empty_oid_set(&status, &oidset);
+	gss_add_oid_set_member(&status, ctx->oid, &oidset);
+
+	ctx->major = gss_import_name(&ctx->minor, &gssbuf,
+	    GSS_C_NT_USER_NAME, &gssname);
+
+	if (!ctx->major)
+		ctx->major = gss_acquire_cred(&ctx->minor,
+		    gssname, 0, oidset, GSS_C_INITIATE,
+		    &ctx->client_creds, NULL, NULL);
+
+	gss_release_name(&status, &gssname);
+	gss_release_oid_set(&status, &oidset);
+
+	if (ctx->major)
+		ssh_gssapi_error(ctx);
+
+	return(ctx->major);
+}
+
+OM_uint32
 ssh_gssapi_sign(Gssctxt *ctx, gss_buffer_t buffer, gss_buffer_t hash)
 {
+	if (ctx == NULL)
+		return -1;
+
 	if ((ctx->major = gss_get_mic(&ctx->minor, ctx->context,
 	    GSS_C_QOP_DEFAULT, buffer, hash)))
 		ssh_gssapi_error(ctx);
@@ -255,6 +473,19 @@ ssh_gssapi_sign(Gssctxt *ctx, gss_buffer
 	return (ctx->major);
 }
 
+/* Priviledged when used by server */
+OM_uint32
+ssh_gssapi_checkmic(Gssctxt *ctx, gss_buffer_t gssbuf, gss_buffer_t gssmic)
+{
+	if (ctx == NULL)
+		return -1;
+
+	ctx->major = gss_verify_mic(&ctx->minor, ctx->context,
+	    gssbuf, gssmic, NULL);
+
+	return (ctx->major);
+}
+
 void
 ssh_gssapi_buildmic(struct sshbuf *b, const char *user, const char *service,
     const char *context, const struct sshbuf *session_id)
@@ -271,11 +502,16 @@ ssh_gssapi_buildmic(struct sshbuf *b, co
 }
 
 int
-ssh_gssapi_check_mechanism(Gssctxt **ctx, gss_OID oid, const char *host)
+ssh_gssapi_check_mechanism(Gssctxt **ctx, gss_OID oid, const char *host,
+    const char *client)
 {
 	gss_buffer_desc token = GSS_C_EMPTY_BUFFER;
 	OM_uint32 major, minor;
 	gss_OID_desc spnego_oid = {6, (void *)"\x2B\x06\x01\x05\x05\x02"};
+	Gssctxt *intctx = NULL;
+
+	if (ctx == NULL)
+		ctx = &intctx;
 
 	/* RFC 4462 says we MUST NOT do SPNEGO */
 	if (oid->length == spnego_oid.length &&
@@ -285,6 +521,10 @@ ssh_gssapi_check_mechanism(Gssctxt **ctx
 	ssh_gssapi_build_ctx(ctx);
 	ssh_gssapi_set_oid(*ctx, oid);
 	major = ssh_gssapi_import_name(*ctx, host);
+
+	if (!GSS_ERROR(major) && client)
+		major = ssh_gssapi_client_identity(*ctx, client);
+
 	if (!GSS_ERROR(major)) {
 		major = ssh_gssapi_init_ctx(*ctx, 0, GSS_C_NO_BUFFER, &token,
 		    NULL);
@@ -294,10 +534,66 @@ ssh_gssapi_check_mechanism(Gssctxt **ctx
 			    GSS_C_NO_BUFFER);
 	}
 
-	if (GSS_ERROR(major))
+	if (GSS_ERROR(major) || intctx != NULL)
 		ssh_gssapi_delete_ctx(ctx);
 
 	return (!GSS_ERROR(major));
 }
 
+int
+ssh_gssapi_credentials_updated(Gssctxt *ctxt) {
+	static gss_name_t saved_name = GSS_C_NO_NAME;
+	static OM_uint32 saved_lifetime = 0;
+	static gss_OID saved_mech = GSS_C_NO_OID;
+	static gss_name_t name;
+	static OM_uint32 last_call = 0;
+	OM_uint32 lifetime, now, major, minor;
+	int equal;
+
+	now = time(NULL);
+
+	if (ctxt) {
+		debug("Rekey has happened - updating saved versions");
+
+		if (saved_name != GSS_C_NO_NAME)
+			gss_release_name(&minor, &saved_name);
+
+		major = gss_inquire_cred(&minor, GSS_C_NO_CREDENTIAL,
+		    &saved_name, &saved_lifetime, NULL, NULL);
+
+		if (!GSS_ERROR(major)) {
+			saved_mech = ctxt->oid;
+		        saved_lifetime+= now;
+		} else {
+			/* Handle the error */
+		}
+		return 0;
+	}
+
+	if (now - last_call < 10)
+		return 0;
+
+	last_call = now;
+
+	if (saved_mech == GSS_C_NO_OID)
+		return 0;
+
+	major = gss_inquire_cred(&minor, GSS_C_NO_CREDENTIAL,
+	    &name, &lifetime, NULL, NULL);
+	if (major == GSS_S_CREDENTIALS_EXPIRED)
+		return 0;
+	else if (GSS_ERROR(major))
+		return 0;
+
+	major = gss_compare_name(&minor, saved_name, name, &equal);
+	gss_release_name(&minor, &name);
+	if (GSS_ERROR(major))
+		return 0;
+
+	if (equal && (saved_lifetime < lifetime + now - 10))
+		return 1;
+
+	return 0;
+}
+
 #endif /* GSSAPI */
diff --color -ruNp a/gss-serv.c b/gss-serv.c
--- a/gss-serv.c	2024-07-01 06:36:28.000000000 +0200
+++ b/gss-serv.c	2024-09-16 11:46:34.692939840 +0200
@@ -1,7 +1,7 @@
 /* $OpenBSD: gss-serv.c,v 1.32 2020/03/13 03:17:07 djm Exp $ */
 
 /*
- * Copyright (c) 2001-2003 Simon Wilkinson. All rights reserved.
+ * Copyright (c) 2001-2009 Simon Wilkinson. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -44,17 +44,19 @@
 #include "session.h"
 #include "misc.h"
 #include "servconf.h"
+#include "uidswap.h"
 
 #include "ssh-gss.h"
+#include "monitor_wrap.h"
 
 extern ServerOptions options;
 
 static ssh_gssapi_client gssapi_client =
-    { GSS_C_EMPTY_BUFFER, GSS_C_EMPTY_BUFFER,
-    GSS_C_NO_CREDENTIAL, NULL, {NULL, NULL, NULL, NULL}};
+    { GSS_C_EMPTY_BUFFER, GSS_C_EMPTY_BUFFER, GSS_C_NO_CREDENTIAL,
+    GSS_C_NO_NAME, NULL, {NULL, NULL, NULL, NULL, NULL}, 0, 0};
 
 ssh_gssapi_mech gssapi_null_mech =
-    { NULL, NULL, {0, NULL}, NULL, NULL, NULL, NULL};
+    { NULL, NULL, {0, NULL}, NULL, NULL, NULL, NULL, NULL};
 
 #ifdef KRB5
 extern ssh_gssapi_mech gssapi_kerberos_mech;
@@ -141,6 +143,29 @@ ssh_gssapi_server_ctx(Gssctxt **ctx, gss
 }
 
 /* Unprivileged */
+char *
+ssh_gssapi_server_mechanisms(void) {
+	if (supported_oids == NULL)
+		ssh_gssapi_prepare_supported_oids();
+	return (ssh_gssapi_kex_mechs(supported_oids,
+	    &ssh_gssapi_server_check_mech, NULL, NULL,
+	    options.gss_kex_algorithms));
+}
+
+/* Unprivileged */
+int
+ssh_gssapi_server_check_mech(Gssctxt **dum, gss_OID oid, const char *data,
+    const char *dummy) {
+	Gssctxt *ctx = NULL;
+	int res;
+
+	res = !GSS_ERROR(mm_ssh_gssapi_server_ctx(&ctx, oid));
+	ssh_gssapi_delete_ctx(&ctx);
+
+	return (res);
+}
+
+/* Unprivileged */
 void
 ssh_gssapi_supported_oids(gss_OID_set *oidset)
 {
@@ -150,7 +175,9 @@ ssh_gssapi_supported_oids(gss_OID_set *o
 	gss_OID_set supported;
 
 	gss_create_empty_oid_set(&min_status, oidset);
-	gss_indicate_mechs(&min_status, &supported);
+
+	if (GSS_ERROR(gss_indicate_mechs(&min_status, &supported)))
+		return;
 
 	while (supported_mechs[i]->name != NULL) {
 		if (GSS_ERROR(gss_test_oid_set_member(&min_status,
@@ -276,8 +303,48 @@ OM_uint32
 ssh_gssapi_getclient(Gssctxt *ctx, ssh_gssapi_client *client)
 {
 	int i = 0;
+	int equal = 0;
+	gss_name_t new_name = GSS_C_NO_NAME;
+	gss_buffer_desc ename = GSS_C_EMPTY_BUFFER;
+
+	if (options.gss_store_rekey && client->used && ctx->client_creds) {
+		if (client->mech->oid.length != ctx->oid->length ||
+		    (memcmp(client->mech->oid.elements,
+		     ctx->oid->elements, ctx->oid->length) !=0)) {
+			debug("Rekeyed credentials have different mechanism");
+			return GSS_S_COMPLETE;
+		}
 
-	gss_buffer_desc ename;
+		if ((ctx->major = gss_inquire_cred_by_mech(&ctx->minor,
+		    ctx->client_creds, ctx->oid, &new_name,
+		    NULL, NULL, NULL))) {
+			ssh_gssapi_error(ctx);
+			return (ctx->major);
+		}
+
+		ctx->major = gss_compare_name(&ctx->minor, client->name,
+		    new_name, &equal);
+
+		if (GSS_ERROR(ctx->major)) {
+			ssh_gssapi_error(ctx);
+			return (ctx->major);
+		}
+
+		if (!equal) {
+			debug("Rekeyed credentials have different name");
+			return GSS_S_COMPLETE;
+		}
+
+		debug("Marking rekeyed credentials for export");
+
+		gss_release_name(&ctx->minor, &client->name);
+		gss_release_cred(&ctx->minor, &client->creds);
+		client->name = new_name;
+		client->creds = ctx->client_creds;
+		ctx->client_creds = GSS_C_NO_CREDENTIAL;
+		client->updated = 1;
+		return GSS_S_COMPLETE;
+	}
 
 	client->mech = NULL;
 
@@ -292,6 +359,13 @@ ssh_gssapi_getclient(Gssctxt *ctx, ssh_g
 	if (client->mech == NULL)
 		return GSS_S_FAILURE;
 
+	if (ctx->client_creds &&
+	    (ctx->major = gss_inquire_cred_by_mech(&ctx->minor,
+	     ctx->client_creds, ctx->oid, &client->name, NULL, NULL, NULL))) {
+		ssh_gssapi_error(ctx);
+		return (ctx->major);
+	}
+
 	if ((ctx->major = gss_display_name(&ctx->minor, ctx->client,
 	    &client->displayname, NULL))) {
 		ssh_gssapi_error(ctx);
@@ -309,6 +383,8 @@ ssh_gssapi_getclient(Gssctxt *ctx, ssh_g
 		return (ctx->major);
 	}
 
+	gss_release_buffer(&ctx->minor, &ename);
+
 	/* We can't copy this structure, so we just move the pointer to it */
 	client->creds = ctx->client_creds;
 	ctx->client_creds = GSS_C_NO_CREDENTIAL;
@@ -319,11 +395,20 @@ ssh_gssapi_getclient(Gssctxt *ctx, ssh_g
 void
 ssh_gssapi_cleanup_creds(void)
 {
-	if (gssapi_client.store.filename != NULL) {
-		/* Unlink probably isn't sufficient */
-		debug("removing gssapi cred file\"%s\"",
-		    gssapi_client.store.filename);
-		unlink(gssapi_client.store.filename);
+	krb5_ccache ccache = NULL;
+	krb5_error_code problem;
+
+	if (gssapi_client.store.data != NULL) {
+		if ((problem = krb5_cc_resolve(gssapi_client.store.data, gssapi_client.store.envval, &ccache))) {
+			debug_f("krb5_cc_resolve(): %.100s",
+				krb5_get_err_text(gssapi_client.store.data, problem));
+		} else if ((problem = krb5_cc_destroy(gssapi_client.store.data, ccache))) {
+			debug_f("krb5_cc_destroy(): %.100s",
+				krb5_get_err_text(gssapi_client.store.data, problem));
+		} else {
+			krb5_free_context(gssapi_client.store.data);
+			gssapi_client.store.data = NULL;
+		}
 	}
 }
 
@@ -356,19 +441,23 @@ ssh_gssapi_do_child(char ***envp, u_int
 
 /* Privileged */
 int
-ssh_gssapi_userok(char *user)
+ssh_gssapi_userok(char *user, struct passwd *pw, int kex)
 {
 	OM_uint32 lmin;
 
+	(void) kex; /* used in privilege separation */
+
 	if (gssapi_client.exportedname.length == 0 ||
 	    gssapi_client.exportedname.value == NULL) {
 		debug("No suitable client data");
 		return 0;
 	}
 	if (gssapi_client.mech && gssapi_client.mech->userok)
-		if ((*gssapi_client.mech->userok)(&gssapi_client, user))
+		if ((*gssapi_client.mech->userok)(&gssapi_client, user)) {
+			gssapi_client.used = 1;
+			gssapi_client.store.owner = pw;
 			return 1;
-		else {
+		} else {
 			/* Destroy delegated credentials if userok fails */
 			gss_release_buffer(&lmin, &gssapi_client.displayname);
 			gss_release_buffer(&lmin, &gssapi_client.exportedname);
@@ -382,14 +471,85 @@ ssh_gssapi_userok(char *user)
 	return (0);
 }
 
-/* Privileged */
-OM_uint32
-ssh_gssapi_checkmic(Gssctxt *ctx, gss_buffer_t gssbuf, gss_buffer_t gssmic)
+/* These bits are only used for rekeying. The unpriviledged child is running
+ * as the user, the monitor is root.
+ *
+ * In the child, we want to :
+ *    *) Ask the monitor to store our credentials into the store we specify
+ *    *) If it succeeds, maybe do a PAM update
+ */
+
+/* Stuff for PAM */
+
+#ifdef USE_PAM
+static int ssh_gssapi_simple_conv(int n, const struct pam_message **msg,
+    struct pam_response **resp, void *data)
 {
-	ctx->major = gss_verify_mic(&ctx->minor, ctx->context,
-	    gssbuf, gssmic, NULL);
+	return (PAM_CONV_ERR);
+}
+#endif
 
-	return (ctx->major);
+void
+ssh_gssapi_rekey_creds(void) {
+	int ok;
+#ifdef USE_PAM
+	int ret;
+	pam_handle_t *pamh = NULL;
+	struct pam_conv pamconv = {ssh_gssapi_simple_conv, NULL};
+	char *envstr;
+#endif
+
+	if (gssapi_client.store.filename == NULL &&
+	    gssapi_client.store.envval == NULL &&
+	    gssapi_client.store.envvar == NULL)
+		return;
+
+	ok = mm_ssh_gssapi_update_creds(&gssapi_client.store);
+
+	if (!ok)
+		return;
+
+	debug("Rekeyed credentials stored successfully");
+
+	/* Actually managing to play with the ssh pam stack from here will
+	 * be next to impossible. In any case, we may want different options
+	 * for rekeying. So, use our own :)
+	 */
+#ifdef USE_PAM	
+	ret = pam_start("sshd-rekey", gssapi_client.store.owner->pw_name,
+ 	    &pamconv, &pamh);
+	if (ret)
+		return;
+
+	xasprintf(&envstr, "%s=%s", gssapi_client.store.envvar,
+	    gssapi_client.store.envval);
+
+	ret = pam_putenv(pamh, envstr);
+	if (!ret)
+		pam_setcred(pamh, PAM_REINITIALIZE_CRED);
+	pam_end(pamh, PAM_SUCCESS);
+#endif
+}
+
+int
+ssh_gssapi_update_creds(ssh_gssapi_ccache *store) {
+	int ok = 0;
+
+	/* Check we've got credentials to store */
+	if (!gssapi_client.updated)
+		return 0;
+
+	gssapi_client.updated = 0;
+
+	temporarily_use_uid(gssapi_client.store.owner);
+	if (gssapi_client.mech && gssapi_client.mech->updatecreds)
+		ok = (*gssapi_client.mech->updatecreds)(store, &gssapi_client);
+	else
+		debug("No update function for this mechanism");
+
+	restore_uid();
+
+	return ok;
 }
 
 /* Privileged */
diff --color -ruNp a/gss-serv-krb5.c b/gss-serv-krb5.c
--- a/gss-serv-krb5.c	2024-07-01 06:36:28.000000000 +0200
+++ b/gss-serv-krb5.c	2024-09-16 11:46:34.692939840 +0200
@@ -1,7 +1,7 @@
 /* $OpenBSD: gss-serv-krb5.c,v 1.9 2018/07/09 21:37:55 markus Exp $ */
 
 /*
- * Copyright (c) 2001-2003 Simon Wilkinson. All rights reserved.
+ * Copyright (c) 2001-2007 Simon Wilkinson. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -120,7 +120,7 @@ ssh_gssapi_krb5_storecreds(ssh_gssapi_cl
 	krb5_error_code problem;
 	krb5_principal princ;
 	OM_uint32 maj_status, min_status;
-	int len;
+	const char *new_ccname, *new_cctype;
 	const char *errmsg;
 
 	if (client->creds == NULL) {
@@ -180,11 +180,26 @@ ssh_gssapi_krb5_storecreds(ssh_gssapi_cl
 		return;
 	}
 
-	client->store.filename = xstrdup(krb5_cc_get_name(krb_context, ccache));
+	new_cctype = krb5_cc_get_type(krb_context, ccache);
+	new_ccname = krb5_cc_get_name(krb_context, ccache);
+
 	client->store.envvar = "KRB5CCNAME";
-	len = strlen(client->store.filename) + 6;
-	client->store.envval = xmalloc(len);
-	snprintf(client->store.envval, len, "FILE:%s", client->store.filename);
+#ifdef USE_CCAPI
+	xasprintf(&client->store.envval, "API:%s", new_ccname);
+	client->store.filename = NULL;
+#else
+	if (new_ccname[0] == ':')
+		new_ccname++;
+	xasprintf(&client->store.envval, "%s:%s", new_cctype, new_ccname);
+	if (strcmp(new_cctype, "DIR") == 0) {
+		char *p;
+		p = strrchr(client->store.envval, '/');
+		if (p)
+			*p = '\0';
+	}
+	if ((strcmp(new_cctype, "FILE") == 0) || (strcmp(new_cctype, "DIR") == 0))
+		client->store.filename = xstrdup(new_ccname);
+#endif
 
 #ifdef USE_PAM
 	if (options.use_pam)
@@ -193,9 +208,76 @@ ssh_gssapi_krb5_storecreds(ssh_gssapi_cl
 
 	krb5_cc_close(krb_context, ccache);
 
+	client->store.data = krb_context;
+
 	return;
 }
 
+int
+ssh_gssapi_krb5_updatecreds(ssh_gssapi_ccache *store,
+    ssh_gssapi_client *client)
+{
+	krb5_ccache ccache = NULL;
+	krb5_principal principal = NULL;
+	char *name = NULL;
+	krb5_error_code problem;
+	OM_uint32 maj_status, min_status;
+
+	if ((problem = krb5_cc_resolve(krb_context, store->envval, &ccache))) {
+                logit("krb5_cc_resolve(): %.100s",
+                    krb5_get_err_text(krb_context, problem));
+                return 0;
+	}
+
+	/* Find out who the principal in this cache is */
+	if ((problem = krb5_cc_get_principal(krb_context, ccache,
+	    &principal))) {
+		logit("krb5_cc_get_principal(): %.100s",
+		    krb5_get_err_text(krb_context, problem));
+		krb5_cc_close(krb_context, ccache);
+		return 0;
+	}
+
+	if ((problem = krb5_unparse_name(krb_context, principal, &name))) {
+		logit("krb5_unparse_name(): %.100s",
+		    krb5_get_err_text(krb_context, problem));
+		krb5_free_principal(krb_context, principal);
+		krb5_cc_close(krb_context, ccache);
+		return 0;
+	}
+
+
+	if (strcmp(name,client->exportedname.value)!=0) {
+		debug("Name in local credentials cache differs. Not storing");
+		krb5_free_principal(krb_context, principal);
+		krb5_cc_close(krb_context, ccache);
+		krb5_free_unparsed_name(krb_context, name);
+		return 0;
+	}
+	krb5_free_unparsed_name(krb_context, name);
+
+	/* Name matches, so lets get on with it! */
+
+	if ((problem = krb5_cc_initialize(krb_context, ccache, principal))) {
+		logit("krb5_cc_initialize(): %.100s",
+		    krb5_get_err_text(krb_context, problem));
+		krb5_free_principal(krb_context, principal);
+		krb5_cc_close(krb_context, ccache);
+		return 0;
+	}
+
+	krb5_free_principal(krb_context, principal);
+
+	if ((maj_status = gss_krb5_copy_ccache(&min_status, client->creds,
+	    ccache))) {
+		logit("gss_krb5_copy_ccache() failed. Sorry!");
+		krb5_cc_close(krb_context, ccache);
+		return 0;
+	}
+
+	return 1;
+}
+
 ssh_gssapi_mech gssapi_kerberos_mech = {
 	"toWM5Slw5Ew8Mqkay+al2g==",
 	"Kerberos",
@@ -203,7 +285,8 @@ ssh_gssapi_mech gssapi_kerberos_mech = {
 	NULL,
 	&ssh_gssapi_krb5_userok,
 	NULL,
-	&ssh_gssapi_krb5_storecreds
+	&ssh_gssapi_krb5_storecreds,
+	&ssh_gssapi_krb5_updatecreds
 };
 
 #endif /* KRB5 */
diff --color -ruNp a/kex.c b/kex.c
--- a/kex.c	2024-07-01 06:36:28.000000000 +0200
+++ b/kex.c	2024-09-16 11:46:34.692939840 +0200
@@ -297,17 +297,37 @@ static int
 kex_compose_ext_info_server(struct ssh *ssh, struct sshbuf *m)
 {
 	int r;
+	int have_key = 0;
+	int ext_count = 2;
+
+#ifdef GSSAPI
+	/*
+	 * Currently GSS KEX don't provide host keys as optional message, so
+	 * no reasons to announce the publickey-hostbound extension
+	 */
+	if (ssh->kex->gss == NULL)
+	    have_key = 1;
+#endif
+	ext_count += have_key;
+
 
 	if (ssh->kex->server_sig_algs == NULL &&
 	    (ssh->kex->server_sig_algs = sshkey_alg_list(0, 1, 1, ',')) == NULL)
 		return SSH_ERR_ALLOC_FAIL;
-	if ((r = sshbuf_put_u32(m, 3)) != 0 ||
+	if ((r = sshbuf_put_u32(m, ext_count)) != 0 ||
 	    (r = sshbuf_put_cstring(m, "server-sig-algs")) != 0 ||
-	    (r = sshbuf_put_cstring(m, ssh->kex->server_sig_algs)) != 0 ||
-	    (r = sshbuf_put_cstring(m,
-	    "publickey-hostbound@openssh.com")) != 0 ||
-	    (r = sshbuf_put_cstring(m, "0")) != 0 ||
-	    (r = sshbuf_put_cstring(m, "ping@openssh.com")) != 0 ||
+	    (r = sshbuf_put_cstring(m, ssh->kex->server_sig_algs)) != 0) {
+		error_fr(r, "compose");
+		return r;
+	}
+	if (have_key) {
+	    if ((r = sshbuf_put_cstring(m, "publickey-hostbound@openssh.com")) != 0 ||
+	        (r = sshbuf_put_cstring(m, "0")) != 0) {
+		    error_fr(r, "compose");
+		    return r;
+		}
+	}
+	if ((r = sshbuf_put_cstring(m, "ping@openssh.com")) != 0 ||
 	    (r = sshbuf_put_cstring(m, "0")) != 0) {
 		error_fr(r, "compose");
 		return r;
@@ -737,6 +757,9 @@ kex_free(struct kex *kex)
 	sshbuf_free(kex->server_version);
 	sshbuf_free(kex->client_pub);
 	sshbuf_free(kex->session_id);
+#ifdef GSSAPI
+	free(kex->gss_host);
+#endif /* GSSAPI */
 	sshbuf_free(kex->initial_sig);
 	sshkey_free(kex->initial_hostkey);
 	free(kex->failed_choice);
diff --color -ruNp a/kexdh.c b/kexdh.c
--- a/kexdh.c	2024-07-01 06:36:28.000000000 +0200
+++ b/kexdh.c	2024-09-16 11:46:34.693939862 +0200
@@ -49,13 +49,23 @@ kex_dh_keygen(struct kex *kex)
 {
 	switch (kex->kex_type) {
 	case KEX_DH_GRP1_SHA1:
+#ifdef GSSAPI
+	case KEX_GSS_GRP1_SHA1:
+#endif
 		kex->dh = dh_new_group1();
 		break;
 	case KEX_DH_GRP14_SHA1:
 	case KEX_DH_GRP14_SHA256:
+#ifdef GSSAPI
+	case KEX_GSS_GRP14_SHA1:
+	case KEX_GSS_GRP14_SHA256:
+#endif
 		kex->dh = dh_new_group14();
 		break;
 	case KEX_DH_GRP16_SHA512:
+#ifdef GSSAPI
+	case KEX_GSS_GRP16_SHA512:
+#endif
 		kex->dh = dh_new_group16();
 		break;
 	case KEX_DH_GRP18_SHA512:
diff --color -ruNp a/kexgen.c b/kexgen.c
--- a/kexgen.c	2024-07-01 06:36:28.000000000 +0200
+++ b/kexgen.c	2024-09-16 11:46:34.693939862 +0200
@@ -44,7 +44,7 @@
 static int input_kex_gen_init(int, u_int32_t, struct ssh *);
 static int input_kex_gen_reply(int type, u_int32_t seq, struct ssh *ssh);
 
-static int
+int
 kex_gen_hash(
     int hash_alg,
     const struct sshbuf *client_version,
diff --color -ruNp a/kexgssc.c b/kexgssc.c
--- a/kexgssc.c	1970-01-01 01:00:00.000000000 +0100
+++ b/kexgssc.c	2024-10-14 15:18:02.491798105 +0200
@@ -0,0 +1,706 @@
+/*
+ * Copyright (c) 2001-2009 Simon Wilkinson. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR `AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "includes.h"
+
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+
+#include "includes.h"
+
+#include <openssl/crypto.h>
+#include <openssl/bn.h>
+
+#include <string.h>
+
+#include "xmalloc.h"
+#include "sshbuf.h"
+#include "ssh2.h"
+#include "sshkey.h"
+#include "cipher.h"
+#include "kex.h"
+#include "log.h"
+#include "packet.h"
+#include "dh.h"
+#include "digest.h"
+#include "ssherr.h"
+
+#include "ssh-gss.h"
+
+static int input_kexgss_hostkey(int, u_int32_t, struct ssh *);
+static int input_kexgss_continue(int, u_int32_t, struct ssh *);
+static int input_kexgss_complete(int, u_int32_t, struct ssh *);
+static int input_kexgss_error(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_group(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_continue(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_complete(int, u_int32_t, struct ssh *);
+
+static int
+kexgss_final(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	struct sshbuf *empty = NULL;
+	struct sshbuf *shared_secret = NULL;
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	size_t hashlen;
+	int r;
+
+	/*
+	 * We _must_ have received a COMPLETE message in reply from the
+	 * server, which will have set server_blob and msg_tok
+	 */
+
+	/* compute shared secret */
+	switch (kex->kex_type) {
+	case KEX_GSS_GRP1_SHA1:
+	case KEX_GSS_GRP14_SHA1:
+	case KEX_GSS_GRP14_SHA256:
+	case KEX_GSS_GRP16_SHA512:
+		r = kex_dh_dec(kex, gss->server_blob, &shared_secret);
+		break;
+	case KEX_GSS_C25519_SHA256:
+		if (sshbuf_ptr(gss->server_blob)[sshbuf_len(gss->server_blob)] & 0x80)
+			fatal("The received key has MSB of last octet set!");
+		r = kex_c25519_dec(kex, gss->server_blob, &shared_secret);
+		break;
+	case KEX_GSS_NISTP256_SHA256:
+		if (sshbuf_len(gss->server_blob) != 65)
+			fatal("The received NIST-P256 key did not match "
+			      "expected length (expected 65, got %zu)",
+			      sshbuf_len(gss->server_blob));
+
+		if (sshbuf_ptr(gss->server_blob)[0] != POINT_CONVERSION_UNCOMPRESSED)
+			fatal("The received NIST-P256 key does not have first octet 0x04");
+
+		r = kex_ecdh_dec(kex, gss->server_blob, &shared_secret);
+		break;
+	default:
+		r = SSH_ERR_INVALID_ARGUMENT;
+		break;
+	}
+	if (r != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		goto out;
+	}
+
+	if ((empty = sshbuf_new()) == NULL) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+
+	hashlen = sizeof(hash);
+	r = kex_gen_hash(kex->hash_alg, kex->client_version,
+			 kex->server_version, kex->my, kex->peer,
+			 (gss->server_host_key_blob ? gss->server_host_key_blob : empty),
+			 kex->client_pub, gss->server_blob, shared_secret,
+			 hash, &hashlen);
+	sshbuf_free(empty);
+	if (r != 0)
+		fatal_f("Unexpected KEX type %d", kex->kex_type);
+
+	gss->buf.value = hash;
+	gss->buf.length = hashlen;
+
+	/* Verify that the hash matches the MIC we just got. */
+	if (GSS_ERROR(ssh_gssapi_checkmic(gss, &gss->buf, &gss->msg_tok)))
+		sshpkt_disconnect(ssh, "Hash's MIC didn't verify");
+
+	gss_release_buffer(&gss->minor, &gss->msg_tok);
+
+	if (kex->gss_deleg_creds)
+		ssh_gssapi_credentials_updated(gss);
+
+	if (gss_kex_context == NULL)
+		gss_kex_context = gss;
+	else
+		ssh_gssapi_delete_ctx(&kex->gss);
+
+	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
+		r = kex_send_newkeys(ssh);
+
+	if (kex->gss != NULL) {
+		sshbuf_free(gss->server_host_key_blob);
+		gss->server_host_key_blob = NULL;
+		sshbuf_free(gss->server_blob);
+		gss->server_blob = NULL;
+	}
+out:
+	explicit_bzero(kex->c25519_client_key, sizeof(kex->c25519_client_key));
+	explicit_bzero(hash, sizeof(hash));
+	sshbuf_free(shared_secret);
+	sshbuf_free(kex->client_pub);
+	kex->client_pub = NULL;
+	return r;
+}
+
+static int
+kexgss_init_ctx(struct ssh *ssh,
+		gss_buffer_desc *token_ptr)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	gss_buffer_desc send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags;
+	int r;
+
+	debug("Calling gss_init_sec_context");
+
+	gss->major = ssh_gssapi_init_ctx(gss, kex->gss_deleg_creds,
+					 token_ptr, &send_tok, &ret_flags);
+
+	if (GSS_ERROR(gss->major)) {
+		/* XXX Useless code: Missing send? */
+		if (send_tok.length != 0) {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0)
+				fatal("sshpkt failed: %s", ssh_err(r));
+		}
+		fatal("gss_init_context failed");
+	}
+
+	/* If we've got an old receive buffer get rid of it */
+	if (token_ptr != GSS_C_NO_BUFFER)
+		gss_release_buffer(&gss->minor, token_ptr);
+
+	if (gss->major == GSS_S_COMPLETE) {
+		/* If mutual state flag is not true, kex fails */
+		if (!(ret_flags & GSS_C_MUTUAL_FLAG))
+			fatal("Mutual authentication failed");
+
+		/* If integ avail flag is not true kex fails */
+		if (!(ret_flags & GSS_C_INTEG_FLAG))
+			fatal("Integrity check failed");
+	}
+
+	/*
+	 * If we have data to send, then the last message that we
+	 * received cannot have been a 'complete'.
+	 */
+	if (send_tok.length != 0) {
+		if (gss->first) {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_INIT)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0 ||
+			    (r = sshpkt_put_stringb(ssh, kex->client_pub)) != 0)
+				fatal("failed to construct packet: %s", ssh_err(r));
+			gss->first = 0;
+		} else {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0)
+				fatal("failed to construct packet: %s", ssh_err(r));
+		}
+		if ((r = sshpkt_send(ssh)) != 0)
+			fatal("failed to send packet: %s", ssh_err(r));
+		gss_release_buffer(&gss->minor, &send_tok);
+
+		/* If we've sent them data, they should reply */
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, &input_kexgss_hostkey);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, &input_kexgss_continue);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, &input_kexgss_complete);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, &input_kexgss_error);
+		return 0;
+	}
+	/* No data, and not complete */
+	if (gss->major != GSS_S_COMPLETE)
+		fatal("Not complete, and no token output");
+
+	if  (gss->major & GSS_S_CONTINUE_NEEDED)
+		return kexgss_init_ctx(ssh, token_ptr);
+
+	return kexgss_final(ssh);
+}
+
+int
+kexgss_client(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	int r;
+
+	/* Initialise our GSSAPI world */
+	ssh_gssapi_build_ctx(&kex->gss);
+	if (ssh_gssapi_id_kex(kex->gss, kex->name, kex->kex_type) == GSS_C_NO_OID)
+		fatal("Couldn't identify host exchange");
+
+	if (ssh_gssapi_import_name(kex->gss, kex->gss_host))
+		fatal("Couldn't import hostname");
+
+	if (kex->gss_client &&
+	    ssh_gssapi_client_identity(kex->gss, kex->gss_client))
+		fatal("Couldn't acquire client credentials");
+
+	/* Step 1 */
+	switch (kex->kex_type) {
+	case KEX_GSS_GRP1_SHA1:
+	case KEX_GSS_GRP14_SHA1:
+	case KEX_GSS_GRP14_SHA256:
+	case KEX_GSS_GRP16_SHA512:
+		r = kex_dh_keypair(kex);
+		break;
+	case KEX_GSS_NISTP256_SHA256:
+		r = kex_ecdh_keypair(kex);
+		break;
+	case KEX_GSS_C25519_SHA256:
+		r = kex_c25519_keypair(kex);
+		break;
+	default:
+		fatal_f("Unexpected KEX type %d", kex->kex_type);
+	}
+	if (r != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		return r;
+	}
+	return kexgss_init_ctx(ssh, GSS_C_NO_BUFFER);
+}
+
+static int
+input_kexgss_hostkey(int type,
+		     u_int32_t seq,
+		     struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	u_char *tmp = NULL;
+	size_t tmp_len = 0;
+	int r;
+
+	debug("Received KEXGSS_HOSTKEY");
+	if (gss->server_host_key_blob)
+		fatal("Server host key received more than once");
+	if ((r = sshpkt_get_string(ssh, &tmp, &tmp_len)) != 0)
+		fatal("Failed to read server host key: %s", ssh_err(r));
+	if ((gss->server_host_key_blob = sshbuf_from(tmp, tmp_len)) == NULL)
+		fatal("sshbuf_from failed");
+	return 0;
+}
+
+static int
+input_kexgss_continue(int type,
+		      u_int32_t seq,
+		      struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok = GSS_C_EMPTY_BUFFER;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, NULL);
+
+	debug("Received GSSAPI_CONTINUE");
+	if (gss->major == GSS_S_COMPLETE)
+		fatal("GSSAPI Continue received from server when complete");
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("Failed to read token: %s", ssh_err(r));
+	if  (!(gss->major & GSS_S_CONTINUE_NEEDED))
+		fatal("Didn't receive a SSH2_MSG_KEXGSS_COMPLETE when I expected it");
+	return kexgss_init_ctx(ssh, &recv_tok);
+}
+
+static int
+input_kexgss_complete(int type,
+		      u_int32_t seq,
+		      struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok = GSS_C_EMPTY_BUFFER;
+	u_char c;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, NULL);
+
+	debug("Received GSSAPI_COMPLETE");
+	if (gss->msg_tok.value != NULL)
+	        fatal("Received GSSAPI_COMPLETE twice?");
+	if ((r = sshpkt_getb_froms(ssh, &gss->server_blob)) != 0 ||
+	    (r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &gss->msg_tok)) != 0)
+		fatal("Failed to read message: %s", ssh_err(r));
+
+	/* Is there a token included? */
+	if ((r = sshpkt_get_u8(ssh, &c)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+	if (c) {
+		if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0)
+			fatal("Failed to read token: %s", ssh_err(r));
+		/* If we're already complete - protocol error */
+		if (gss->major == GSS_S_COMPLETE)
+			sshpkt_disconnect(ssh, "Protocol error: received token when complete");
+	} else {
+		if (gss->major != GSS_S_COMPLETE)
+			sshpkt_disconnect(ssh, "Protocol error: did not receive final token");
+	}
+	if ((r = sshpkt_get_end(ssh)) != 0)
+		fatal("Expecting end of packet.");
+
+	if  (gss->major & GSS_S_CONTINUE_NEEDED)
+		return kexgss_init_ctx(ssh, &recv_tok);
+
+	gss_release_buffer(&gss->minor, &recv_tok);
+	return kexgss_final(ssh);
+}
+
+static int
+input_kexgss_error(int type,
+		   u_int32_t seq,
+		   struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	u_char *msg;
+	int r;
+
+	debug("Received Error");
+	if ((r = sshpkt_get_u32(ssh, &gss->major)) != 0 ||
+	    (r = sshpkt_get_u32(ssh, &gss->minor)) != 0 ||
+	    (r = sshpkt_get_string(ssh, &msg, NULL)) != 0 ||
+	    (r = sshpkt_get_string(ssh, NULL, NULL)) != 0 || /* lang tag */
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt_get failed: %s", ssh_err(r));
+	fatal("GSSAPI Error: \n%.400s", msg);
+	return 0;
+}
+
+/*******************************************************/
+/******************** KEXGSSGEX ************************/
+/*******************************************************/
+
+int
+kexgssgex_client(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	int r;
+
+	/* Initialise our GSSAPI world */
+	ssh_gssapi_build_ctx(&kex->gss);
+	if (ssh_gssapi_id_kex(kex->gss, kex->name, kex->kex_type) == GSS_C_NO_OID)
+		fatal("Couldn't identify host exchange");
+
+	if (ssh_gssapi_import_name(kex->gss, kex->gss_host))
+		fatal("Couldn't import hostname");
+
+	if (kex->gss_client &&
+	    ssh_gssapi_client_identity(kex->gss, kex->gss_client))
+		fatal("Couldn't acquire client credentials");
+
+	debug("Doing group exchange");
+	kex->min = DH_GRP_MIN;
+	kex->max = DH_GRP_MAX;
+	kex->nbits = dh_estimate(kex->dh_need * 8);
+
+	if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_GROUPREQ)) != 0 ||
+	    (r = sshpkt_put_u32(ssh, kex->min)) != 0 ||
+	    (r = sshpkt_put_u32(ssh, kex->nbits)) != 0 ||
+	    (r = sshpkt_put_u32(ssh, kex->max)) != 0 ||
+	    (r = sshpkt_send(ssh)) != 0)
+		fatal("Failed to construct a packet: %s", ssh_err(r));
+
+	debug("Wait SSH2_MSG_KEXGSS_GROUP");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_GROUP, &input_kexgssgex_group);
+	return 0;
+}
+
+static int
+kexgssgex_final(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	struct sshbuf *buf = NULL;
+	struct sshbuf *empty = NULL;
+	struct sshbuf *shared_secret = NULL;
+	BIGNUM *dh_server_pub = NULL;
+	const BIGNUM *pub_key, *dh_p, *dh_g;
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	size_t hashlen;
+	int r = SSH_ERR_INTERNAL_ERROR;
+
+	/*
+	 * We _must_ have received a COMPLETE message in reply from the
+	 * server, which will have set server_blob and msg_tok
+	 */
+
+	/* 7. C verifies that the key Q_S is valid */
+	/* 8. C computes shared secret */
+	if ((buf = sshbuf_new()) == NULL ||
+	    (r = sshbuf_put_stringb(buf, gss->server_blob)) != 0 ||
+	    (r = sshbuf_get_bignum2(buf, &dh_server_pub)) != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		goto out;
+	}
+	sshbuf_free(buf);
+	buf = NULL;
+
+	if ((shared_secret = sshbuf_new()) == NULL) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+
+	if ((r = kex_dh_compute_key(kex, dh_server_pub, shared_secret)) != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		goto out;
+	}
+
+	if ((empty = sshbuf_new()) == NULL) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+
+	DH_get0_key(kex->dh, &pub_key, NULL);
+	DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);
+	hashlen = sizeof(hash);
+	r = kexgex_hash(kex->hash_alg, kex->client_version,
+			kex->server_version, kex->my, kex->peer,
+			(gss->server_host_key_blob ? gss->server_host_key_blob : empty),
+			kex->min, kex->nbits, kex->max, dh_p, dh_g, pub_key,
+			dh_server_pub, sshbuf_ptr(shared_secret), sshbuf_len(shared_secret),
+			hash, &hashlen);
+	sshbuf_free(empty);
+	if (r != 0)
+		fatal("Failed to calculate hash: %s", ssh_err(r));
+
+	gss->buf.value = hash;
+	gss->buf.length = hashlen;
+
+	/* Verify that the hash matches the MIC we just got. */
+	if (GSS_ERROR(ssh_gssapi_checkmic(gss, &gss->buf, &gss->msg_tok)))
+		sshpkt_disconnect(ssh, "Hash's MIC didn't verify");
+
+	gss_release_buffer(&gss->minor, &gss->msg_tok);
+
+	if (kex->gss_deleg_creds)
+		ssh_gssapi_credentials_updated(gss);
+
+	if (gss_kex_context == NULL)
+		gss_kex_context = gss;
+	else
+		ssh_gssapi_delete_ctx(&kex->gss);
+
+	/* Finally derive the keys and send them */
+	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
+		r = kex_send_newkeys(ssh);
+
+	if (kex->gss != NULL) {
+		sshbuf_free(gss->server_host_key_blob);
+		gss->server_host_key_blob = NULL;
+		sshbuf_free(gss->server_blob);
+		gss->server_blob = NULL;
+	}
+out:
+	explicit_bzero(hash, sizeof(hash));
+	DH_free(kex->dh);
+	kex->dh = NULL;
+	BN_clear_free(dh_server_pub);
+	sshbuf_free(shared_secret);
+	return r;
+}
+
+static int
+kexgssgex_init_ctx(struct ssh *ssh,
+		   gss_buffer_desc *token_ptr)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	const BIGNUM *pub_key;
+	gss_buffer_desc send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags;
+	int r;
+
+	/* Step 2 - call GSS_Init_sec_context() */
+	debug("Calling gss_init_sec_context");
+
+	gss->major = ssh_gssapi_init_ctx(gss, kex->gss_deleg_creds,
+					 token_ptr, &send_tok, &ret_flags);
+
+	if (GSS_ERROR(gss->major)) {
+		/* XXX Useless code: Missing send? */
+		if (send_tok.length != 0) {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0)
+				fatal("sshpkt failed: %s", ssh_err(r));
+		}
+		fatal("gss_init_context failed");
+	}
+
+	/* If we've got an old receive buffer get rid of it */
+	if (token_ptr != GSS_C_NO_BUFFER)
+		gss_release_buffer(&gss->minor, token_ptr);
+
+	if (gss->major == GSS_S_COMPLETE) {
+		/* If mutual state flag is not true, kex fails */
+		if (!(ret_flags & GSS_C_MUTUAL_FLAG))
+			fatal("Mutual authentication failed");
+
+		/* If integ avail flag is not true kex fails */
+		if (!(ret_flags & GSS_C_INTEG_FLAG))
+			fatal("Integrity check failed");
+	}
+
+	/*
+	 * If we have data to send, then the last message that we
+	 * received cannot have been a 'complete'.
+	 */
+	if (send_tok.length != 0) {
+		if (gss->first) {
+	                DH_get0_key(kex->dh, &pub_key, NULL);
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_INIT)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0 ||
+			    (r = sshpkt_put_bignum2(ssh, pub_key)) != 0)
+				fatal("failed to construct packet: %s", ssh_err(r));
+			gss->first = 0;
+		} else {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok.value, send_tok.length)) != 0)
+				fatal("failed to construct packet: %s", ssh_err(r));
+		}
+		if ((r = sshpkt_send(ssh)) != 0)
+			fatal("failed to send packet: %s", ssh_err(r));
+		gss_release_buffer(&gss->minor, &send_tok);
+
+		/* If we've sent them data, they should reply */
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, &input_kexgss_hostkey);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, &input_kexgssgex_continue);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, &input_kexgssgex_complete);
+		ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, &input_kexgss_error);
+		return 0;
+	}
+	/* No data, and not complete */
+	if (gss->major != GSS_S_COMPLETE)
+		fatal("Not complete, and no token output");
+
+	if  (gss->major & GSS_S_CONTINUE_NEEDED)
+		return kexgssgex_init_ctx(ssh, token_ptr);
+
+	return kexgssgex_final(ssh);
+}
+
+static int
+input_kexgssgex_group(int type,
+		      u_int32_t seq,
+		      struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	BIGNUM *p = NULL;
+	BIGNUM *g = NULL;
+	int r;
+
+	debug("Received SSH2_MSG_KEXGSS_GROUP");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_GROUP, NULL);
+
+	if ((r = sshpkt_get_bignum2(ssh, &p)) != 0 ||
+	    (r = sshpkt_get_bignum2(ssh, &g)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("shpkt_get_bignum2 failed: %s", ssh_err(r));
+
+	if (BN_num_bits(p) < kex->min || BN_num_bits(p) > kex->max)
+		fatal("GSSGRP_GEX group out of range: %d !< %d !< %d",
+		    kex->min, BN_num_bits(p), kex->max);
+
+	if ((kex->dh = dh_new_group(g, p)) == NULL)
+		fatal("dn_new_group() failed");
+	p = g = NULL; /* belong to kex->dh now */
+
+	if ((r = dh_gen_key(kex->dh, kex->we_need * 8)) != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		DH_free(kex->dh);
+		kex->dh = NULL;
+		return r;
+	}
+
+	return kexgssgex_init_ctx(ssh, GSS_C_NO_BUFFER);
+}
+
+static int
+input_kexgssgex_continue(int type,
+			 u_int32_t seq,
+			 struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok = GSS_C_EMPTY_BUFFER;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, NULL);
+
+	debug("Received GSSAPI_CONTINUE");
+	if (gss->major == GSS_S_COMPLETE)
+		fatal("GSSAPI Continue received from server when complete");
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("Failed to read token: %s", ssh_err(r));
+	if  (!(gss->major & GSS_S_CONTINUE_NEEDED))
+		fatal("Didn't receive a SSH2_MSG_KEXGSS_COMPLETE when I expected it");
+	return kexgssgex_init_ctx(ssh, &recv_tok);
+}
+
+static int
+input_kexgssgex_complete(int type,
+		      u_int32_t seq,
+		      struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok = GSS_C_EMPTY_BUFFER;
+	u_char c;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_HOSTKEY, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_COMPLETE, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_ERROR, NULL);
+
+	debug("Received GSSAPI_COMPLETE");
+	if (gss->msg_tok.value != NULL)
+	        fatal("Received GSSAPI_COMPLETE twice?");
+	if ((r = sshpkt_getb_froms(ssh, &gss->server_blob)) != 0 ||
+	    (r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &gss->msg_tok)) != 0)
+		fatal("Failed to read message: %s", ssh_err(r));
+
+	/* Is there a token included? */
+	if ((r = sshpkt_get_u8(ssh, &c)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+	if (c) {
+		if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0)
+			fatal("Failed to read token: %s", ssh_err(r));
+		/* If we're already complete - protocol error */
+		if (gss->major == GSS_S_COMPLETE)
+			sshpkt_disconnect(ssh, "Protocol error: received token when complete");
+	} else {
+		if (gss->major != GSS_S_COMPLETE)
+			sshpkt_disconnect(ssh, "Protocol error: did not receive final token");
+	}
+	if ((r = sshpkt_get_end(ssh)) != 0)
+		fatal("Expecting end of packet.");
+
+	if  (gss->major & GSS_S_CONTINUE_NEEDED)
+		return kexgssgex_init_ctx(ssh, &recv_tok);
+
+	gss_release_buffer(&gss->minor, &recv_tok);
+	return kexgssgex_final(ssh);
+}
+
+#endif /* defined(GSSAPI) && defined(WITH_OPENSSL) */
diff --color -ruNp a/kexgsss.c b/kexgsss.c
--- a/kexgsss.c	1970-01-01 01:00:00.000000000 +0100
+++ b/kexgsss.c	2024-10-14 15:18:02.491798105 +0200
@@ -0,0 +1,601 @@
+/*
+ * Copyright (c) 2001-2009 Simon Wilkinson. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR `AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "includes.h"
+
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+
+#include <string.h>
+
+#include <openssl/crypto.h>
+#include <openssl/bn.h>
+
+#include "xmalloc.h"
+#include "sshbuf.h"
+#include "ssh2.h"
+#include "sshkey.h"
+#include "cipher.h"
+#include "kex.h"
+#include "log.h"
+#include "packet.h"
+#include "dh.h"
+#include "ssh-gss.h"
+#include "monitor_wrap.h"
+#include "misc.h"      /* servconf.h needs misc.h for struct ForwardOptions */
+#include "servconf.h"
+#include "ssh-gss.h"
+#include "digest.h"
+#include "ssherr.h"
+
+extern ServerOptions options;
+
+static int input_kexgss_init(int, u_int32_t, struct ssh *);
+static int input_kexgss_continue(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_groupreq(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_init(int, u_int32_t, struct ssh *);
+static int input_kexgssgex_continue(int, u_int32_t, struct ssh *);
+
+int
+kexgss_server(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	gss_OID oid;
+	char *mechs;
+
+	/* Initialise GSSAPI */
+
+	/* If we're rekeying, privsep means that some of the private structures
+	 * in the GSSAPI code are no longer available. This kludges them back
+	 * into life
+	 */
+	if (!ssh_gssapi_oid_table_ok()) {
+		mechs = ssh_gssapi_server_mechanisms();
+		free(mechs);
+	}
+
+	debug2_f("Identifying %s", kex->name);
+	oid = ssh_gssapi_id_kex(NULL, kex->name, kex->kex_type);
+	if (oid == GSS_C_NO_OID)
+		fatal("Unknown gssapi mechanism");
+
+	debug2_f("Acquiring credentials");
+
+	if (GSS_ERROR(mm_ssh_gssapi_server_ctx(&kex->gss, oid)))
+		fatal("Unable to acquire credentials for the server");
+
+	ssh_gssapi_build_ctx(&kex->gss);
+	if (kex->gss == NULL)
+		fatal("Unable to allocate memory for gss context");
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, &input_kexgss_init);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, &input_kexgss_continue);
+	debug("Wait SSH2_MSG_KEXGSS_INIT");
+	return 0;
+}
+
+static inline void
+kexgss_accept_ctx(struct ssh *ssh,
+		  gss_buffer_desc *recv_tok,
+		  gss_buffer_desc *send_tok,
+		  OM_uint32 *ret_flags)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	int r;
+
+	gss->major = mm_ssh_gssapi_accept_ctx(gss, recv_tok, send_tok, ret_flags);
+	gss_release_buffer(&gss->minor, recv_tok);
+
+	if (gss->major != GSS_S_COMPLETE && send_tok->length == 0)
+		fatal("Zero length token output when incomplete");
+
+	if (gss->buf.value == NULL)
+		fatal("No client public key");
+
+	if (gss->major & GSS_S_CONTINUE_NEEDED) {
+		debug("Sending GSSAPI_CONTINUE");
+		if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+		    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0 ||
+		    (r = sshpkt_send(ssh)) != 0)
+			fatal("sshpkt failed: %s", ssh_err(r));
+		gss_release_buffer(&gss->minor, send_tok);
+	}
+}
+
+static inline int
+kexgss_final(struct ssh *ssh,
+	     gss_buffer_desc *send_tok,
+	     OM_uint32 *ret_flags)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	gss_buffer_desc msg_tok;
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	size_t hashlen;
+	struct sshbuf *shared_secret = NULL;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+
+	if (GSS_ERROR(gss->major)) {
+		if (send_tok->length > 0) {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0 ||
+			    (r = sshpkt_send(ssh)) != 0)
+				fatal("sshpkt failed: %s", ssh_err(r));
+		}
+		fatal("accept_ctx died");
+	}
+
+	if (!(*ret_flags & GSS_C_MUTUAL_FLAG))
+		fatal("Mutual Authentication flag wasn't set");
+
+	if (!(*ret_flags & GSS_C_INTEG_FLAG))
+		fatal("Integrity flag wasn't set");
+
+	if (GSS_ERROR(mm_ssh_gssapi_sign(gss, &gss->buf, &msg_tok)))
+		fatal("Couldn't get MIC");
+
+	if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_COMPLETE)) != 0 ||
+	    (r = sshpkt_put_stringb(ssh, gss->server_pubkey)) != 0 ||
+	    (r = sshpkt_put_string(ssh, msg_tok.value, msg_tok.length)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	if (send_tok->length != 0) {
+		if ((r = sshpkt_put_u8(ssh, 1)) != 0 || /* true */
+		    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0)
+			fatal("sshpkt failed: %s", ssh_err(r));
+	} else {
+		if ((r = sshpkt_put_u8(ssh, 0)) != 0) /* false */
+			fatal("sshpkt failed: %s", ssh_err(r));
+	}
+	if ((r = sshpkt_send(ssh)) != 0)
+		fatal("sshpkt_send failed: %s", ssh_err(r));
+
+	gss_release_buffer(&gss->minor, send_tok);
+	gss_release_buffer(&gss->minor, &msg_tok);
+
+	hashlen = gss->hashlen;
+	memcpy(hash, gss->hash, hashlen);
+	explicit_bzero(gss->hash, sizeof(gss->hash));
+	shared_secret = gss->shared_secret;
+	gss->shared_secret = NULL;
+
+	if (gss_kex_context == NULL)
+		gss_kex_context = gss;
+	else
+		ssh_gssapi_delete_ctx(&kex->gss);
+
+	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
+		r = kex_send_newkeys(ssh);
+
+	/* If this was a rekey, then save out any delegated credentials we
+	 * just exchanged.  */
+	if (options.gss_store_rekey)
+		ssh_gssapi_rekey_creds();
+
+	if (kex->gss != NULL) {
+		sshbuf_free(gss->server_pubkey);
+		gss->server_pubkey = NULL;
+	}
+	explicit_bzero(hash, sizeof(hash));
+	sshbuf_free(shared_secret);
+	return r;
+}
+
+static int
+input_kexgss_init(int type,
+		  u_int32_t seq,
+		  struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	struct sshbuf *empty;
+	struct sshbuf *client_pubkey = NULL;
+	gss_buffer_desc recv_tok, send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags = 0;
+	int r;
+
+	debug("SSH2_MSG_KEXGSS_INIT received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, NULL);
+
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_getb_froms(ssh, &client_pubkey)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	switch (kex->kex_type) {
+	case KEX_GSS_GRP1_SHA1:
+	case KEX_GSS_GRP14_SHA1:
+	case KEX_GSS_GRP14_SHA256:
+	case KEX_GSS_GRP16_SHA512:
+		r = kex_dh_enc(kex, client_pubkey, &gss->server_pubkey, &gss->shared_secret);
+		break;
+	case KEX_GSS_NISTP256_SHA256:
+		r = kex_ecdh_enc(kex, client_pubkey, &gss->server_pubkey, &gss->shared_secret);
+		break;
+	case KEX_GSS_C25519_SHA256:
+		r = kex_c25519_enc(kex, client_pubkey, &gss->server_pubkey, &gss->shared_secret);
+		break;
+	default:
+		fatal_f("Unexpected KEX type %d", kex->kex_type);
+	}
+	if (r != 0) {
+		sshbuf_free(client_pubkey);
+		gss_release_buffer(&gss->minor, &recv_tok);
+		ssh_gssapi_delete_ctx(&kex->gss);
+		return r;
+	}
+
+	/* Send SSH_MSG_KEXGSS_HOSTKEY here, if we want */
+
+	if ((empty = sshbuf_new()) == NULL) {
+		sshbuf_free(client_pubkey);
+		gss_release_buffer(&gss->minor, &recv_tok);
+		ssh_gssapi_delete_ctx(&kex->gss);
+		return SSH_ERR_ALLOC_FAIL;
+	}
+
+	/* Calculate the hash early so we can free the
+	 * client_pubkey, which has reference to the parent
+	 * buffer state->incoming_packet
+	 */
+	gss->hashlen = sizeof(gss->hash);
+	r = kex_gen_hash(kex->hash_alg, kex->client_version, kex->server_version,
+			 kex->peer, kex->my, empty, client_pubkey, gss->server_pubkey,
+			 gss->shared_secret, gss->hash, &gss->hashlen);
+	sshbuf_free(empty);
+	sshbuf_free(client_pubkey);
+	if (r != 0) {
+		gss_release_buffer(&gss->minor, &recv_tok);
+		ssh_gssapi_delete_ctx(&kex->gss);
+		return r;
+	}
+
+	gss->buf.value = gss->hash;
+	gss->buf.length = gss->hashlen;
+
+	kexgss_accept_ctx(ssh, &recv_tok, &send_tok, &ret_flags);
+	if (gss->major & GSS_S_CONTINUE_NEEDED)
+		return 0;
+
+	return kexgss_final(ssh, &send_tok, &ret_flags);
+}
+
+static int
+input_kexgss_continue(int type,
+		      u_int32_t seq,
+		      struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok, send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags = 0;
+	int r;
+
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	kexgss_accept_ctx(ssh, &recv_tok, &send_tok, &ret_flags);
+	if (gss->major & GSS_S_CONTINUE_NEEDED)
+		return 0;
+
+	return kexgss_final(ssh, &send_tok, &ret_flags);
+}
+
+/*******************************************************/
+/******************** KEXGSSGEX ************************/
+/*******************************************************/
+
+int
+kexgssgex_server(struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	gss_OID oid;
+	char *mechs;
+
+	/* Initialise GSSAPI */
+
+	/* If we're rekeying, privsep means that some of the private structures
+	 * in the GSSAPI code are no longer available. This kludges them back
+	 * into life
+	 */
+	if (!ssh_gssapi_oid_table_ok()) {
+		mechs = ssh_gssapi_server_mechanisms();
+		free(mechs);
+	}
+
+	debug2_f("Identifying %s", kex->name);
+	oid = ssh_gssapi_id_kex(NULL, kex->name, kex->kex_type);
+	if (oid == GSS_C_NO_OID)
+		fatal("Unknown gssapi mechanism");
+
+	debug2_f("Acquiring credentials");
+
+	if (GSS_ERROR(mm_ssh_gssapi_server_ctx(&kex->gss, oid)))
+		fatal("Unable to acquire credentials for the server");
+
+	ssh_gssapi_build_ctx(&kex->gss);
+	if (kex->gss == NULL)
+		fatal("Unable to allocate memory for gss context");
+
+	debug("Doing group exchange");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_GROUPREQ, &input_kexgssgex_groupreq);
+	return 0;
+}
+
+static inline void
+kexgssgex_accept_ctx(struct ssh *ssh,
+		     gss_buffer_desc *recv_tok,
+		     gss_buffer_desc *send_tok,
+		     OM_uint32 *ret_flags)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	int r;
+
+	gss->major = mm_ssh_gssapi_accept_ctx(gss, recv_tok, send_tok, ret_flags);
+	gss_release_buffer(&gss->minor, recv_tok);
+
+	if (gss->major != GSS_S_COMPLETE && send_tok->length == 0)
+		fatal("Zero length token output when incomplete");
+
+	if (gss->dh_client_pub == NULL)
+		fatal("No client public key");
+
+	if (gss->major & GSS_S_CONTINUE_NEEDED) {
+		debug("Sending GSSAPI_CONTINUE");
+		if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+		    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0 ||
+		    (r = sshpkt_send(ssh)) != 0)
+			fatal("sshpkt failed: %s", ssh_err(r));
+		gss_release_buffer(&gss->minor, send_tok);
+	}
+}
+
+static inline int
+kexgssgex_final(struct ssh *ssh,
+		gss_buffer_desc *send_tok,
+		OM_uint32 *ret_flags)
+{
+	struct kex *kex = ssh->kex;
+	Gssctxt *gss = kex->gss;
+	gss_buffer_desc msg_tok;
+	u_char hash[SSH_DIGEST_MAX_LENGTH];
+	size_t hashlen;
+	const BIGNUM *pub_key, *dh_p, *dh_g;
+	struct sshbuf *shared_secret = NULL;
+	struct sshbuf *empty = NULL;
+	int r;
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, NULL);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, NULL);
+
+	if (GSS_ERROR(gss->major)) {
+		if (send_tok->length > 0) {
+			if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_CONTINUE)) != 0 ||
+			    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0 ||
+			    (r = sshpkt_send(ssh)) != 0)
+				fatal("sshpkt failed: %s", ssh_err(r));
+		}
+		fatal("accept_ctx died");
+	}
+
+	if (!(*ret_flags & GSS_C_MUTUAL_FLAG))
+		fatal("Mutual Authentication flag wasn't set");
+
+	if (!(*ret_flags & GSS_C_INTEG_FLAG))
+		fatal("Integrity flag wasn't set");
+
+	/* calculate shared secret */
+	shared_secret = sshbuf_new();
+	if (shared_secret == NULL) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+	if ((r = kex_dh_compute_key(kex, gss->dh_client_pub, shared_secret)) != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		goto out;
+	}
+
+	if ((empty = sshbuf_new()) == NULL) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		r = SSH_ERR_ALLOC_FAIL;
+		goto out;
+	}
+
+	DH_get0_key(kex->dh, &pub_key, NULL);
+	DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);
+	hashlen = sizeof(hash);
+	r = kexgex_hash(kex->hash_alg, kex->client_version, kex->server_version,
+			kex->peer, kex->my, empty, kex->min, kex->nbits, kex->max, dh_p, dh_g,
+			gss->dh_client_pub, pub_key, sshbuf_ptr(shared_secret),
+			sshbuf_len(shared_secret), hash, &hashlen);
+	sshbuf_free(empty);
+	if (r != 0)
+		fatal("kexgex_hash failed: %s", ssh_err(r));
+
+	gss->buf.value = hash;
+	gss->buf.length = hashlen;
+
+	if (GSS_ERROR(mm_ssh_gssapi_sign(gss, &gss->buf, &msg_tok)))
+		fatal("Couldn't get MIC");
+
+	if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_COMPLETE)) != 0 ||
+	    (r = sshpkt_put_bignum2(ssh, pub_key)) != 0 ||
+	    (r = sshpkt_put_string(ssh, msg_tok.value, msg_tok.length)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	if (send_tok->length != 0) {
+		if ((r = sshpkt_put_u8(ssh, 1)) != 0 || /* true */
+		    (r = sshpkt_put_string(ssh, send_tok->value, send_tok->length)) != 0)
+			fatal("sshpkt failed: %s", ssh_err(r));
+	} else {
+		if ((r = sshpkt_put_u8(ssh, 0)) != 0) /* false */
+			fatal("sshpkt failed: %s", ssh_err(r));
+	}
+	if ((r = sshpkt_send(ssh)) != 0)
+		fatal("sshpkt_send failed: %s", ssh_err(r));
+
+	gss_release_buffer(&gss->minor, send_tok);
+	gss_release_buffer(&gss->minor, &msg_tok);
+
+	if (gss_kex_context == NULL)
+		gss_kex_context = gss;
+	else
+		ssh_gssapi_delete_ctx(&kex->gss);
+
+	/* Finally derive the keys and send them */
+	if ((r = kex_derive_keys(ssh, hash, hashlen, shared_secret)) == 0)
+		r = kex_send_newkeys(ssh);
+
+	/* If this was a rekey, then save out any delegated credentials we
+	 * just exchanged.  */
+	if (options.gss_store_rekey)
+		ssh_gssapi_rekey_creds();
+
+	if (kex->gss != NULL)
+		BN_clear_free(gss->dh_client_pub);
+
+out:
+	explicit_bzero(hash, sizeof(hash));
+	DH_free(kex->dh);
+	kex->dh = NULL;
+	sshbuf_free(shared_secret);
+	return r;
+}
+
+static int
+input_kexgssgex_groupreq(int type,
+			 u_int32_t seq,
+			 struct ssh *ssh)
+{
+	struct kex *kex = ssh->kex;
+	const BIGNUM *dh_p, *dh_g;
+	int min = -1, max = -1, nbits = -1;
+	int cmin = -1, cmax = -1; /* client proposal */
+	int r;
+
+	/* 5. S generates an ephemeral key pair (do the allocations early) */
+
+	debug("SSH2_MSG_KEXGSS_GROUPREQ received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_GROUPREQ, NULL);
+
+	/* store client proposal to provide valid signature */
+	if ((r = sshpkt_get_u32(ssh, &cmin)) != 0 ||
+	    (r = sshpkt_get_u32(ssh, &nbits)) != 0 ||
+	    (r = sshpkt_get_u32(ssh, &cmax)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	kex->nbits = nbits;
+	kex->min = cmin;
+	kex->max = cmax;
+	min = MAX(DH_GRP_MIN, cmin);
+	max = MIN(DH_GRP_MAX, cmax);
+	nbits = MAXIMUM(DH_GRP_MIN, nbits);
+	nbits = MINIMUM(DH_GRP_MAX, nbits);
+
+	if (max < min || nbits < min || max < nbits)
+		fatal("GSS_GEX, bad parameters: %d !< %d !< %d", min, nbits, max);
+
+	kex->dh = mm_choose_dh(min, nbits, max);
+	if (kex->dh == NULL) {
+		sshpkt_disconnect(ssh, "Protocol error: no matching group found");
+		fatal("Protocol error: no matching group found");
+	}
+
+	DH_get0_pqg(kex->dh, &dh_p, NULL, &dh_g);
+	if ((r = sshpkt_start(ssh, SSH2_MSG_KEXGSS_GROUP)) != 0 ||
+	    (r = sshpkt_put_bignum2(ssh, dh_p)) != 0 ||
+	    (r = sshpkt_put_bignum2(ssh, dh_g)) != 0 ||
+	    (r = sshpkt_send(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	if ((r = ssh_packet_write_wait(ssh)) != 0)
+		fatal("ssh_packet_write_wait: %s", ssh_err(r));
+
+	/* Compute our exchange value in parallel with the client */
+	if ((r = dh_gen_key(kex->dh, kex->we_need * 8)) != 0) {
+		ssh_gssapi_delete_ctx(&kex->gss);
+		DH_free(kex->dh);
+		kex->dh = NULL;
+		return r;
+	}
+
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, &input_kexgssgex_init);
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_CONTINUE, &input_kexgssgex_continue);
+	debug("Wait SSH2_MSG_KEXGSS_INIT");
+	return 0;
+}
+
+static int
+input_kexgssgex_init(int type,
+		     u_int32_t seq,
+		     struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok, send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags = 0;
+	int r;
+
+	debug("SSH2_MSG_KEXGSS_INIT received");
+	ssh_dispatch_set(ssh, SSH2_MSG_KEXGSS_INIT, NULL);
+
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_get_bignum2(ssh, &gss->dh_client_pub)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	/* Send SSH_MSG_KEXGSS_HOSTKEY here, if we want */
+
+	kexgssgex_accept_ctx(ssh, &recv_tok, &send_tok, &ret_flags);
+	if (gss->major & GSS_S_CONTINUE_NEEDED)
+		return 0;
+
+	return kexgssgex_final(ssh, &send_tok, &ret_flags);
+}
+
+static int
+input_kexgssgex_continue(int type,
+			 u_int32_t seq,
+			 struct ssh *ssh)
+{
+	Gssctxt *gss = ssh->kex->gss;
+	gss_buffer_desc recv_tok, send_tok = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ret_flags = 0;
+	int r;
+
+	if ((r = ssh_gssapi_sshpkt_get_buffer_desc(ssh, &recv_tok)) != 0 ||
+	    (r = sshpkt_get_end(ssh)) != 0)
+		fatal("sshpkt failed: %s", ssh_err(r));
+
+	kexgssgex_accept_ctx(ssh, &recv_tok, &send_tok, &ret_flags);
+	if (gss->major & GSS_S_CONTINUE_NEEDED)
+		return 0;
+
+	return kexgssgex_final(ssh, &send_tok, &ret_flags);
+}
+
+#endif /* defined(GSSAPI) && defined(WITH_OPENSSL) */
diff --color -ruNp a/kex.h b/kex.h
--- a/kex.h	2024-07-01 06:36:28.000000000 +0200
+++ b/kex.h	2024-09-16 11:46:34.710940224 +0200
@@ -29,6 +29,10 @@
 #include "mac.h"
 #include "crypto_api.h"
 
+#ifdef GSSAPI
+# include "ssh-gss.h" /* Gssctxt */
+#endif
+
 #ifdef WITH_OPENSSL
 # include <openssl/bn.h>
 # include <openssl/dh.h>
@@ -102,6 +106,15 @@ enum kex_exchange {
 	KEX_C25519_SHA256,
 	KEX_KEM_SNTRUP761X25519_SHA512,
 	KEX_KEM_MLKEM768X25519_SHA256,
+#ifdef GSSAPI
+	KEX_GSS_GRP1_SHA1,
+	KEX_GSS_GRP14_SHA1,
+	KEX_GSS_GRP14_SHA256,
+	KEX_GSS_GRP16_SHA512,
+	KEX_GSS_GEX_SHA1,
+	KEX_GSS_NISTP256_SHA256,
+	KEX_GSS_C25519_SHA256,
+#endif
 	KEX_MAX
 };
 
@@ -164,6 +177,13 @@ struct kex {
 	u_int	flags;
 	int	hash_alg;
 	int	ec_nid;
+#ifdef GSSAPI
+	Gssctxt *gss;
+	int	gss_deleg_creds;
+	int	gss_trust_dns;
+	char    *gss_host;
+	char	*gss_client;
+#endif
 	char	*failed_choice;
 	int	(*verify_host_key)(struct sshkey *, struct ssh *);
 	struct sshkey *(*load_host_public_key)(int, int, struct ssh *);
@@ -189,8 +209,10 @@ int	 kex_hash_from_name(const char *);
 int	 kex_nid_from_name(const char *);
 int	 kex_names_valid(const char *);
 char	*kex_alg_list(char);
+char	*kex_gss_alg_list(char);
 char	*kex_names_cat(const char *, const char *);
 int	 kex_has_any_alg(const char *, const char *);
+int	 kex_gss_names_valid(const char *);
 int	 kex_assemble_names(char **, const char *, const char *);
 void	 kex_proposal_populate_entries(struct ssh *, char *prop[PROPOSAL_MAX],
     const char *, const char *, const char *, const char *, const char *);
@@ -224,6 +246,12 @@ int	 kexgex_client(struct ssh *);
 int	 kexgex_server(struct ssh *);
 int	 kex_gen_client(struct ssh *);
 int	 kex_gen_server(struct ssh *);
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+int	 kexgssgex_client(struct ssh *);
+int	 kexgssgex_server(struct ssh *);
+int	 kexgss_client(struct ssh *);
+int	 kexgss_server(struct ssh *);
+#endif
 
 int	 kex_dh_keypair(struct kex *);
 int	 kex_dh_enc(struct kex *, const struct sshbuf *, struct sshbuf **,
@@ -256,6 +284,12 @@ int	 kexgex_hash(int, const struct sshbu
     const BIGNUM *, const u_char *, size_t,
     u_char *, size_t *);
 
+int	 kex_gen_hash(int hash_alg, const struct sshbuf *client_version,
+    const struct sshbuf *server_version, const struct sshbuf *client_kexinit,
+    const struct sshbuf *server_kexinit, const struct sshbuf *server_host_key_blob,
+    const struct sshbuf *client_pub, const struct sshbuf *server_pub,
+    const struct sshbuf *shared_secret, u_char *hash, size_t *hashlen);
+
 void	kexc25519_keygen(u_char key[CURVE25519_SIZE], u_char pub[CURVE25519_SIZE])
 	__attribute__((__bounded__(__minbytes__, 1, CURVE25519_SIZE)))
 	__attribute__((__bounded__(__minbytes__, 2, CURVE25519_SIZE)));
diff --color -ruNp a/kex-names.c b/kex-names.c
--- a/kex-names.c	2024-07-01 06:36:28.000000000 +0200
+++ b/kex-names.c	2024-09-16 11:46:34.694939883 +0200
@@ -45,6 +45,10 @@
 #include "ssherr.h"
 #include "xmalloc.h"
 
+#ifdef GSSAPI
+#include "ssh-gss.h"
+#endif
+
 struct kexalg {
 	char *name;
 	u_int type;
@@ -83,15 +87,28 @@ static const struct kexalg kexalgs[] = {
 #endif /* HAVE_EVP_SHA256 || !WITH_OPENSSL */
 	{ NULL, 0, -1, -1},
 };
+static const struct kexalg gss_kexalgs[] = {
+#ifdef GSSAPI
+	{ KEX_GSS_GEX_SHA1_ID, KEX_GSS_GEX_SHA1, 0, SSH_DIGEST_SHA1 },
+	{ KEX_GSS_GRP1_SHA1_ID, KEX_GSS_GRP1_SHA1, 0, SSH_DIGEST_SHA1 },
+	{ KEX_GSS_GRP14_SHA1_ID, KEX_GSS_GRP14_SHA1, 0, SSH_DIGEST_SHA1 },
+	{ KEX_GSS_GRP14_SHA256_ID, KEX_GSS_GRP14_SHA256, 0, SSH_DIGEST_SHA256 },
+	{ KEX_GSS_GRP16_SHA512_ID, KEX_GSS_GRP16_SHA512, 0, SSH_DIGEST_SHA512 },
+	{ KEX_GSS_NISTP256_SHA256_ID, KEX_GSS_NISTP256_SHA256,
+	    NID_X9_62_prime256v1, SSH_DIGEST_SHA256 },
+	{ KEX_GSS_C25519_SHA256_ID, KEX_GSS_C25519_SHA256, 0, SSH_DIGEST_SHA256 },
+#endif
+	{ NULL, 0, -1, -1},
+};
 
-char *
-kex_alg_list(char sep)
+static char *
+kex_alg_list_internal(char sep, const struct kexalg *algs)
 {
 	char *ret = NULL, *tmp;
 	size_t nlen, rlen = 0;
 	const struct kexalg *k;
 
-	for (k = kexalgs; k->name != NULL; k++) {
+	for (k = algs; k->name != NULL; k++) {
 		if (ret != NULL)
 			ret[rlen++] = sep;
 		nlen = strlen(k->name);
@@ -106,6 +123,18 @@ kex_alg_list(char sep)
 	return ret;
 }
 
+char *
+kex_alg_list(char sep)
+{
+	return kex_alg_list_internal(sep, kexalgs);
+}
+
+char *
+kex_gss_alg_list(char sep)
+{
+	return kex_alg_list_internal(sep, gss_kexalgs);
+}
+
 static const struct kexalg *
 kex_alg_by_name(const char *name)
 {
@@ -115,6 +144,10 @@ kex_alg_by_name(const char *name)
 		if (strcmp(k->name, name) == 0)
 			return k;
 	}
+	for (k = gss_kexalgs; k->name != NULL; k++) {
+		if (strncmp(k->name, name, strlen(k->name)) == 0)
+			return k;
+	}
 	return NULL;
 }
 
@@ -328,3 +361,26 @@ kex_assemble_names(char **listp, const c
 	free(ret);
 	return r;
 }
+ 
+/* Validate GSS KEX method name list */
+int
+kex_gss_names_valid(const char *names)
+{
+	char *s, *cp, *p;
+
+	if (names == NULL || *names == '\0')
+		return 0;
+	s = cp = xstrdup(names);
+	for ((p = strsep(&cp, ",")); p && *p != '\0';
+	    (p = strsep(&cp, ","))) {
+		if (strncmp(p, "gss-", 4) != 0
+		  || kex_alg_by_name(p) == NULL) {
+			error("Unsupported KEX algorithm \"%.100s\"", p);
+			free(s);
+			return 0;
+		}
+	}
+	debug3("gss kex names ok: [%s]", names);
+	free(s);
+	return 1;
+}
diff --color -ruNp a/Makefile.in b/Makefile.in
--- a/Makefile.in	2024-09-16 11:45:56.868133454 +0200
+++ b/Makefile.in	2024-09-16 11:46:34.695939904 +0200
@@ -114,6 +114,7 @@ LIBSSH_OBJS=${LIBOPENSSH_OBJS} \
 	kex.o kex-names.o kexdh.o kexgex.o kexecdh.o kexc25519.o \
 	kexgexc.o kexgexs.o \
 	kexsntrup761x25519.o kexmlkem768x25519.o sntrup761.o kexgen.o \
+	kexgssc.o \
 	sftp-realpath.o platform-pledge.o platform-tracing.o platform-misc.o \
 	sshbuf-io.o
 
@@ -135,7 +136,7 @@ SSHD_SESSION_OBJS=sshd-session.o auth-rh
 	auth2-chall.o groupaccess.o \
 	auth-bsdauth.o auth2-hostbased.o auth2-kbdint.o \
 	auth2-none.o auth2-passwd.o auth2-pubkey.o auth2-pubkeyfile.o \
-	monitor.o monitor_wrap.o auth-krb5.o \
+	monitor.o monitor_wrap.o auth-krb5.o kexgsss.o \
 	auth2-gss.o gss-serv.o gss-serv-krb5.o \
 	loginrec.o auth-pam.o auth-shadow.o auth-sia.o \
 	sftp-server.o sftp-common.o \
@@ -529,7 +530,7 @@ regress-prep:
 	    ln -s `cd $(srcdir) && pwd`/regress/Makefile `pwd`/regress/Makefile
 
 REGRESSLIBS=libssh.a $(LIBCOMPAT)
-TESTLIBS=$(LIBS) $(CHANNELLIBS)
+TESTLIBS=$(LIBS) $(CHANNELLIBS) $(GSSLIBS)
 
 regress/modpipe$(EXEEXT): $(srcdir)/regress/modpipe.c $(REGRESSLIBS)
 	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $(srcdir)/regress/modpipe.c \
diff --color -ruNp a/monitor.c b/monitor.c
--- a/monitor.c	2024-09-16 11:45:56.861133305 +0200
+++ b/monitor.c	2024-09-16 11:46:34.696939926 +0200
@@ -143,6 +143,8 @@ int mm_answer_gss_setup_ctx(struct ssh *
 int mm_answer_gss_accept_ctx(struct ssh *, int, struct sshbuf *);
 int mm_answer_gss_userok(struct ssh *, int, struct sshbuf *);
 int mm_answer_gss_checkmic(struct ssh *, int, struct sshbuf *);
+int mm_answer_gss_sign(struct ssh *, int, struct sshbuf *);
+int mm_answer_gss_updatecreds(struct ssh *, int, struct sshbuf *);
 #endif
 
 #ifdef SSH_AUDIT_EVENTS
@@ -219,11 +221,18 @@ struct mon_table mon_dispatch_proto20[]
     {MONITOR_REQ_GSSSTEP, 0, mm_answer_gss_accept_ctx},
     {MONITOR_REQ_GSSUSEROK, MON_ONCE|MON_AUTHDECIDE, mm_answer_gss_userok},
     {MONITOR_REQ_GSSCHECKMIC, MON_ONCE, mm_answer_gss_checkmic},
+    {MONITOR_REQ_GSSSIGN, MON_ONCE, mm_answer_gss_sign},
 #endif
     {0, 0, NULL}
 };
 
 struct mon_table mon_dispatch_postauth20[] = {
+#ifdef GSSAPI
+    {MONITOR_REQ_GSSSETUP, 0, mm_answer_gss_setup_ctx},
+    {MONITOR_REQ_GSSSTEP, 0, mm_answer_gss_accept_ctx},
+    {MONITOR_REQ_GSSSIGN, 0, mm_answer_gss_sign},
+    {MONITOR_REQ_GSSUPCREDS, 0, mm_answer_gss_updatecreds},
+#endif
 #ifdef WITH_OPENSSL
     {MONITOR_REQ_MODULI, 0, mm_answer_moduli},
 #endif
@@ -292,6 +301,10 @@ monitor_child_preauth(struct ssh *ssh, s
 	/* Permit requests for moduli and signatures */
 	monitor_permit(mon_dispatch, MONITOR_REQ_MODULI, 1);
 	monitor_permit(mon_dispatch, MONITOR_REQ_SIGN, 1);
+#ifdef GSSAPI
+	/* and for the GSSAPI key exchange */
+	monitor_permit(mon_dispatch, MONITOR_REQ_GSSSETUP, 1);
+#endif
 
 	/* The first few requests do not require asynchronous access */
 	while (!authenticated) {
@@ -344,8 +357,15 @@ monitor_child_preauth(struct ssh *ssh, s
 		if (ent->flags & (MON_AUTHDECIDE|MON_ALOG)) {
 			auth_log(ssh, authenticated, partial,
 			    auth_method, auth_submethod);
-			if (!partial && !authenticated)
+			if (!partial && !authenticated) {
+#ifdef GSSAPI
+				/* If gssapi-with-mic failed, MONITOR_REQ_GSSCHECKMIC is disabled.
+				 * We have to reenable it to try again for gssapi-keyex */
+				if (strcmp(auth_method, "gssapi-with-mic") == 0 && options.gss_keyex)
+					monitor_permit(mon_dispatch, MONITOR_REQ_GSSCHECKMIC, 1);
+#endif
 				authctxt->failures++;
+			}
 			if (authenticated || partial) {
 				auth2_update_session_info(authctxt,
 				    auth_method, auth_submethod);
@@ -413,6 +433,10 @@ monitor_child_postauth(struct ssh *ssh,
 	monitor_permit(mon_dispatch, MONITOR_REQ_MODULI, 1);
 	monitor_permit(mon_dispatch, MONITOR_REQ_SIGN, 1);
 	monitor_permit(mon_dispatch, MONITOR_REQ_TERM, 1);
+#ifdef GSSAPI
+	/* and for the GSSAPI key exchange */
+	monitor_permit(mon_dispatch, MONITOR_REQ_GSSSETUP, 1);
+#endif
 
 	if (auth_opts->permit_pty_flag) {
 		monitor_permit(mon_dispatch, MONITOR_REQ_PTY, 1);
@@ -1793,6 +1817,17 @@ monitor_apply_keystate(struct ssh *ssh,
 # ifdef OPENSSL_HAS_ECC
 	kex->kex[KEX_ECDH_SHA2] = kex_gen_server;
 # endif
+# ifdef GSSAPI
+	if (options.gss_keyex) {
+		kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_server;
+		kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_server;
+		kex->kex[KEX_GSS_GRP14_SHA256] = kexgss_server;
+		kex->kex[KEX_GSS_GRP16_SHA512] = kexgss_server;
+		kex->kex[KEX_GSS_GEX_SHA1] = kexgssgex_server;
+		kex->kex[KEX_GSS_NISTP256_SHA256] = kexgss_server;
+		kex->kex[KEX_GSS_C25519_SHA256] = kexgss_server;
+	}
+# endif
 #endif /* WITH_OPENSSL */
 	kex->kex[KEX_C25519_SHA256] = kex_gen_server;
 	kex->kex[KEX_KEM_SNTRUP761X25519_SHA512] = kex_gen_server;
@@ -1885,8 +1920,8 @@ mm_answer_gss_setup_ctx(struct ssh *ssh,
 	u_char *p;
 	int r;
 
-	if (!options.gss_authentication)
-		fatal_f("GSSAPI authentication not enabled");
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
 
 	if ((r = sshbuf_get_string(m, &p, &len)) != 0)
 		fatal_fr(r, "parse");
@@ -1918,8 +1953,8 @@ mm_answer_gss_accept_ctx(struct ssh *ssh
 	OM_uint32 flags = 0; /* GSI needs this */
 	int r;
 
-	if (!options.gss_authentication)
-		fatal_f("GSSAPI authentication not enabled");
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
 
 	if ((r = ssh_gssapi_get_buffer_desc(m, &in)) != 0)
 		fatal_fr(r, "ssh_gssapi_get_buffer_desc");
@@ -1939,6 +1974,7 @@ mm_answer_gss_accept_ctx(struct ssh *ssh
 		monitor_permit(mon_dispatch, MONITOR_REQ_GSSSTEP, 0);
 		monitor_permit(mon_dispatch, MONITOR_REQ_GSSUSEROK, 1);
 		monitor_permit(mon_dispatch, MONITOR_REQ_GSSCHECKMIC, 1);
+		monitor_permit(mon_dispatch, MONITOR_REQ_GSSSIGN, 1);
 	}
 	return (0);
 }
@@ -1950,8 +1986,8 @@ mm_answer_gss_checkmic(struct ssh *ssh,
 	OM_uint32 ret;
 	int r;
 
-	if (!options.gss_authentication)
-		fatal_f("GSSAPI authentication not enabled");
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
 
 	if ((r = ssh_gssapi_get_buffer_desc(m, &gssbuf)) != 0 ||
 	    (r = ssh_gssapi_get_buffer_desc(m, &mic)) != 0)
@@ -1977,13 +2013,17 @@ mm_answer_gss_checkmic(struct ssh *ssh,
 int
 mm_answer_gss_userok(struct ssh *ssh, int sock, struct sshbuf *m)
 {
-	int r, authenticated;
+	int r, authenticated, kex;
 	const char *displayname;
 
-	if (!options.gss_authentication)
-		fatal_f("GSSAPI authentication not enabled");
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
 
-	authenticated = authctxt->valid && ssh_gssapi_userok(authctxt->user);
+	if ((r = sshbuf_get_u32(m, &kex)) != 0)
+		fatal_fr(r, "buffer error");
+
+	authenticated = authctxt->valid &&
+	    ssh_gssapi_userok(authctxt->user, authctxt->pw, kex);
 
 	sshbuf_reset(m);
 	if ((r = sshbuf_put_u32(m, authenticated)) != 0)
@@ -1992,7 +2032,11 @@ mm_answer_gss_userok(struct ssh *ssh, in
 	debug3_f("sending result %d", authenticated);
 	mm_request_send(sock, MONITOR_ANS_GSSUSEROK, m);
 
-	auth_method = "gssapi-with-mic";
+	if (kex) {
+		auth_method = "gssapi-keyex";
+	} else {
+		auth_method = "gssapi-with-mic";
+	}
 
 	if ((displayname = ssh_gssapi_displayname()) != NULL)
 		auth2_record_info(authctxt, "%s", displayname);
@@ -2000,5 +2044,84 @@ mm_answer_gss_userok(struct ssh *ssh, in
 	/* Monitor loop will terminate if authenticated */
 	return (authenticated);
 }
+
+int
+mm_answer_gss_sign(struct ssh *ssh, int socket, struct sshbuf *m)
+{
+	gss_buffer_desc data;
+	gss_buffer_desc hash = GSS_C_EMPTY_BUFFER;
+	OM_uint32 major, minor;
+	size_t len;
+	u_char *p = NULL;
+	int r;
+
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
+
+	if ((r = sshbuf_get_string(m, &p, &len)) != 0)
+		fatal_fr(r, "buffer error");
+	data.value = p;
+	data.length = len;
+	/* Lengths of SHA-1, SHA-256 and SHA-512 hashes that are used */
+	if (data.length != 20 && data.length != 32 && data.length != 64)
+		fatal_f("data length incorrect: %d", (int) data.length);
+
+	/* Save the session ID on the first time around */
+	if (session_id2_len == 0) {
+		session_id2_len = data.length;
+		session_id2 = xmalloc(session_id2_len);
+		memcpy(session_id2, data.value, session_id2_len);
+	}
+	major = ssh_gssapi_sign(gsscontext, &data, &hash);
+
+	free(data.value);
+
+	sshbuf_reset(m);
+
+	if ((r = sshbuf_put_u32(m, major)) != 0 ||
+	    (r = sshbuf_put_string(m, hash.value, hash.length)) != 0)
+		fatal_fr(r, "buffer error");
+
+	mm_request_send(socket, MONITOR_ANS_GSSSIGN, m);
+
+	gss_release_buffer(&minor, &hash);
+
+	/* Turn on getpwnam permissions */
+	monitor_permit(mon_dispatch, MONITOR_REQ_PWNAM, 1);
+
+	/* And credential updating, for when rekeying */
+	monitor_permit(mon_dispatch, MONITOR_REQ_GSSUPCREDS, 1);
+
+	return (0);
+}
+
+int
+mm_answer_gss_updatecreds(struct ssh *ssh, int socket, struct sshbuf *m) {
+	ssh_gssapi_ccache store;
+	int r, ok;
+
+	if (!options.gss_authentication && !options.gss_keyex)
+		fatal_f("GSSAPI not enabled");
+
+	if ((r = sshbuf_get_string(m, (u_char **)&store.filename, NULL)) != 0 ||
+	    (r = sshbuf_get_string(m, (u_char **)&store.envvar, NULL)) != 0 ||
+	    (r = sshbuf_get_string(m, (u_char **)&store.envval, NULL)) != 0)
+		fatal_fr(r, "buffer error");
+
+	ok = ssh_gssapi_update_creds(&store);
+
+	free(store.filename);
+	free(store.envvar);
+	free(store.envval);
+
+	sshbuf_reset(m);
+	if ((r = sshbuf_put_u32(m, ok)) != 0)
+		fatal_fr(r, "buffer error");
+
+	mm_request_send(socket, MONITOR_ANS_GSSUPCREDS, m);
+
+	return(0);
+}
+
 #endif /* GSSAPI */
 
diff --color -ruNp a/monitor.h b/monitor.h
--- a/monitor.h	2024-09-16 11:45:56.861133305 +0200
+++ b/monitor.h	2024-09-16 11:46:34.696939926 +0200
@@ -67,6 +67,8 @@ enum monitor_reqtype {
 	MONITOR_REQ_PAM_FREE_CTX = 110, MONITOR_ANS_PAM_FREE_CTX = 111,
 	MONITOR_REQ_AUDIT_EVENT = 112, MONITOR_REQ_AUDIT_COMMAND = 113,
 
+	MONITOR_REQ_GSSSIGN = 150, MONITOR_ANS_GSSSIGN = 151,
+	MONITOR_REQ_GSSUPCREDS = 152, MONITOR_ANS_GSSUPCREDS = 153,
 };
 
 struct ssh;
diff --color -ruNp a/monitor_wrap.c b/monitor_wrap.c
--- a/monitor_wrap.c	2024-09-16 11:45:56.862133326 +0200
+++ b/monitor_wrap.c	2024-09-16 11:46:34.697939947 +0200
@@ -1075,13 +1075,15 @@ mm_ssh_gssapi_checkmic(Gssctxt *ctx, gss
 }
 
 int
-mm_ssh_gssapi_userok(char *user)
+mm_ssh_gssapi_userok(char *user, struct passwd *pw, int kex)
 {
 	struct sshbuf *m;
 	int r, authenticated = 0;
 
 	if ((m = sshbuf_new()) == NULL)
 		fatal_f("sshbuf_new failed");
+	if ((r = sshbuf_put_u32(m, kex)) != 0)
+		fatal_fr(r, "buffer error");
 
 	mm_request_send(pmonitor->m_recvfd, MONITOR_REQ_GSSUSEROK, m);
 	mm_request_receive_expect(pmonitor->m_recvfd,
@@ -1094,6 +1096,59 @@ mm_ssh_gssapi_userok(char *user)
 	debug3_f("user %sauthenticated", authenticated ? "" : "not ");
 	return (authenticated);
 }
+
+OM_uint32
+mm_ssh_gssapi_sign(Gssctxt *ctx, gss_buffer_desc *data, gss_buffer_desc *hash)
+{
+	struct sshbuf *m;
+	OM_uint32 major;
+	int r;
+
+	if ((m = sshbuf_new()) == NULL)
+		fatal_f("sshbuf_new failed");
+	if ((r = sshbuf_put_string(m, data->value, data->length)) != 0)
+		fatal_fr(r, "buffer error");
+
+	mm_request_send(pmonitor->m_recvfd, MONITOR_REQ_GSSSIGN, m);
+	mm_request_receive_expect(pmonitor->m_recvfd, MONITOR_ANS_GSSSIGN, m);
+
+	if ((r = sshbuf_get_u32(m, &major)) != 0 ||
+	    (r = ssh_gssapi_get_buffer_desc(m, hash)) != 0)
+		fatal_fr(r, "buffer error");
+
+	sshbuf_free(m);
+
+	return (major);
+}
+
+int
+mm_ssh_gssapi_update_creds(ssh_gssapi_ccache *store)
+{
+	struct sshbuf *m;
+	int r, ok;
+
+	if ((m = sshbuf_new()) == NULL)
+		fatal_f("sshbuf_new failed");
+
+	if ((r = sshbuf_put_cstring(m,
+	    store->filename ? store->filename : "")) != 0 ||
+	    (r = sshbuf_put_cstring(m,
+	    store->envvar ? store->envvar : "")) != 0 ||
+	    (r = sshbuf_put_cstring(m,
+	    store->envval ? store->envval : "")) != 0)
+		fatal_fr(r, "buffer error");
+
+	mm_request_send(pmonitor->m_recvfd, MONITOR_REQ_GSSUPCREDS, m);
+	mm_request_receive_expect(pmonitor->m_recvfd, MONITOR_ANS_GSSUPCREDS, m);
+
+	if ((r = sshbuf_get_u32(m, &ok)) != 0)
+		fatal_fr(r, "buffer error");
+
+	sshbuf_free(m);
+
+	return (ok);
+}
+
 #endif /* GSSAPI */
 
 /*
diff --color -ruNp a/monitor_wrap.h b/monitor_wrap.h
--- a/monitor_wrap.h	2024-09-16 11:45:56.862133326 +0200
+++ b/monitor_wrap.h	2024-09-16 11:46:34.697939947 +0200
@@ -67,8 +67,10 @@ void mm_decode_activate_server_options(s
 OM_uint32 mm_ssh_gssapi_server_ctx(Gssctxt **, gss_OID);
 OM_uint32 mm_ssh_gssapi_accept_ctx(Gssctxt *,
    gss_buffer_desc *, gss_buffer_desc *, OM_uint32 *);
-int mm_ssh_gssapi_userok(char *user);
+int mm_ssh_gssapi_userok(char *user, struct passwd *, int kex);
 OM_uint32 mm_ssh_gssapi_checkmic(Gssctxt *, gss_buffer_t, gss_buffer_t);
+OM_uint32 mm_ssh_gssapi_sign(Gssctxt *, gss_buffer_t, gss_buffer_t);
+int mm_ssh_gssapi_update_creds(ssh_gssapi_ccache *);
 #endif
 
 #ifdef USE_PAM
diff --color -ruNp a/readconf.c b/readconf.c
--- a/readconf.c	2024-07-01 06:36:28.000000000 +0200
+++ b/readconf.c	2024-09-16 11:46:34.699939990 +0200
@@ -70,6 +70,7 @@
 #include "uidswap.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "ssh-gss.h"
 
 /* Format of the configuration file:
 
@@ -164,6 +165,8 @@ typedef enum {
 	oClearAllForwardings, oNoHostAuthenticationForLocalhost,
 	oEnableSSHKeysign, oRekeyLimit, oVerifyHostKeyDNS, oConnectTimeout,
 	oAddressFamily, oGssAuthentication, oGssDelegateCreds,
+	oGssTrustDns, oGssKeyEx, oGssClientIdentity, oGssRenewalRekey,
+	oGssServerIdentity, oGssKexAlgorithms,
 	oServerAliveInterval, oServerAliveCountMax, oIdentitiesOnly,
 	oSendEnv, oSetEnv, oControlPath, oControlMaster, oControlPersist,
 	oHashKnownHosts,
@@ -210,10 +213,22 @@ static struct {
 	/* Sometimes-unsupported options */
 #if defined(GSSAPI)
 	{ "gssapiauthentication", oGssAuthentication },
+	{ "gssapikeyexchange", oGssKeyEx },
 	{ "gssapidelegatecredentials", oGssDelegateCreds },
+	{ "gssapitrustdns", oGssTrustDns },
+	{ "gssapiclientidentity", oGssClientIdentity },
+	{ "gssapiserveridentity", oGssServerIdentity },
+	{ "gssapirenewalforcesrekey", oGssRenewalRekey },
+	{ "gssapikexalgorithms", oGssKexAlgorithms },
 # else
 	{ "gssapiauthentication", oUnsupported },
+	{ "gssapikeyexchange", oUnsupported },
 	{ "gssapidelegatecredentials", oUnsupported },
+	{ "gssapitrustdns", oUnsupported },
+	{ "gssapiclientidentity", oUnsupported },
+	{ "gssapiserveridentity", oUnsupported },
+	{ "gssapirenewalforcesrekey", oUnsupported },
+	{ "gssapikexalgorithms", oUnsupported },
 #endif
 #ifdef ENABLE_PKCS11
 	{ "pkcs11provider", oPKCS11Provider },
@@ -1227,10 +1242,42 @@ parse_time:
 		intptr = &options->gss_authentication;
 		goto parse_flag;
 
+	case oGssKeyEx:
+		intptr = &options->gss_keyex;
+		goto parse_flag;
+
 	case oGssDelegateCreds:
 		intptr = &options->gss_deleg_creds;
 		goto parse_flag;
 
+	case oGssTrustDns:
+		intptr = &options->gss_trust_dns;
+		goto parse_flag;
+
+	case oGssClientIdentity:
+		charptr = &options->gss_client_identity;
+		goto parse_string;
+
+	case oGssServerIdentity:
+		charptr = &options->gss_server_identity;
+		goto parse_string;
+
+	case oGssRenewalRekey:
+		intptr = &options->gss_renewal_rekey;
+		goto parse_flag;
+
+	case oGssKexAlgorithms:
+		arg = argv_next(&ac, &av);
+		if (!arg || *arg == '\0')
+			fatal("%.200s line %d: Missing argument.",
+			    filename, linenum);
+		if (!kex_gss_names_valid(arg))
+			fatal("%.200s line %d: Bad GSSAPI KexAlgorithms '%s'.",
+			    filename, linenum, arg ? arg : "<NONE>");
+		if (*activep && options->gss_kex_algorithms == NULL)
+			options->gss_kex_algorithms = xstrdup(arg);
+		break;
+
 	case oBatchMode:
 		intptr = &options->batch_mode;
 		goto parse_flag;
@@ -2542,7 +2589,13 @@ initialize_options(Options * options)
 	options->fwd_opts.streamlocal_bind_unlink = -1;
 	options->pubkey_authentication = -1;
 	options->gss_authentication = -1;
+	options->gss_keyex = -1;
 	options->gss_deleg_creds = -1;
+	options->gss_trust_dns = -1;
+	options->gss_renewal_rekey = -1;
+	options->gss_client_identity = NULL;
+	options->gss_server_identity = NULL;
+	options->gss_kex_algorithms = NULL;
 	options->password_authentication = -1;
 	options->kbd_interactive_authentication = -1;
 	options->kbd_interactive_devices = NULL;
@@ -2705,8 +2758,18 @@ fill_default_options(Options * options)
 		options->pubkey_authentication = SSH_PUBKEY_AUTH_ALL;
 	if (options->gss_authentication == -1)
 		options->gss_authentication = 0;
+	if (options->gss_keyex == -1)
+		options->gss_keyex = 0;
 	if (options->gss_deleg_creds == -1)
 		options->gss_deleg_creds = 0;
+	if (options->gss_trust_dns == -1)
+		options->gss_trust_dns = 0;
+	if (options->gss_renewal_rekey == -1)
+		options->gss_renewal_rekey = 0;
+#ifdef GSSAPI
+	if (options->gss_kex_algorithms == NULL)
+		options->gss_kex_algorithms = strdup(GSS_KEX_DEFAULT_KEX);
+#endif
 	if (options->password_authentication == -1)
 		options->password_authentication = 1;
 	if (options->kbd_interactive_authentication == -1)
@@ -3533,7 +3596,14 @@ dump_client_config(Options *o, const cha
 	dump_cfg_fmtint(oGatewayPorts, o->fwd_opts.gateway_ports);
 #ifdef GSSAPI
 	dump_cfg_fmtint(oGssAuthentication, o->gss_authentication);
+	dump_cfg_fmtint(oGssKeyEx, o->gss_keyex);
 	dump_cfg_fmtint(oGssDelegateCreds, o->gss_deleg_creds);
+	dump_cfg_fmtint(oGssTrustDns, o->gss_trust_dns);
+	dump_cfg_fmtint(oGssRenewalRekey, o->gss_renewal_rekey);
+	dump_cfg_string(oGssClientIdentity, o->gss_client_identity);
+	dump_cfg_string(oGssServerIdentity, o->gss_server_identity);
+	dump_cfg_string(oGssKexAlgorithms, o->gss_kex_algorithms ?
+	    o->gss_kex_algorithms : GSS_KEX_DEFAULT_KEX);
 #endif /* GSSAPI */
 	dump_cfg_fmtint(oHashKnownHosts, o->hash_known_hosts);
 	dump_cfg_fmtint(oHostbasedAuthentication, o->hostbased_authentication);
diff --color -ruNp a/readconf.h b/readconf.h
--- a/readconf.h	2024-07-01 06:36:28.000000000 +0200
+++ b/readconf.h	2024-09-16 11:46:34.699939990 +0200
@@ -40,7 +40,13 @@ typedef struct {
 	int     pubkey_authentication;	/* Try ssh2 pubkey authentication. */
 	int     hostbased_authentication;	/* ssh2's rhosts_rsa */
 	int     gss_authentication;	/* Try GSS authentication */
+	int     gss_keyex;		/* Try GSS key exchange */
 	int     gss_deleg_creds;	/* Delegate GSS credentials */
+	int	gss_trust_dns;		/* Trust DNS for GSS canonicalization */
+	int	gss_renewal_rekey;	/* Credential renewal forces rekey */
+	char    *gss_client_identity;   /* Principal to initiate GSSAPI with */
+	char    *gss_server_identity;   /* GSSAPI target principal */
+	char    *gss_kex_algorithms;	/* GSSAPI kex methods to be offered by client. */
 	int     password_authentication;	/* Try password
 						 * authentication. */
 	int     kbd_interactive_authentication; /* Try keyboard-interactive auth. */
diff --color -ruNp a/servconf.c b/servconf.c
--- a/servconf.c	2024-07-01 06:36:28.000000000 +0200
+++ b/servconf.c	2024-09-16 11:46:34.700940011 +0200
@@ -68,6 +68,7 @@
 #include "auth.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "ssh-gss.h"
 
 #if !defined(SSHD_PAM_SERVICE)
 # define SSHD_PAM_SERVICE		"sshd"
@@ -137,8 +138,11 @@ initialize_server_options(ServerOptions
 	options->kerberos_ticket_cleanup = -1;
 	options->kerberos_get_afs_token = -1;
 	options->gss_authentication=-1;
+	options->gss_keyex = -1;
 	options->gss_cleanup_creds = -1;
 	options->gss_strict_acceptor = -1;
+	options->gss_store_rekey = -1;
+	options->gss_kex_algorithms = NULL;
 	options->password_authentication = -1;
 	options->kbd_interactive_authentication = -1;
 	options->permit_empty_passwd = -1;
@@ -376,10 +380,18 @@ fill_default_server_options(ServerOption
 		options->kerberos_get_afs_token = 0;
 	if (options->gss_authentication == -1)
 		options->gss_authentication = 0;
+	if (options->gss_keyex == -1)
+		options->gss_keyex = 0;
 	if (options->gss_cleanup_creds == -1)
 		options->gss_cleanup_creds = 1;
 	if (options->gss_strict_acceptor == -1)
 		options->gss_strict_acceptor = 1;
+	if (options->gss_store_rekey == -1)
+		options->gss_store_rekey = 0;
+#ifdef GSSAPI
+	if (options->gss_kex_algorithms == NULL)
+		options->gss_kex_algorithms = strdup(GSS_KEX_DEFAULT_KEX);
+#endif
 	if (options->password_authentication == -1)
 		options->password_authentication = 1;
 	if (options->kbd_interactive_authentication == -1)
@@ -558,6 +570,7 @@ typedef enum {
 	sPerSourcePenalties, sPerSourcePenaltyExemptList,
 	sClientAliveInterval, sClientAliveCountMax, sAuthorizedKeysFile,
 	sGssAuthentication, sGssCleanupCreds, sGssStrictAcceptor,
+	sGssKeyEx, sGssKexAlgorithms, sGssStoreRekey,
 	sAcceptEnv, sSetEnv, sPermitTunnel,
 	sMatch, sPermitOpen, sPermitListen, sForceCommand, sChrootDirectory,
 	sUsePrivilegeSeparation, sAllowAgentForwarding,
@@ -643,12 +656,22 @@ static struct {
 #ifdef GSSAPI
 	{ "gssapiauthentication", sGssAuthentication, SSHCFG_ALL },
 	{ "gssapicleanupcredentials", sGssCleanupCreds, SSHCFG_GLOBAL },
+	{ "gssapicleanupcreds", sGssCleanupCreds, SSHCFG_GLOBAL },
 	{ "gssapistrictacceptorcheck", sGssStrictAcceptor, SSHCFG_GLOBAL },
+	{ "gssapikeyexchange", sGssKeyEx, SSHCFG_GLOBAL },
+	{ "gssapistorecredentialsonrekey", sGssStoreRekey, SSHCFG_GLOBAL },
+	{ "gssapikexalgorithms", sGssKexAlgorithms, SSHCFG_GLOBAL },
 #else
 	{ "gssapiauthentication", sUnsupported, SSHCFG_ALL },
 	{ "gssapicleanupcredentials", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapicleanupcreds", sUnsupported, SSHCFG_GLOBAL },
 	{ "gssapistrictacceptorcheck", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapikeyexchange", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapistorecredentialsonrekey", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapikexalgorithms", sUnsupported, SSHCFG_GLOBAL },
 #endif
+	{ "gssusesessionccache", sUnsupported, SSHCFG_GLOBAL },
+	{ "gssapiusesessioncredcache", sUnsupported, SSHCFG_GLOBAL },
 	{ "passwordauthentication", sPasswordAuthentication, SSHCFG_ALL },
 	{ "kbdinteractiveauthentication", sKbdInteractiveAuthentication, SSHCFG_ALL },
 	{ "challengeresponseauthentication", sKbdInteractiveAuthentication, SSHCFG_ALL }, /* alias */
@@ -1585,6 +1608,10 @@ process_server_config_line_depth(ServerO
 		intptr = &options->gss_authentication;
 		goto parse_flag;
 
+	case sGssKeyEx:
+		intptr = &options->gss_keyex;
+		goto parse_flag;
+
 	case sGssCleanupCreds:
 		intptr = &options->gss_cleanup_creds;
 		goto parse_flag;
@@ -1593,6 +1620,22 @@ process_server_config_line_depth(ServerO
 		intptr = &options->gss_strict_acceptor;
 		goto parse_flag;
 
+	case sGssStoreRekey:
+		intptr = &options->gss_store_rekey;
+		goto parse_flag;
+
+	case sGssKexAlgorithms:
+		arg = argv_next(&ac, &av);
+		if (!arg || *arg == '\0')
+			fatal("%.200s line %d: Missing argument.",
+			    filename, linenum);
+		if (!kex_gss_names_valid(arg))
+			fatal("%.200s line %d: Bad GSSAPI KexAlgorithms '%s'.",
+			    filename, linenum, arg ? arg : "<NONE>");
+		if (*activep && options->gss_kex_algorithms == NULL)
+			options->gss_kex_algorithms = xstrdup(arg);
+		break;
+
 	case sPasswordAuthentication:
 		intptr = &options->password_authentication;
 		goto parse_flag;
@@ -3178,6 +3221,10 @@ dump_config(ServerOptions *o)
 #ifdef GSSAPI
 	dump_cfg_fmtint(sGssAuthentication, o->gss_authentication);
 	dump_cfg_fmtint(sGssCleanupCreds, o->gss_cleanup_creds);
+	dump_cfg_fmtint(sGssKeyEx, o->gss_keyex);
+	dump_cfg_fmtint(sGssStrictAcceptor, o->gss_strict_acceptor);
+	dump_cfg_fmtint(sGssStoreRekey, o->gss_store_rekey);
+	dump_cfg_string(sGssKexAlgorithms, o->gss_kex_algorithms);
 #endif
 	dump_cfg_fmtint(sPasswordAuthentication, o->password_authentication);
 	dump_cfg_fmtint(sKbdInteractiveAuthentication,
diff --color -ruNp a/servconf.h b/servconf.h
--- a/servconf.h	2024-07-01 06:36:28.000000000 +0200
+++ b/servconf.h	2024-09-16 11:46:34.700940011 +0200
@@ -149,8 +149,11 @@ typedef struct {
 	int     kerberos_get_afs_token;		/* If true, try to get AFS token if
 						 * authenticated with Kerberos. */
 	int     gss_authentication;	/* If true, permit GSSAPI authentication */
+	int     gss_keyex;		/* If true, permit GSSAPI key exchange */
 	int     gss_cleanup_creds;	/* If true, destroy cred cache on logout */
 	int     gss_strict_acceptor;	/* If true, restrict the GSSAPI acceptor name */
+	int 	gss_store_rekey;
+	char   *gss_kex_algorithms;	/* GSSAPI kex methods to be offered by client. */
 	int     password_authentication;	/* If true, permit password
 						 * authentication. */
 	int     kbd_interactive_authentication;	/* If true, permit */
diff --color -ruNp a/session.c b/session.c
--- a/session.c	2024-09-16 11:45:56.866133411 +0200
+++ b/session.c	2024-09-16 11:46:34.701940032 +0200
@@ -2674,13 +2674,19 @@ do_cleanup(struct ssh *ssh, Authctxt *au
 
 #ifdef KRB5
 	if (options.kerberos_ticket_cleanup &&
-	    authctxt->krb5_ctx)
+	    authctxt->krb5_ctx) {
+		temporarily_use_uid(authctxt->pw);
 		krb5_cleanup_proc(authctxt);
+		restore_uid();
+	}
 #endif
 
 #ifdef GSSAPI
-	if (options.gss_cleanup_creds)
+	if (options.gss_cleanup_creds) {
+		temporarily_use_uid(authctxt->pw);
 		ssh_gssapi_cleanup_creds();
+		restore_uid();
+	}
 #endif
 
 	/* remove agent socket */
diff --color -ruNp a/ssh.1 b/ssh.1
--- a/ssh.1	2024-09-16 11:45:56.875133603 +0200
+++ b/ssh.1	2024-09-16 11:46:34.701940032 +0200
@@ -536,7 +536,13 @@ For full details of the options listed b
 .It GatewayPorts
 .It GlobalKnownHostsFile
 .It GSSAPIAuthentication
+.It GSSAPIKeyExchange
+.It GSSAPIClientIdentity
 .It GSSAPIDelegateCredentials
+.It GSSAPIKexAlgorithms
+.It GSSAPIRenewalForcesRekey
+.It GSSAPIServerIdentity
+.It GSSAPITrustDns
 .It HashKnownHosts
 .It Host
 .It HostbasedAcceptedAlgorithms
@@ -624,6 +630,8 @@ flag),
 (supported message integrity codes),
 .Ar kex
 (key exchange algorithms),
+.Ar kex-gss
+(GSSAPI key exchange algorithms),
 .Ar key
 (key types),
 .Ar key-ca-sign
diff --color -ruNp a/ssh.c b/ssh.c
--- a/ssh.c	2024-07-01 06:36:28.000000000 +0200
+++ b/ssh.c	2024-09-16 11:46:34.702940054 +0200
@@ -827,6 +827,8 @@ main(int ac, char **av)
 			else if (strcmp(optarg, "kex") == 0 ||
 			    strcasecmp(optarg, "KexAlgorithms") == 0)
 				cp = kex_alg_list('\n');
+			else if (strcmp(optarg, "kex-gss") == 0)
+				cp = kex_gss_alg_list('\n');
 			else if (strcmp(optarg, "key") == 0)
 				cp = sshkey_alg_list(0, 0, 0, '\n');
 			else if (strcmp(optarg, "key-cert") == 0)
@@ -857,8 +859,8 @@ main(int ac, char **av)
 			} else if (strcmp(optarg, "help") == 0) {
 				cp = xstrdup(
 				    "cipher\ncipher-auth\ncompression\nkex\n"
-				    "key\nkey-cert\nkey-plain\nkey-sig\nmac\n"
-				    "protocol-version\nsig");
+				    "kex-gss\nkey\nkey-cert\nkey-plain\n"
+				    "key-sig\nmac\nprotocol-version\nsig");
 			}
 			if (cp == NULL)
 				fatal("Unsupported query \"%s\"", optarg);
diff --color -ruNp a/ssh_config b/ssh_config
--- a/ssh_config	2024-09-16 11:45:56.884133795 +0200
+++ b/ssh_config	2024-09-16 11:46:34.702940054 +0200
@@ -24,6 +24,8 @@
 #   HostbasedAuthentication no
 #   GSSAPIAuthentication no
 #   GSSAPIDelegateCredentials no
+#   GSSAPIKeyExchange no
+#   GSSAPITrustDNS no
 #   BatchMode no
 #   CheckHostIP no
 #   AddressFamily any
diff --color -ruNp a/ssh_config.5 b/ssh_config.5
--- a/ssh_config.5	2024-07-01 06:36:28.000000000 +0200
+++ b/ssh_config.5	2024-09-16 11:46:34.703940075 +0200
@@ -938,10 +938,68 @@ The default is
 Specifies whether user authentication based on GSSAPI is allowed.
 The default is
 .Cm no .
+.It Cm GSSAPIClientIdentity
+If set, specifies the GSSAPI client identity that ssh should use when
+connecting to the server. The default is unset, which means that the default
+identity will be used.
 .It Cm GSSAPIDelegateCredentials
 Forward (delegate) credentials to the server.
 The default is
 .Cm no .
+.It Cm GSSAPIKeyExchange
+Specifies whether key exchange based on GSSAPI may be used. When using
+GSSAPI key exchange the server need not have a host key.
+The default is
+.Dq no .
+.It Cm GSSAPIRenewalForcesRekey
+If set to
+.Dq yes
+then renewal of the client's GSSAPI credentials will force the rekeying of the
+ssh connection. With a compatible server, this will delegate the renewed
+credentials to a session on the server.
+.Pp
+Checks are made to ensure that credentials are only propagated when the new
+credentials match the old ones on the originating client and where the
+receiving server still has the old set in its cache.
+.Pp
+The default is
+.Dq no .
+.Pp
+For this to work
+.Cm GSSAPIKeyExchange
+needs to be enabled in the server and also used by the client.
+.It Cm GSSAPIServerIdentity
+If set, specifies the GSSAPI server identity that ssh should expect when
+connecting to the server. The default is unset, which means that the
+expected GSSAPI server identity will be determined from the target
+hostname.
+.It Cm GSSAPITrustDns
+Set to
+.Dq yes
+to indicate that the DNS is trusted to securely canonicalize
+the name of the host being connected to. If
+.Dq no ,
+the hostname entered on the
+command line will be passed untouched to the GSSAPI library.
+The default is
+.Dq no .
+.It Cm GSSAPIKexAlgorithms
+The list of key exchange algorithms that are offered for GSSAPI
+key exchange. Possible values are
+.Bd -literal -offset 3n
+gss-gex-sha1-,
+gss-group1-sha1-,
+gss-group14-sha1-,
+gss-group14-sha256-,
+gss-group16-sha512-,
+gss-nistp256-sha256-,
+gss-curve25519-sha256-
+.Ed
+.Pp
+The default is
+.Dq gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,
+gss-curve25519-sha256-,gss-group14-sha1-,gss-gex-sha1- .
+This option only applies to connections using GSSAPI.
 .It Cm HashKnownHosts
 Indicates that
 .Xr ssh 1
diff --color -ruNp a/sshconnect2.c b/sshconnect2.c
--- a/sshconnect2.c	2024-07-01 06:36:28.000000000 +0200
+++ b/sshconnect2.c	2024-09-16 11:46:34.703940075 +0200
@@ -222,6 +222,11 @@ ssh_kex2(struct ssh *ssh, char *host, st
 	char *all_key, *hkalgs = NULL;
 	int r, use_known_hosts_order = 0;
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+	char *orig = NULL, *gss = NULL;
+	char *gss_host = NULL;
+#endif
+
 	xxx_host = host;
 	xxx_hostaddr = hostaddr;
 	xxx_conn_info = cinfo;
@@ -255,6 +260,42 @@ ssh_kex2(struct ssh *ssh, char *host, st
 	    compression_alg_list(options.compression),
 	    hkalgs ? hkalgs : options.hostkeyalgorithms);
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+	if (options.gss_keyex) {
+		/* Add the GSSAPI mechanisms currently supported on this
+		 * client to the key exchange algorithm proposal */
+		orig = myproposal[PROPOSAL_KEX_ALGS];
+
+		if (options.gss_server_identity) {
+			gss_host = xstrdup(options.gss_server_identity);
+		} else if (options.gss_trust_dns) {
+			gss_host = remote_hostname(ssh);
+			/* Fall back to specified host if we are using proxy command
+			 * and can not use DNS on that socket */
+			if (strcmp(gss_host, "UNKNOWN") == 0) {
+				free(gss_host);
+				gss_host = xstrdup(host);
+			}
+		} else {
+			gss_host = xstrdup(host);
+		}
+
+		gss = ssh_gssapi_client_mechanisms(gss_host,
+		    options.gss_client_identity, options.gss_kex_algorithms);
+		if (gss) {
+			debug("Offering GSSAPI proposal: %s", gss);
+			xasprintf(&myproposal[PROPOSAL_KEX_ALGS],
+			    "%s,%s", gss, orig);
+
+			/* If we've got GSSAPI algorithms, then we also support the
+			 * 'null' hostkey, as a last resort */
+			orig = myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS];
+			xasprintf(&myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS],
+			    "%s,null", orig);
+		}
+	}
+#endif
+
 	free(hkalgs);
 
 	/* start key exchange */
@@ -271,15 +312,45 @@ ssh_kex2(struct ssh *ssh, char *host, st
 # ifdef OPENSSL_HAS_ECC
 	ssh->kex->kex[KEX_ECDH_SHA2] = kex_gen_client;
 # endif
-#endif
+# ifdef GSSAPI
+	if (options.gss_keyex) {
+		ssh->kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_client;
+		ssh->kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_client;
+		ssh->kex->kex[KEX_GSS_GRP14_SHA256] = kexgss_client;
+		ssh->kex->kex[KEX_GSS_GRP16_SHA512] = kexgss_client;
+		ssh->kex->kex[KEX_GSS_GEX_SHA1] = kexgssgex_client;
+		ssh->kex->kex[KEX_GSS_NISTP256_SHA256] = kexgss_client;
+		ssh->kex->kex[KEX_GSS_C25519_SHA256] = kexgss_client;
+	}
+# endif
+#endif /* WITH_OPENSSL */
 	ssh->kex->kex[KEX_C25519_SHA256] = kex_gen_client;
 	ssh->kex->kex[KEX_KEM_SNTRUP761X25519_SHA512] = kex_gen_client;
 	ssh->kex->kex[KEX_KEM_MLKEM768X25519_SHA256] = kex_gen_client;
 	ssh->kex->verify_host_key=&verify_host_key_callback;
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+	if (options.gss_keyex) {
+		ssh->kex->gss_deleg_creds = options.gss_deleg_creds;
+		ssh->kex->gss_trust_dns = options.gss_trust_dns;
+		ssh->kex->gss_client = options.gss_client_identity;
+		ssh->kex->gss_host = gss_host;
+	}
+#endif
+
 	ssh_dispatch_run_fatal(ssh, DISPATCH_BLOCK, &ssh->kex->done);
 	kex_proposal_free_entries(myproposal);
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+	/* repair myproposal after it was crumpled by the */
+	/* ext-info removal above */
+	if (gss) {
+		orig = myproposal[PROPOSAL_KEX_ALGS];
+		xasprintf(&myproposal[PROPOSAL_KEX_ALGS],
+		    "%s,%s", gss, orig);
+		free(gss);
+	}
+#endif
 #ifdef DEBUG_KEXDH
 	/* send 1st encrypted/maced/compressed message */
 	if ((r = sshpkt_start(ssh, SSH2_MSG_IGNORE)) != 0 ||
@@ -368,6 +439,7 @@ static int input_gssapi_response(int typ
 static int input_gssapi_token(int type, u_int32_t, struct ssh *);
 static int input_gssapi_error(int, u_int32_t, struct ssh *);
 static int input_gssapi_errtok(int, u_int32_t, struct ssh *);
+static int userauth_gsskeyex(struct ssh *);
 #endif
 
 void	userauth(struct ssh *, char *);
@@ -384,6 +456,11 @@ static char *authmethods_get(void);
 
 Authmethod authmethods[] = {
 #ifdef GSSAPI
+	{"gssapi-keyex",
+		userauth_gsskeyex,
+		NULL,
+		&options.gss_keyex,
+		NULL},
 	{"gssapi-with-mic",
 		userauth_gssapi,
 		userauth_gssapi_cleanup,
@@ -755,12 +832,32 @@ userauth_gssapi(struct ssh *ssh)
 	OM_uint32 min;
 	int r, ok = 0;
 	gss_OID mech = NULL;
+	char *gss_host = NULL;
+
+	if (options.gss_server_identity) {
+		gss_host = xstrdup(options.gss_server_identity);
+	} else if (options.gss_trust_dns) {
+		gss_host = remote_hostname(ssh);
+		/* Fall back to specified host if we are using proxy command
+		 * and can not use DNS on that socket */
+		if (strcmp(gss_host, "UNKNOWN") == 0) {
+			free(gss_host);
+			gss_host = xstrdup(authctxt->host);
+		}
+	} else {
+		gss_host = xstrdup(authctxt->host);
+	}
 
 	/* Try one GSSAPI method at a time, rather than sending them all at
 	 * once. */
 
 	if (authctxt->gss_supported_mechs == NULL)
-		gss_indicate_mechs(&min, &authctxt->gss_supported_mechs);
+		if (GSS_ERROR(gss_indicate_mechs(&min,
+		    &authctxt->gss_supported_mechs))) {
+			authctxt->gss_supported_mechs = NULL;
+			free(gss_host);
+			return 0;
+		}
 
 	/* Check to see whether the mechanism is usable before we offer it */
 	while (authctxt->mech_tried < authctxt->gss_supported_mechs->count &&
@@ -769,13 +866,15 @@ userauth_gssapi(struct ssh *ssh)
 		    elements[authctxt->mech_tried];
 		/* My DER encoding requires length<128 */
 		if (mech->length < 128 && ssh_gssapi_check_mechanism(&gssctxt,
-		    mech, authctxt->host)) {
+		    mech, gss_host, options.gss_client_identity)) {
 			ok = 1; /* Mechanism works */
 		} else {
 			authctxt->mech_tried++;
 		}
 	}
 
+	free(gss_host);
+
 	if (!ok || mech == NULL)
 		return 0;
 
@@ -1009,6 +1108,55 @@ input_gssapi_error(int type, u_int32_t p
 	free(lang);
 	return r;
 }
+
+int
+userauth_gsskeyex(struct ssh *ssh)
+{
+	struct sshbuf *b = NULL;
+	Authctxt *authctxt = ssh->authctxt;
+	gss_buffer_desc gssbuf;
+	gss_buffer_desc mic = GSS_C_EMPTY_BUFFER;
+	OM_uint32 ms;
+	int r;
+
+	static int attempt = 0;
+	if (attempt++ >= 1)
+		return (0);
+
+	if (gss_kex_context == NULL) {
+		debug("No valid Key exchange context");
+		return (0);
+	}
+
+	if ((b = sshbuf_new()) == NULL)
+		fatal_f("sshbuf_new failed");
+
+	ssh_gssapi_buildmic(b, authctxt->server_user, authctxt->service,
+	    "gssapi-keyex", ssh->kex->session_id);
+
+	if ((gssbuf.value = sshbuf_mutable_ptr(b)) == NULL)
+		fatal_f("sshbuf_mutable_ptr failed");
+	gssbuf.length = sshbuf_len(b);
+
+	if (GSS_ERROR(ssh_gssapi_sign(gss_kex_context, &gssbuf, &mic))) {
+		sshbuf_free(b);
+		return (0);
+	}
+
+	if ((r = sshpkt_start(ssh, SSH2_MSG_USERAUTH_REQUEST)) != 0 ||
+	    (r = sshpkt_put_cstring(ssh, authctxt->server_user)) != 0 ||
+	    (r = sshpkt_put_cstring(ssh, authctxt->service)) != 0 ||
+	    (r = sshpkt_put_cstring(ssh, authctxt->method->name)) != 0 ||
+	    (r = sshpkt_put_string(ssh, mic.value, mic.length)) != 0 ||
+	    (r = sshpkt_send(ssh)) != 0)
+		fatal_fr(r, "parsing");
+
+	sshbuf_free(b);
+	gss_release_buffer(&ms, &mic);
+
+	return (1);
+}
+
 #endif /* GSSAPI */
 
 static int
diff --color -ruNp a/sshd.c b/sshd.c
--- a/sshd.c	2024-07-01 06:36:28.000000000 +0200
+++ b/sshd.c	2024-09-16 11:46:34.704940096 +0200
@@ -1551,7 +1551,8 @@ main(int ac, char **av)
 		free(fp);
 	}
 	accumulate_host_timing_secret(cfg, NULL);
-	if (!sensitive_data.have_ssh2_key) {
+	/* The GSSAPI key exchange can run without a host key */
+	if (!sensitive_data.have_ssh2_key && !options.gss_keyex) {
 		logit("sshd: no hostkeys available -- exiting.");
 		exit(1);
 	}
diff --color -ruNp a/sshd_config b/sshd_config
--- a/sshd_config	2024-09-16 11:45:56.888133880 +0200
+++ b/sshd_config	2024-09-16 11:46:34.704940096 +0200
@@ -77,6 +77,8 @@ AuthorizedKeysFile	.ssh/authorized_keys
 # GSSAPI options
 #GSSAPIAuthentication no
 #GSSAPICleanupCredentials yes
+#GSSAPIStrictAcceptorCheck yes
+#GSSAPIKeyExchange no
 
 # Set this to 'yes' to enable PAM authentication, account processing,
 # and session processing. If this is enabled, PAM authentication will
diff --color -ruNp a/sshd_config.5 b/sshd_config.5
--- a/sshd_config.5	2024-09-16 11:45:56.885133816 +0200
+++ b/sshd_config.5	2024-09-16 11:46:34.704940096 +0200
@@ -739,6 +739,11 @@ Specifies whether to automatically destr
 on logout.
 The default is
 .Cm yes .
+.It Cm GSSAPIKeyExchange
+Specifies whether key exchange based on GSSAPI is allowed. GSSAPI key exchange
+doesn't rely on ssh keys to verify host identity.
+The default is
+.Cm no .
 .It Cm GSSAPIStrictAcceptorCheck
 Determines whether to be strict about the identity of the GSSAPI acceptor
 a client authenticates against.
@@ -753,6 +758,32 @@ machine's default store.
 This facility is provided to assist with operation on multi homed machines.
 The default is
 .Cm yes .
+.It Cm GSSAPIStoreCredentialsOnRekey
+Controls whether the user's GSSAPI credentials should be updated following a
+successful connection rekeying. This option can be used to accepted renewed
+or updated credentials from a compatible client. The default is
+.Dq no .
+.Pp
+For this to work
+.Cm GSSAPIKeyExchange
+needs to be enabled in the server and also used by the client.
+.It Cm GSSAPIKexAlgorithms
+The list of key exchange algorithms that are accepted by GSSAPI
+key exchange. Possible values are
+.Bd -literal -offset 3n
+gss-gex-sha1-,
+gss-group1-sha1-,
+gss-group14-sha1-,
+gss-group14-sha256-,
+gss-group16-sha512-,
+gss-nistp256-sha256-,
+gss-curve25519-sha256-
+.Ed
+.Pp
+The default is
+.Dq gss-group14-sha256-,gss-group16-sha512-,gss-nistp256-sha256-,
+gss-curve25519-sha256-,gss-group14-sha1-,gss-gex-sha1- .
+This option only applies to connections using GSSAPI.
 .It Cm HostbasedAcceptedAlgorithms
 Specifies the signature algorithms that will be accepted for hostbased
 authentication as a list of comma-separated patterns.
diff --color -ruNp a/sshd-session.c b/sshd-session.c
--- a/sshd-session.c	2024-09-16 11:45:56.888133880 +0200
+++ b/sshd-session.c	2024-09-16 11:46:34.705940118 +0200
@@ -660,8 +660,8 @@ notify_hostkeys(struct ssh *ssh)
 	}
 	debug3_f("sent %u hostkeys", nkeys);
 	if (nkeys == 0)
-		fatal_f("no hostkeys");
-	if ((r = sshpkt_send(ssh)) != 0)
+		debug3_f("no hostkeys");
+	else if ((r = sshpkt_send(ssh)) != 0)
 		sshpkt_fatal(ssh, r, "%s: send", __func__);
 	sshbuf_free(buf);
 }
@@ -1180,8 +1180,9 @@ main(int ac, char **av)
 			break;
 		}
 	}
-	if (!have_key)
-		fatal("internal error: monitor received no hostkeys");
+	/* The GSSAPI key exchange can run without a host key */
+	if (!have_key && !options.gss_keyex)
+		fatal("internal error: monitor received no hostkeys and GSS KEX is not configured");
 
 	/* Ensure that umask disallows at least group and world write */
 	new_umask = umask(0077) | 0022;
@@ -1462,6 +1463,48 @@ do_ssh2_kex(struct ssh *ssh)
 
 	free(hkalgs);
 
+#if defined(GSSAPI) && defined(WITH_OPENSSL)
+	{
+	char *orig;
+	char *gss = NULL;
+	char *newstr = NULL;
+	orig = myproposal[PROPOSAL_KEX_ALGS];
+
+	/*
+	 * If we don't have a host key, then there's no point advertising
+	 * the other key exchange algorithms
+	 */
+
+	if (strlen(myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS]) == 0)
+		orig = NULL;
+
+	if (options.gss_keyex)
+		gss = ssh_gssapi_server_mechanisms();
+	else
+		gss = NULL;
+
+	if (gss && orig)
+		xasprintf(&newstr, "%s,%s", gss, orig);
+	else if (gss)
+		xasprintf(&newstr, "%s,%s", gss, "kex-strict-s-v00@openssh.com");
+	else if (orig)
+		newstr = orig;
+
+	/*
+	 * If we've got GSSAPI mechanisms, then we've got the 'null' host
+	 * key alg, but we can't tell people about it unless its the only
+	 * host key algorithm we support
+	 */
+	if (gss && (strlen(myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS])) == 0)
+		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] = xstrdup("null");
+
+	if (newstr)
+		myproposal[PROPOSAL_KEX_ALGS] = newstr;
+	else
+		fatal("No supported key exchange algorithms");
+	}
+#endif
+
 	/* start key exchange */
 	if ((r = kex_setup(ssh, myproposal)) != 0)
 		fatal_r(r, "kex_setup");
@@ -1479,7 +1522,18 @@ do_ssh2_kex(struct ssh *ssh)
  #ifdef OPENSSL_HAS_ECC
 	kex->kex[KEX_ECDH_SHA2] = kex_gen_server;
  #endif
-#endif
+# ifdef GSSAPI
+	if (options.gss_keyex) {
+		kex->kex[KEX_GSS_GRP1_SHA1] = kexgss_server;
+		kex->kex[KEX_GSS_GRP14_SHA1] = kexgss_server;
+		kex->kex[KEX_GSS_GRP14_SHA256] = kexgss_server;
+		kex->kex[KEX_GSS_GRP16_SHA512] = kexgss_server;
+		kex->kex[KEX_GSS_GEX_SHA1] = kexgssgex_server;
+		kex->kex[KEX_GSS_NISTP256_SHA256] = kexgss_server;
+		kex->kex[KEX_GSS_C25519_SHA256] = kexgss_server;
+	}
+# endif
+#endif /* WITH_OPENSSL */
 	kex->kex[KEX_C25519_SHA256] = kex_gen_server;
 	kex->kex[KEX_KEM_SNTRUP761X25519_SHA512] = kex_gen_server;
  	kex->kex[KEX_KEM_MLKEM768X25519_SHA256] = kex_gen_server;
diff --color -ruNp a/ssh-gss.h b/ssh-gss.h
--- a/ssh-gss.h	2024-07-01 06:36:28.000000000 +0200
+++ b/ssh-gss.h	2024-09-16 11:46:34.710940224 +0200
@@ -61,10 +61,36 @@
 
 #define SSH_GSS_OIDTYPE 0x06
 
+#define SSH2_MSG_KEXGSS_INIT                            30
+#define SSH2_MSG_KEXGSS_CONTINUE                        31
+#define SSH2_MSG_KEXGSS_COMPLETE                        32
+#define SSH2_MSG_KEXGSS_HOSTKEY                         33
+#define SSH2_MSG_KEXGSS_ERROR                           34
+#define SSH2_MSG_KEXGSS_GROUPREQ			40
+#define SSH2_MSG_KEXGSS_GROUP				41
+#define KEX_GSS_GRP1_SHA1_ID				"gss-group1-sha1-"
+#define KEX_GSS_GRP14_SHA1_ID				"gss-group14-sha1-"
+#define KEX_GSS_GRP14_SHA256_ID			"gss-group14-sha256-"
+#define KEX_GSS_GRP16_SHA512_ID			"gss-group16-sha512-"
+#define KEX_GSS_GEX_SHA1_ID				"gss-gex-sha1-"
+#define KEX_GSS_NISTP256_SHA256_ID			"gss-nistp256-sha256-"
+#define KEX_GSS_C25519_SHA256_ID			"gss-curve25519-sha256-"
+
+#define        GSS_KEX_DEFAULT_KEX \
+	KEX_GSS_GRP14_SHA256_ID "," \
+	KEX_GSS_GRP16_SHA512_ID	"," \
+	KEX_GSS_NISTP256_SHA256_ID "," \
+	KEX_GSS_C25519_SHA256_ID "," \
+	KEX_GSS_GRP14_SHA1_ID "," \
+	KEX_GSS_GEX_SHA1_ID
+
+#include "digest.h" /* SSH_DIGEST_MAX_LENGTH */
+
 typedef struct {
 	char *filename;
 	char *envvar;
 	char *envval;
+	struct passwd *owner;
 	void *data;
 } ssh_gssapi_ccache;
 
@@ -72,8 +98,11 @@ typedef struct {
 	gss_buffer_desc displayname;
 	gss_buffer_desc exportedname;
 	gss_cred_id_t creds;
+	gss_name_t name;
 	struct ssh_gssapi_mech_struct *mech;
 	ssh_gssapi_ccache store;
+	int used;
+	int updated;
 } ssh_gssapi_client;
 
 typedef struct ssh_gssapi_mech_struct {
@@ -84,6 +113,7 @@ typedef struct ssh_gssapi_mech_struct {
 	int (*userok) (ssh_gssapi_client *, char *);
 	int (*localname) (ssh_gssapi_client *, char **);
 	void (*storecreds) (ssh_gssapi_client *);
+	int (*updatecreds) (ssh_gssapi_ccache *, ssh_gssapi_client *);
 } ssh_gssapi_mech;
 
 typedef struct {
@@ -94,10 +124,21 @@ typedef struct {
 	gss_OID		oid; /* client */
 	gss_cred_id_t	creds; /* server */
 	gss_name_t	client; /* server */
-	gss_cred_id_t	client_creds; /* server */
+	gss_cred_id_t	client_creds; /* both */
+	struct sshbuf *shared_secret; /* both */
+	struct sshbuf *server_pubkey; /* server */
+	struct sshbuf *server_blob; /* client */
+	struct sshbuf *server_host_key_blob; /* client */
+	gss_buffer_desc msg_tok; /* client */
+	gss_buffer_desc buf; /* both */
+	u_char hash[SSH_DIGEST_MAX_LENGTH]; /* both */
+	size_t hashlen; /* both */
+	int first; /* client */
+	BIGNUM *dh_client_pub; /* server (gex) */
 } Gssctxt;
 
 extern ssh_gssapi_mech *supported_mechs[];
+extern Gssctxt *gss_kex_context;
 
 int  ssh_gssapi_check_oid(Gssctxt *, void *, size_t);
 void ssh_gssapi_set_oid_data(Gssctxt *, void *, size_t);
@@ -108,6 +149,7 @@ OM_uint32 ssh_gssapi_test_oid_supported(
 
 struct sshbuf;
 int ssh_gssapi_get_buffer_desc(struct sshbuf *, gss_buffer_desc *);
+int ssh_gssapi_sshpkt_get_buffer_desc(struct ssh *, gss_buffer_desc *);
 
 OM_uint32 ssh_gssapi_import_name(Gssctxt *, const char *);
 OM_uint32 ssh_gssapi_init_ctx(Gssctxt *, int,
@@ -122,17 +164,33 @@ void ssh_gssapi_delete_ctx(Gssctxt **);
 OM_uint32 ssh_gssapi_sign(Gssctxt *, gss_buffer_t, gss_buffer_t);
 void ssh_gssapi_buildmic(struct sshbuf *, const char *,
     const char *, const char *, const struct sshbuf *);
-int ssh_gssapi_check_mechanism(Gssctxt **, gss_OID, const char *);
+int ssh_gssapi_check_mechanism(Gssctxt **, gss_OID, const char *, const char *);
+OM_uint32 ssh_gssapi_client_identity(Gssctxt *, const char *);
+int ssh_gssapi_credentials_updated(Gssctxt *);
 
 /* In the server */
+typedef int ssh_gssapi_check_fn(Gssctxt **, gss_OID, const char *,
+    const char *);
+char *ssh_gssapi_client_mechanisms(const char *, const char *, const char *);
+char *ssh_gssapi_kex_mechs(gss_OID_set, ssh_gssapi_check_fn *, const char *,
+    const char *, const char *);
+gss_OID ssh_gssapi_id_kex(Gssctxt *, char *, int);
+int ssh_gssapi_server_check_mech(Gssctxt **,gss_OID, const char *,
+    const char *);
 OM_uint32 ssh_gssapi_server_ctx(Gssctxt **, gss_OID);
-int ssh_gssapi_userok(char *name);
+int ssh_gssapi_userok(char *name, struct passwd *, int kex);
 OM_uint32 ssh_gssapi_checkmic(Gssctxt *, gss_buffer_t, gss_buffer_t);
 void ssh_gssapi_do_child(char ***, u_int *);
 void ssh_gssapi_cleanup_creds(void);
 void ssh_gssapi_storecreds(void);
 const char *ssh_gssapi_displayname(void);
 
+char *ssh_gssapi_server_mechanisms(void);
+int ssh_gssapi_oid_table_ok(void);
+
+int ssh_gssapi_update_creds(ssh_gssapi_ccache *store);
+void ssh_gssapi_rekey_creds(void);
+
 #endif /* GSSAPI */
 
 #endif /* _SSH_GSS_H */
diff --color -ruNp a/sshkey.c b/sshkey.c
--- a/sshkey.c	2024-07-01 06:36:28.000000000 +0200
+++ b/sshkey.c	2024-09-16 11:46:34.706940139 +0200
@@ -131,6 +131,75 @@ extern const struct sshkey_impl sshkey_x
 extern const struct sshkey_impl sshkey_xmss_cert_impl;
 #endif
 
+static int ssh_gss_equal(const struct sshkey *, const struct sshkey *)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_serialize_public(const struct sshkey *, struct sshbuf *,
+	enum sshkey_serialize_rep)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_deserialize_public(const char *, struct sshbuf *,
+	     struct sshkey *)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_serialize_private(const struct sshkey *, struct sshbuf *,
+	     enum sshkey_serialize_rep)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_deserialize_private(const char *, struct sshbuf *,
+	     struct sshkey *)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_copy_public(const struct sshkey *, struct sshkey *)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static int ssh_gss_verify(const struct sshkey *, const u_char *, size_t,
+	    const u_char *, size_t, const char *, u_int,
+	    struct sshkey_sig_details **)
+{
+	return SSH_ERR_FEATURE_UNSUPPORTED;
+}
+
+static const struct sshkey_impl_funcs sshkey_gss_funcs = {
+	/* .size = */		NULL,
+	/* .alloc = */		NULL,
+	/* .cleanup = */	NULL,
+	/* .equal = */		ssh_gss_equal,
+	/* .ssh_serialize_public = */ ssh_gss_serialize_public,
+	/* .ssh_deserialize_public = */ ssh_gss_deserialize_public,
+	/* .ssh_serialize_private = */ ssh_gss_serialize_private,
+	/* .ssh_deserialize_private = */ ssh_gss_deserialize_private,
+	/* .generate = */	NULL,
+	/* .copy_public = */	ssh_gss_copy_public,
+	/* .sign = */		NULL,
+	/* .verify = */		ssh_gss_verify,
+};
+
+/* The struct is intentionally dummy and has no gss calls */
+static const struct sshkey_impl sshkey_gss_kex_impl = {
+	/* .name = */		"null",
+	/* .shortname = */	"null",
+	/* .sigalg = */		NULL,
+	/* .type = */		KEY_NULL,
+	/* .nid = */		0,
+	/* .cert = */		0,
+	/* .sigonly = */	0,
+	/* .keybits = */	0, /* FIXME */
+	/* .funcs = */		&sshkey_gss_funcs,
+};
+
 const struct sshkey_impl * const keyimpls[] = {
 	&sshkey_ed25519_impl,
 	&sshkey_ed25519_cert_impl,
@@ -169,6 +238,7 @@ const struct sshkey_impl * const keyimpl
 	&sshkey_xmss_impl,
 	&sshkey_xmss_cert_impl,
 #endif
+	&sshkey_gss_kex_impl,
 	NULL
 };
 
@@ -324,7 +394,7 @@ sshkey_alg_list(int certs_only, int plai
 
 	for (i = 0; keyimpls[i] != NULL; i++) {
 		impl = keyimpls[i];
-		if (impl->name == NULL)
+		if (impl->name == NULL || impl->type == KEY_NULL)
 			continue;
 		if (!include_sigonly && impl->sigonly)
 			continue;
diff --color -ruNp a/sshkey.h b/sshkey.h
--- a/sshkey.h	2024-07-01 06:36:28.000000000 +0200
+++ b/sshkey.h	2024-09-16 11:46:34.706940139 +0200
@@ -71,6 +71,7 @@ enum sshkey_types {
 	KEY_ECDSA_SK_CERT,
 	KEY_ED25519_SK,
 	KEY_ED25519_SK_CERT,
+	KEY_NULL,
 	KEY_UNSPEC
 };