verify upstream signatures in %prep, unless bootstrapping

Per the guidelines¹, verify upstream signatures, unless we are in
bootstrap mode.

The fingerprints of the keys contained in signature_key.asc were checked
against the upstream page (https://gnupg.org/signature_key.html).  One
downside is that we are unable to verify signatures made with only the
brainpool key.  The hope is that such releases are relatively rare and
the benefit of automated signature verification outweighs the hassle of
handling such releases.  For these releases, set skip_verify to 1, as
we've done here.  Afterward, reset it to 0.

¹ https://docs.fedoraproject.org/en-US/packaging-guidelines/#_source_file_verification

gnupg2.spec

@@ -1,11 +1,19 @@
+%bcond_with bootstrap
+
+# Releases are occasionally signed with a brainpool key, which we cannot
+# (currently) use.  In such cases, set skip_verify to 1 and manually verify
+# the upstream source.
+%global skip_verify 1
+
 Summary: Utility for secure communication and data storage
 Name:    gnupg2
 Version: 2.3.7
-Release: 4%{?dist}
+Release: 5%{?dist}
 
 License: GPLv3+
 Source0: https://gnupg.org/ftp/gcrypt/%{?pre:alpha/}gnupg/gnupg-%{version}%{?pre}.tar.bz2
 Source1: https://gnupg.org/ftp/gcrypt/%{?pre:alpha/}gnupg/gnupg-%{version}%{?pre}.tar.bz2.sig
+Source2: https://gnupg.org/signature_key.asc
 # needed for compatibility with system FIPS mode
 Patch3:  gnupg-2.1.10-secmem.patch
 # non-upstreamable patch adding file-is-digest option needed for Copr
@@ -34,6 +42,10 @@ BuildRequires: bzip2-devel
 BuildRequires: curl-devel
 BuildRequires: docbook-utils
 BuildRequires: gettext
+%if %{without bootstrap}
+# Require gnupg2 to verify sources, unless bootstrapping
+BuildRequires: gnupg2
+%endif
 BuildRequires: libassuan-devel >= 2.5.0
 BuildRequires: libgcrypt-devel >= 1.9.1
 BuildRequires: libgpg-error-devel >= 1.41
@@ -95,6 +107,9 @@ package adds support for smart cards and S/MIME encryption and signing
 to the base GnuPG package
 
 %prep
+%if %{without bootstrap} && ! 0%{?skip_verify}
+%{gpgverify} --keyring='%{SOURCE2}' --signature='%{SOURCE1}' --data='%{SOURCE0}'
+%endif
 %setup -q -n gnupg-%{version}
 
 %patch3 -p1 -b .secmem
@@ -209,6 +224,9 @@ make -k check
 
 
 %changelog
+* Mon Oct 17 2022 Todd Zullinger <tmz@pobox.com> - 2.3.7-5
+- verify upstream signatures in %%prep, unless bootstrapping
+
 * Wed Oct 05 2022 Todd Zullinger <tmz@pobox.com> - 2.3.7-4
 - update BR/R versions for libassuan, libgpg-error, and libksba
 - drop with/without unversioned_gpg, last used with fedora-29

signature_key.asc

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