Signing RPMs is supposed to be easy:

gpg --list-keys
cat > ~/.rpmmacros < <EOL
%_topdir /home/rob/rpmbuild
%debug_package %{nil}
%_signature gpg
%_gpg_name Rob A <>
%{__gpg} \
gpg --digest-algo=sha1 --batch --no-verbose --no-armor \
--force-v3-sigs --passphrase-fd 3 --no-secmem-warning -u "%{_gpg_name}" \
-sbo %{__signature_filename} %{__plaintext_filename}
rpm --resign

But there are a lot of caveats:

  1. If you run this on RHEL or CentOS or Scientific Linux 6.x and have an RSA, rather than a DSA, GPG key, any older systems (5.x or 4.x) won’t be able to properly decode the signatures. If you have a DSA key (and only v6+), the --force-v3-sigs is enough.
  2. If you run this on RHEL or CentOS 5.x, you need to remove the “–force-v3-sigs” option from the .rpmmacros line
  3. is a bit wrong – RSA keys don’t always work, DSA do (there’s apparently a RHN KB article about this, if you have support and licenses to read it. I don’t right now).
  4. RHEL / CentOS / Scientific Linux 7 won’t accept (without warning) RPMs signed with weak keys (or weak digests, in some cases, like sha1)

So, based on the systems you’re trying to build your package for, ensure the signing key you’re using and the digest algorithm you’re using, are supported across all the versions you expect to support… or build multiple RPMs, and put them in separate repos for each OS version.