I think I may have just seen the light! The key I am trying to encrypt to is a PGP-2.x generated one. These keys have no algo preferences on them which probably results in some incompatibility problems. I’ve just tried with SHA-256 and received a similar error.

Thanks for the feedback!

I actually do not receive that warning when I use SHA256 ; it seems to be specific to you using SHA512.

I read that warning as you are prefering SHA512, but maybe your recipient does not, so you would then be forcing them to.

My man page for gpg has this blurb:

http://linuxcommand.org/man_pages/gpg1.html

INTEROPERABILITY WITH OTHER OPENPGP PROGRAMS
GnuPG tries to be a very flexible implementation of the OpenPGP stan-
dard. In particular, GnuPG implements many of the optional parts of
the standard, such as the SHA-512 hash, and the ZLIB and BZIP2 compres-
sion algorithms. It is important to be aware that not all OpenPGP pro-
grams implement these optional algorithms and that by forcing their use
via the –cipher-algo, –digest-algo, –cert-digest-algo, or –com-
press-algo options in GnuPG, it is possible to create a perfectly valid
OpenPGP message, but one that cannot be read by the intended recipient.

So it would seem that gpg is just warning you that your recipient may not be able to handle your message.

One problem I have encountered is when I want to encrypt and sign a message to a user who doesn’t support my preferred algo. I get:

gpg: WARNING: forcing digest algorithm SHA512 (10) violates recipient preferences

I can’t figure out why I’m forcing this algo, my entry in gpg.conf (digest-algo) should only select my preferred option, not force it. Any thoughts on how to resolve this?