GnuPG 1.03, RSA/IDEA, and cipher-algo preferences

[Glenn Leavell]

On Wed, Sep 27, 2000 at 07:59:52AM +0200, Huels, Ralf KSV wrote:

# By creating a Key ;-) Seriously, the algorithm preferences are stored 
# in the self-signature of a user ID (You can review them by running
# gpg --list-packets on a public key file or using the pref command in
# the --edit-key function). I donŽt know, whether algrithm preferences
# are possible at all with RSA keys/v3 sigs.
# 
# Currently the preferences 10/4/3 (Twofish, Blowfish, Cast5) are 
# hard-wired into the GnuPG key creation process. 
# YouŽd have to go source-diving to change them (g10/keygen.c, IIRC).

Thanks -- this clears up a lot for me.  From reading everyone's responses
and testing things with various keys, I've figured out what I've
really been trying to ask:

If I want to use gpg to encrypt something to someone's old RSA/IDEA key, I
can do so by including "--cipher-algo idea", and it works fine.  However,
if I leave off this option, the person trying to decrypt the message
(using PGP) "Unsupported packet format - you need a newer version of PGP
for this file."  But if I look at their public key with --list-packets,
I see that their RSA key *does* have a preference for algorithm 1, which
is IDEA.  So, why do I need to be explicit about wanting to use IDEA
with the --cipher-algo option?  Why doesn't gpg pick up on this preference
based on the public key itself?

Thanks,
Glenn

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