interoperability

[vedaal]

> > Leigh S. Jones, KR6X wrote:
> Message: 10
> From: "Leigh S. Jones, KR6X" <kr6x@kr6x.com>
> To: <gnupg-users@gnupg.org>
> Subject: Re: Making it simple
> Date: Tue, 7 May 2002 14:17:15 -0700
...
>  If the encrypted data will be encrypted to your
> keys, then GnuPG 1.0.7 allows you to select your preference
> of encryption algorithms.  PGP 6.x or 7.x will encrypt to the
> preferred algorithms on your key.  GnuPG will decrypt this
> data OK.  Thank you, OpenPGP Foundation.
>
> Of course, if you created your keys on PGP, the preferred
> algorithm on the key will be "idea".  You can export a key
> like this from PGP to GnuPG and be able to send messages
> one way only -- from GnuPG to PGP.  PGP will encrypt to
> your "idea" preference, and you'll need to add "idea" to
> your GnuPG installation to be able to decrypt.
>
> But if you change the key preferences from "idea" to, say,
> AES, then "gpg -a --export <key>" the public key back to PGP,
> then PGP will start encrypting keys to AES.  Interoperability
> restored.  Send the public key to the keyserver and everyone
> with fairly up-to-date software revisions will be able to find
> out which algorithm you prefer.
...

there still remains a minor potential interoperability problem:

if a pgp user encrypts a message to a recipient's dh key, and (by default)
to his own v3 rsa key,
(inconvenient to change a long-standing key that has come to be known and
trusted by many correspondents)
and the recipient's dh key was generated in pgp
{and therefore 'allows'  the idea cipher for that key, even if it is not the
preferred one}
then the message will have 'idea' as the symmetric cipher for 'both' keys,
even if the pgp user specifies a different algorithm,
as long as the 'idea' algorithm is not disabled

now, if the recipient was a previous pgp user, and moved to gnupg, and took
the pgp generated dh keys with him,
but does not use the 'idea' module, in gnupg,
then the message will not be decryptable in gnupg

this can easily be worked around by having the pgp user (sender) temporarily
disable the 'idea' cipher for messages to those recipients where this may be
an issue

this does not apply to gnupg users whose keys were generated in gnupg
without the idea module,
as then pgp will automatically pick a symmetric cipher common to both keys,
and not use idea

hth,

vedaal