Subject: Key Algorithms?

From: Ben Laurie <[email-masked]>
Date: Thu, 19 May 2005 21:37:20 +0100

Key algorithms ... these are used in various contexts, and there's a
list in 9.1 - some of these are clearly unsuitable in some contexts -
for example, one would not expect to see RSA Encrypt-Only (3) in a
signature. But I can't find any language saying anything about
this. Are there any rules?

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From: Jon Callas <[email-masked]>
Date: Fri, 20 May 2005 15:32:33 -0700

> this. Are there any rules?

All of these are deprecated or disallowed.

12.4. RSA

    There are algorithm types for RSA-signature-only, and
    RSA-encrypt-only keys. These types are deprecated. The "key flags"
    subpacket in a signature is a much better way to express the same
    idea, and generalizes it to all algorithms. An implementation SHOULD
    NOT create such a key, but MAY interpret it.

[...]

12.7. Reserved Algorithm Numbers

    A number of algorithm IDs have been reserved for algorithms that
    would be useful to use in an OpenPGP implementation, yet there are
    issues that prevent an implementer from actually implementing the
    algorithm. These are marked in the Public Algorithms section as
    "(reserved for)".

[...]

    Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
    with a public key identifier of 20. These are no longer permitted.
    An implementation MUST NOT generate such keys. An implementation
    MUST NOT generate Elgamal signatures.