Subject: PKININT: A signature using a symmetric key?

From: "Geoffrey Elgey" <[email-masked]>
Date: Thu, 7 Jan 1999 10:20:53 -0000

G'day,

Another PKINIT question...

On section 3.4 of the latest draft (pk-init-07) discusses how a user may
retrieve a private key from the KDC. It is mentioned that there is a
symmetric key K2 shared between the user and the KDC, and that this key is
used to sign the PKAuthenticator, as follows:

SignedPKAuth ::= SEQUENCE {
        pkAuth                  [0] PKAuthenticator,
        pkAuthSig               [1] Signature
                                    -- of pkAuth
                                    -- using the symmetric key K2
}

while a Signature is defined in section 3.2 as follows:

Signature ::= SEQUENCE {
        signatureAlgorithm      [0] SignatureAlgorithmIdentifier,
        pkcsSignature           [1] BIT STRING
                                    -- octet-aligned big-endian bit
                                    -- string (encrypted with signer's
                                    -- private key)
}

I'm a bit confused about the concept of using the terms 'signature' and
'symmetric key' together - is the 'Signature' structure above meant to be a
generalized coupling of encrypted data and encryption algorithm, using some
known key (symmetric or private/public), or is it meant as a verifiable
signature in the normal sense (ie can only be generated by owner of the
private key, but can be verified by anyone with the public key) ?

Perhaps a different term than 'Signature' should be used to avoid ambiguity
here (ie MAC)? Or another structure that contains an AlgorithmIdentifier and
bit string, albiet more generalized than a signature?

I understand the intended effect of the SignedPKAuth structure - basically
to provide a hash of the authenticator (using a derivable symmetric key) for
verification by the KDC. However the naming ambiguity wreaks havoc on our
PKINIT implementation, as our Signature classes require PublicKey &
PrivateKey class instances, so creating a signature using a symmetric key
stuffs that up...

Any thoughts/corrections appreciated.

Cheers,
Geoff

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