While exhaustively testing all possible combinations of PEM modes, I came
across the following case for which the behaviour is not particularly
well-defined by the RFC: MIC-CLEAR with symmetric key management.
This combination is meaningful: it can be used when two users who share
a symmetric key want integrity but don't care about confidentiality or
non-repudiation.
The main problem is with the definition of the Key-Info field: it has to contain
both a DEK and a MIC, but with MIC-CLEAR there is no DEK. This can be kludged
by having the DEK zero length, like so:
Key-Info: DES-ECB,RSA-MD2,,6da84f3c23486f48fc15841a7fc06295
Is this a reasonable thing to do?
Mike