Peter,
The text in 4.6.2.1 states that the primary example of needing
to replicate Originator-ID fields is for mixed symmetric/asymmetric
key management circumstances. When we first developed the PEM specs,
this was the motivation for that provision. However, as we expanded
PEM to accommodate the possibility of other public key algorithms, we
realized that sending to a set of recipients using different public
key signature algorithms also provides a rationale for this facility.
The text encompasses this latter requirement for multiple
Originator-ID fields (and matching MIC-Info fields), though it does
not explicitly cite it as an example.
My point to Sandy, and to Ned when I respond his message, is
that this is the only allowed use for multiple signatures on a message
and it is a well-defined in the specification. The argument, from my
perspective, was over whether PEM provided a general facility for
multiple signatures, specifically applied by different
originators. The answer to that question is no.
Steve