At PGP Corporation we are planning on adding a notation packet to newly
generated keys. It is intended to let key holders indicate whether they
can receive PGP/MIME email, or an alternate format used by PGP, or both.
Although PGP/MIME provides a powerful and flexible mechanism for
dealing with multiparts, some mail processing software does not lend
itself to automating PGP processing of MIME messages. As a result we
have implemented alternatives which use RFC2440 PGP message formats
(non-PGP/MIME). For non-multipart messages, this involves simply
processing the message body per RFC 2440. For multiparts, we process
each message part and attachment separately, applying the appropriate
cryptographic transformations for privacy and authenticity to each
part, while retaining the overall structure of the message.
The new notation packet will allow key holders to specify whether they
prefer to receive email in PGP/MIME format or in this other format,
which we call "partitioned". Here is the format of the notation packet.
The notation packet does not have the critical bit set.
The flags field of the notation packet will have the human-readable bit
set, which is 0x80 in the first octet of the four octet flags field.
The name field of the notation packet will be
"preferred-email-encoding(_at_)pgp(_dot_)com". This follows the RFC2440bis
convention for a "private" name in the namespace assocated with pgp.com.
The value field of the notation packet will be a single instance, or
a comma separated list, of the keywords: "pgpmime" and "partitioned".
If there is a single item, that is the format the key holder wants
to receive in; if there are multiple items, he can handle all (both)
formats and they are listed in order of preference.
An example preferred-email-encoding notation packet will have the
Flags: 0x80, 0x00, 0x00, 0x00
This would mean that the key holder can handle both PGP/MIME and
partitioned formats, but that he prefers to receive PGP/MIME.
If in the future a new PGP email format becomes popular then it is
possible that new keywords could appear in the value field. It is
recommended that software ignore any keywords which it does not recognize
and make its format choice based on the keywords that it understands.
I will be happy to answer any questions about this new notation packet.