-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 4,MIC-CLEAR
Content-Domain: RFC822
Originator-ID-Asymmetric: MFMxCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJNRDE
kMCIGA1UEChMbVHJ1c3RlZCBJbmZvcm1hdGlvbiBTeXN0ZW1zMREwDwYDVQQLEwh
HbGVud29vZA==,03
MIC-Info: RSA-MD5,RSA,ZvYI9yeICrn3g+LSK4KCUG0CS5hP1FDubFaiq86wMiU
40ai2exB4kPKvh1MbB0BDetCwWv/0XkbSAWEn23xEm8BKb2i9g60E8omm8Fy0D/N
mZIaeIXY3ktY11JnF0A4R
When TIS/PEM is invoked under MH, it "de-enhances" a PEM message that
is (usually) in the user's inbox and puts the result back into the
user's inbox. If that message has a PEM body within it, the user can
invoke PEM processing again.
In the integration of MIME and PEM we're working on, one of the modes
will be to save the name of the sender and other information, e.g. how
and when the signature was checked, as a separate but related body
part. This could be done recursively, but it's messy if it's desired
that annotations be recomputable and that signatures apply to
uninterpreted body parts.
Let S(u,x) stand for a message with text x signed by user u.
Let A(u,x,t) stand for a de-enhanced message with information
t generated by the "de-enhancement" process, e.g. the
distinguished name of the sender, time stamp information, etc.
An implicit constraint is that t is completely redundant. At
any time, t can be discarded and regenerated.
Suppose the user receives S(u1,S(u2,m)). If only one step is
applied, the result is A(u1,S(u2,m),t1). This is fine.
However, if we also dig into nested messages, we probably want
something like A(u1,A(u2,m,t2),t1). However, the annotation
t1 properly applies to S(u2,m), not to A(u2,m,t2), so
A(u1,A(u2,m,t2),t1) is not correct.
One method of recursively processing a message which keeps annotations
and leaves the original message intact requires mirroring the message
structure in an auxiliary structure. This requires at least some
replication of information, and may result in exponential increase in
message length. This approach would likely make the current PEM, MIME
and X.400 formats look efficient.
Steve
-----END PRIVACY-ENHANCED MESSAGE-----