On 10 Nov 1992 16:00:12 -0700 (PDT), Ned Freed
<NED(_at_)SIGURD(_dot_)INNOSOFT(_dot_)COM> said:
Ned> Most MIME UAs use Content-Type information to decide how to
Ned> display messages. The most general of them use a table that
Ned> specifies an appropriate filter to invoke. In the case of
Ned> message/pem, the filter would be some kind of PEM facility that
Ned> reads the PEM information.
Ned> In other words, the presence of this headers makes it trivial to
Ned> integrate PEM facilties into MIME UAs.
I agree that having a message/pem Content-Type would be highly
valuable. However it will not be trivial to provide it in a way that
preserves interoperability across gateways.
Gateways may perform a variety of transformations on plain text
messages, such as character code translation, whitespace
removal/padding, etc. When a message passes through a gateway, the
gateway can perform any necessary transformations on the encrypted
representation of a message, but not on the the underlying plain text
message.
The inability of gateways to manipulate the underlying plain text of a
message may result in the message being difficult or impossible to read
after being decrypted. A solution to this problem would be for
gateways to add a parameter to the Content-Type line to indicate the
transformation the gateway would have performed if the message had not
been encrypted. If the UA is then able to perform all of the
transformation that the gateway(s) were not able to perform, then the
message will be as readable as it would have been if it had been sent
as plain text.