PEM provides authenticated identities for message recipients and originators expressed in the form of distinguished names. Mail systems in which PEM is employed may employ identifiers other than DNs as the primary means of identifying recipients or originators. Thus, in order to benefit from these authentication facilities, each PEM implementation must employ some means of binding native mail system identifiers to distinguished names in a fashion which does not undermine this basic PEM functionality.
For example, if a human user interacts directly with PEM, then the full DN of the originator of any message received using PEM should be displayed for the user. Merely displaying the PEM-protected message content, containing an originator name from the native mail system, does not provide equivalent security functionality and could allow spoofing. If the recipient of a message is a forwarding agent such as a list exploder or mail relay, display of the originator's DN is not a relevant requirement. In all cases the essential requirement is that the ultimate recipient of a PEM message be able to ascertain the identity of the originator based on the PEM certification system, not on unauthenticated identification information, e.g., extracted from the native message system.
Conversely, for the originator of an ENCRYPTED message, it is important that recipient identities be linked to the DNs as expressed in PEM certificates. This can be effected in a variety of ways by the PEM implementation, e.g., by display of recipient DNs upon message submission or by a tightly controlled binding between local aliases and the DNs. Here too, if the originator is a forwarding process this linkage might be effected via various mechanisms not applicable to direct human interaction. Again, the essential requirement is to avoid procedures which might undermine the authentication services provided by PEM.
As described above, it is a local matter how and what certification information is displayed for a human user in the course of submission or delivery of a PEM message. Nonetheless all PEM implementations must provide a user with the ability to display a full certification path for any certificate employed in PEM upon demand. Implementors are urged to not overwhelm the user with certification path information which might confuse him or distract him from the critical information cited above.