*Proof of submission* allows a message originator to obtain proof that
its MTA submitted a message for delivery to the intended recipient(s). This
service counters the threat of masquerade (i.e., impersonation of an MTA to
an MTS user). It is provided on a per-message basis using symmetric or
asymmetric encryption techniques.

The message originator requests the service by submitting a *proof of
submission* request with a message. The originator's MTA returns the proof
in an acknowledgment. The proof is computed as a function of
the submitted message arguments (i.e., the submitted message without the
content), the message identifier (which is added by the MTA), and the
time the message was submitted.

To generate the proof using an asymmetric encryption algorithm, the MTA signs
the acknowledgment using its private key. The message originator
validates the signature using the MTA's public key certificate. This
certificate may be registered with the originator, transferred in the
acknowledgment, or obtained by some other means. An asymmetric *proof
of submission* also provides *non-repudiation of submission* (see
sec. 11.6.6).

If the message originator transferred a symmetric encryption key to the MTA
during the authentication process (see sec. 11.6.2), the MTA can
compute the *proof of submission* using this key. A symmetric *proof
of submission* does not provide *non-repudiation of submission*.

Fri Oct 7 16:17:21 EDT 1994