appendixa.tex

早期freebsd实现

% appendix A

\appendix{A}{An MH Session}

\tagdiagram{A1-1}{Sending Encrypted Mail}{sendmail}
In the following,
the user \eg{Marshall\ T.\ Rose} logged onto host \eg{udel-dewey},
wishes to send a message to a user known as
the \eg{UCI\ Portal} (a system maintenance account).
As shown in Figure~\sendmail, line~1,
the user first establishes a mapping between the name \eg{UCI\ Portal} and
the address {\tx uci@udel-dewey}.
Once this mapping is performed,
it remains in effect until the user indicates otherwise to the \TMA/.
When the \pgm{tma} program is invoked,
it consults the \TMA/ database to see if that user is known.
If not,
it contacts the \KDS/ to ask for the \KDS/ ID associated with the user.
If the response is successful (in this case, the \KDS/ ID is \eg{3}),
then the \TMA/ updates its database.
The \pgm{tma} program indicates in its output the \KDS/ ID associated with
the user,
along with all known addresses (in this case, only one).
So, once the name to address mapping has been described the user,
the user agent, \MH/, deals only with the address,
while the trusted mail agent deals with the name and \KDS/ ID aspects of the
user.

Next, the \pgm{comp} program is invoked to compose a new draft on line~5.
The user addresses the local user \eg{uci} in the To: field,
and indicates that a plaintext copy should be kept in the folder \eg{+outbox}.
After entering the subject and text of the draft,
the user enters \whatnow/ level on line~13.
At this point,
the user directs \MH/ to send the draft in encrypted form.
The resulting output is verbose (a default for \pgm{send} for this user)
but instructive.
Initially,
all addresses in the draft are verified on lines~14 to~17.
Two forms of verification occur:
first, the \MTS/ is asked to verify the address as much as possible.
For local addresses,
the \MTS/ decides if the name has a maildrop associated with it.
For remote addresses,
the \MTS/ decides if the host is known to it.
The second type of verification occurs with the \TMA/.
For all addresses,
the \TMA/ is asked if it can find a mapping from the address to a \KDS/ ID.

The reason \MH/ goes to all this trouble is a philosophical issue.
Since the copy of the encrypted draft is different for each recipient,
\pgm{post} tries to verify that all recipients can be successfully posted
prior to actually posting the different ciphertext versions of the draft.
This behavior is not optimal in terms of cycles,
but is perhaps ``correct'' from a \UA/ perspective.

Finally, the draft is actually posted, and the folder carbon-copy is filed.

\tagdiagram{A1-2}{Receiving Encrypted Mail}{recvmail}
Some time later, the UCI portal is informed that new mail has arrived.
As shown in Figure~\recvmail,
the \pgm{inc} program is run.
The \eg{E} prior to the date of the message indicates that \pgm{inc} has
detected the message to be encrypted.
Since the user did not inhibit \pgm{inc} from deciphering the message,
it proceeds to do so.

\tagdiagram{A1-3}{Message Prior to Decryption}{before}
\tagdiagram{A1-4}{Message After Decryption}{after}
Finally, it may be instructive to see what the encrypted message looked like
when it was delivered to the portal's maildrop,
and the final message after deciphering.
Figures~\before\ and~\after\ show these respectively.
In particular,
note that the \eg{X-KDS-ID:} field has been introduced in Figure~\after\
after successfully deciphering the message.
The presence of this field authenticates the sender of the message.