No information follows the unique-id in the unique-id listing.

          The unique-id of a message is an arbitrary server-determined
          string, consisting of characters in the range 0x21 to 0x7E,
          which uniquely identifies a message within a maildrop and
          which persists across sessions. The server should never reuse
          an unique-id in a given maildrop, for as long as the entity
          using the unique-id exists.

          Note that messages marked as deleted are not listed.

      Possible Responses:
          +OK unique-id listing follows
          -ERR no such message

      Examples:
          C: UIDL
          S: +OK
          S: 1 whqtswO00WBw418f9t5JxYwZ
          S: 2 QhdPYR:00WBw1Ph7x7
          S: .
             ...
          C: UIDL 2
          S: +OK 2 QhdPYR:00WBw1Ph7x7
             ...
          C: UIDL 3
          S: -ERR no such message, only 2 messages in maildrop

      APOP name digest

         Arguments:
             a string identifying a mailbox and a MD5 digest string
             (both required)

         Restrictions:
             may only be given in the AUTHORIZATION state after the POP3
             greeting

         Discussion:
             Normally, each POP3 session starts with a USER/PASS
             exchange.  This results in a server/user-id specific
             password being sent in the clear on the network.  For
             intermittent use of POP3, this may not introduce a sizable
             risk.  However, many POP3 client implementations connect to
             the POP3 server on a regular basis -- to check for new
             mail.  Further the interval of session initiation may be on
             the order of five minutes.  Hence, the risk of password
             capture is greatly enhanced.



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             An alternate method of authentication is required which
             provides for both origin authentication and replay
             protection, but which does not involve sending a password
             in the clear over the network.  The APOP command provides
             this functionality.

             A POP3 server which implements the APOP command will
             include a timestamp in its banner greeting.  The syntax of
             the timestamp corresponds to the `msg-id' in [RFC822], and
             MUST be different each time the POP3 server issues a banner
             greeting.  For example, on a UNIX implementation in which a
             separate UNIX process is used for each instance of a POP3
             server, the syntax of the timestamp might be:

                <process-ID.clock@hostname>

             where `process-ID' is the decimal value of the process's
             PID, clock is the decimal value of the system clock, and
             hostname is the fully-qualified domain-name corresponding
             to the host where the POP3 server is running.

             The POP3 client makes note of this timestamp, and then
             issues the APOP command.  The `name' parameter has
             identical semantics to the `name' parameter of the USER
             command. The `digest' parameter is calculated by applying
             the MD5 algorithm [RFC1321] to a string consisting of the
             timestamp (including angle-brackets) followed by a shared
             secret.  This shared secret is a string known only to the
             POP3 client and server.  Great care should be taken to
             prevent unauthorized disclosure of the secret, as knowledge
             of the secret will allow any entity to successfully
             masquerade as the named user.  The `digest' parameter
             itself is a 16-octet value which is sent in hexadecimal
             format, using lower-case ASCII characters.

             When the POP3 server receives the APOP command, it verifies
             the digest provided.  If the digest is correct, the POP3
             server issues a positive response, and the POP3 session
             enters the TRANSACTION state.  Otherwise, a negative
             response is issued and the POP3 session remains in the
             AUTHORIZATION state.

             Note that as the length of the shared secret increases, so
             does the difficulty of deriving it.  As such, shared
             secrets should be long strings (considerably longer than
             the 8-character example shown below).





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RFC 1725                          POP3                     November 1994


         Possible Responses:
             +OK maildrop locked and ready
             -ERR permission denied

         Examples:
             S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
             C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
             S: +OK maildrop has 1 message (369 octets)

             In this example, the shared  secret  is  the  string  `tan-
             staaf'.  Hence, the MD5 algorithm is applied to the string

                <1896.697170952@dbc.mtview.ca.us>tanstaaf

             which produces a digest value of

                c4c9334bac560ecc979e58001b3e22fb

8. POP3 Command Summary

   Minimal POP3 Commands:

      USER name               valid in the AUTHORIZATION state
      PASS string
      QUIT

      STAT                    valid in the TRANSACTION state
      LIST [msg]
      RETR msg
      DELE msg
      NOOP
      RSET

      QUIT                    valid in the UPDATE state

   Optional POP3 Commands:

      APOP name digest        valid in the AUTHORIZATION state

      TOP msg n               valid in the TRANSACTION state
      UIDL [msg]

   POP3 Replies:

      +OK
      -ERR





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RFC 1725                          POP3                     November 1994


   Note that with the exception of the STAT, LIST, and UIDL commands,
   the reply given by the POP3 server to any command is significant only
   to "+OK" and "-ERR".  Any text occurring after this reply may be
   ignored by the client.

9. Example POP3 Session

   S: <wait for connection on TCP port 110>
   C: <open connection>
   S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
   C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
   S:    +OK mrose's maildrop has 2 messages (320 octets)
   C:    STAT
   S:    +OK 2 320
   C:    LIST
   S:    +OK 2 messages (320 octets)
   S:    1 120
   S:    2 200
   S:    .
   C:    RETR 1
   S:    +OK 120 octets
   S:    <the POP3 server sends message 1>
   S:    .
   C:    DELE 1
   S:    +OK message 1 deleted
   C:    RETR 2
   S:    +OK 200 octets
   S:    <the POP3 server sends message 2>
   S:    .
   C:    DELE 2
   S:    +OK message 2 deleted
   C:    QUIT
   S:    +OK dewey POP3 server signing off (maildrop empty)
   C:  <close connection>
   S:  <wait for next connection>

10. Message Format

   All messages transmitted during a POP3 session are assumed to conform
   to the standard for the format of Internet text messages [RFC822].

   It is important to note that the octet count for a message on the
   server host may differ from the octet count assigned to that message
   due to local conventions for designating end-of-line.  Usually,
   during the AUTHORIZATION state of the POP3 session, the POP3 server
   can calculate the size of each message in octets when it opens the
   maildrop.  For example, if the POP3 server host internally represents
   end-of-line as a single character, then the POP3 server simply counts



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RFC 1725                          POP3                     November 1994


   each occurrence of this character in a message as two octets.  Note
   that lines in the message which start with the termination octet need
   not be counted twice, since the POP3 client will remove all byte-
   stuffed termination characters when it receives a multi-line
   response.

11. References

   [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
       821, USC/Information Sciences Institute, August 1982.

   [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet Text
       Messages", STD 11, RFC 822, University of Delaware, August 1982.

   [RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", RFC 1321,
       MIT Laboratory for Computer Science, April, 1992.

12. Security Considerations

   It is conjectured that use of the APOP command provides origin
   identification and replay protection for a POP3 session.
   Accordingly, a POP3 server which implements both the PASS and APOP
   commands must not allow both methods of access for a given user; that
   is, for a given "USER name" either the PASS or APOP command is
   allowed, but not both.

   Further, note that as the length of the shared secret increases, so
   does the difficulty of deriving it.

   Servers that answer -ERR to the USER command are giving potential
   attackers clues about which names are valid

   Use of the PASS command sends passwords in the clear over the
   network.

   Use of the RETR and TOP commands sends mail in the clear over the
   network.

   Otherwise, security issues are not discussed in this memo.

13. Acknowledgements

   The POP family has a long and checkered history.  Although primarily
   a minor revision to RFC 1460, POP3 is based on the ideas presented in
   RFCs 918, 937, and 1081.

   In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
   provided significant comments on the APOP command.



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RFC 1725                          POP3                     November 1994


14. Authors' Addresses

   John G. Myers
   Carnegie-Mellon University
   5000 Forbes Ave
   Pittsburgh, PA 15213

   EMail: jgm+@cmu.edu


   Marshall T. Rose
   Dover Beach Consulting, Inc.
   420 Whisman Court
   Mountain View, CA  94043-2186

   EMail: mrose@dbc.mtview.ca.us



































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