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Network Working Group                                   Bob ThomasRequest for Comments: 644                               BBN-TENEX                                                        Jul 1974On the Problem of Signature Authentication for Network MailThis note describes the problem of signature authentication for networkmail, presents a general approach to the problem and proposes a specificimplementation of that approach.1. The Problem   The problem we wish to consider is:      How can the recipient of a network mail message be "certain"      that the signature (e.g., the name in the "FROM" field) is      authentic; that is, that the message is really from whom it      claims to be?We are interested in the problem of signature authenticity in the networkcontext.  For purposes of this note we shall assume a solution to thesignature authentication problem for local mail (i.e., messages from oneuser to another within a single host).  That is, we assume that for anyhost, either the host regards the problem as important and has a mechanismfor guaranteeing signatures on local mail or that the host does not regard theproblem as important and does not guarantee signature authentication.  Itshould become clear how this assumption relates to our approach to the networksignature problem.We shall discuss our approach using the following simple model for networkmail:      To send net mail a user invokes a mail sending process (SP) on      his local host (SH).  The process SP acts on behalf of the user      to deliver the message to an appropriate mailbox at the      receiving host (RH).  It does that by interacting with a      receiving process (RP) that runs on host RH.  RP accepts the      message from SP and deposits it in the appropriate mailbox.In the current implementation of network mail, the receiving process RP istypically an FTP server process.  For the current TENEX implementation themail sending process SP is either a process running SNDMSG or a "background"MAILER process which sends "queued" (previously posted but undelivered) mail.2.  An ApproachWe seek a solution which will allow RP, the receiving process, to markthe signature on messages it receives as authenticated or not withrespect to SH, the sending host.  If RP can so mark incoming messages,a user reading his mail at RH would be able to see the signature on eachmessage as authenticated or not with respect to the host of origin.  Theauthenticity of the signature on a piece of mail is understood to beresponsibility of the originating host.  The credibility a user gives aparticular message which is marked as authentic can be based on the user'sown estimate of the source host's user authentication and access controlmechanisms.                                     -1-

    The success of this approach depends upon two things:    a.  Users develop estimates of the security of various host user        authentication and access control mechanisms.  We have seen that        users who are concerned about data privacy and security are        already doing this within the ARPANET.    b.  The existence of a mechanism which RP, the receiving process,        can use to distinguish mail authenticated with respect to the        sending host from mail that has not been authenticated by the        sending host.  That is, a mechanism is required which will allow a        properly authorized (by the sending host) mail sending process to        identify itself as such to the mail receiving process.  The        receiving process can then mark mail from such an authenticated        process as authentic.  Nonauthorized processes (e.g., a user        process attempting to pose as an authorized mail sending process)        may try to send mail to mailboxes at RH; in such a case the        receiving process has the option of refusing to accept the message        or accepting them marking them as unauthenticated.3.  Proposed Implementation of ApproachThe use of passwords is one possible way to accomplish sending processauthentication.  Only an authorized sending process would know the passwordand thus be able to properly identify itself to a mail receiving process.We reject the password mechanism as operationally impractical for the followingreasons:    a.  Use of a password requires that the password be stored in        the sending program or be accessible to it in some way thereby        increasing the likelihood that the privacy of such a password will        be compromised.    b.  If a password is compromised, it must be changed at both        sending and receiving hosts; a synchronization problem.    c.  Truly secure mail would probably require passwords for each        pair of hosts; this requires N*N passwords for an N host network.As an alternative to the use of passwords as a means for processauthentication, we propose that authentication be based on thecommunication path itself between the sending and receiving process.In the ARPANET, a communication path is uniquely identified by its twoends: the send host-socket pair and the receive host-socket pair.  Aprocess can accurately determine the host-socket pair at the remote endof a communication path.  We propose that the receiving processconsider the sending process to be a properly authorized (by thesending host) sender of mail only if the sending end of thecommunication path is (one of) the socket(s) reserved for transmissionof authenticated mail.  The mail sending socket(s) would be reservedby prior host agreement.                                   -2-

The responsibility of the sending host is to allow only authorizedmail sending processes to access the mail sending socket(s).  Theresponsibility of the user concerned about the authenticity of hismail is to understand that mail marked as authentic means that thesending host has determined the identity of the sender and that thesignature on such mail is only as good or bad as the user authenticationand access control procedures of the sending host.4.  Additional Remarks    a.  The use of sockets for process authentication is not a new concept        within the ARPANET.  By host agreement, the TELNET logger process        responds to connections to socket #1, the FTP logger process to socket        #3, etc.  In fact, the privacy of net mail depends upon how well the        host controls access to the FTP logger socket; that is, the        authenticity of the mail receiving process is based upon that fact        that it is the process reached by ICP'ing to socket #3.  This note        proposes that the same mechanism be used to provide authentication of        mail sending processes.    b.  Planned TENEX Experiment        A set of sockets has been assigned for mail transmission.  They are       (all numbers are decimal)           ICP "from" socket - 232           FTP user command sockets:  receive, send = 234, 235           Default data transfer (user, send) socket = 237        We intend to modify the TENEX mail sending, receiving and reading        software as suggested above.  Mail sent by TENEX to remote hosts        which is authentic (with respect to TENEX) will be sent by initiating        the ICP to the remote FTP server socket 232.  Mail received from        remote hosts will be marked as authentic only if the ICP to the TENEX        FTP server was initiated from remote socket 232.  The TENEX mail        reading software will indicate for each message whether or not the        signature on the message was source authenticated.    c.  Contention for the Mail Sending Socket        Depending upon the implementation of the sending host's NCP and        its mail net sending software, it may be the case that several users        concurrently sending network mail may be competing for the single        ICP "from" socket.  If socket contention turns out to be a serious        problem in practice, a set of ICP "from" sockets could be reserved        for authenticated network mail.    d.  The local mail signature authentication problem is nearly independent        of the network mail signature authentication problem as we have        discussed it.  For example, the following observations can be made:                                     -3-

        1.  The local users of a host which does not authenticate local mail            probably should not expect the host to reliably deliver            authenticated network  mail to them.  Because local mail is not            authenticated, it is likely that a malicious local user could            add to other users' mail boxes forged messages which are formatted            identically to net mail and are marked as authentic in the way            the host's mail receiving process marks mail.        2. A host that has strong user authentication procedures and           authenticates local mail is not necessarily a reliable source           of authenticated network mail.  In order to be a reliable source,           it must limit access to the net mail transmission socket(s) to           authorized mail sending processes.       3.  A host which does not support local authentic mail could be a           reliable source of authentic net mail.                                   -4-