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          forgeries - how easy it is to forge mail

          An electronic mail message can easily be forged.  Almost
          everything in it, including the return address, is
          completely under the control of the sender.

          An electronic mail message can be manually traced to its
          origin if (1) all system administrators of intermediate
          machines are both cooperative and competent, (2) the sender
          did not break low-level TCP/IP security, and (3) all
          intermediate machines are secure.

          Users of cryptography can automatically ensure the integrity
          and secrecy of their mail messages, as long as the sending
          and receiving machines are secure.

          Like postal mail, electronic mail can be created entirely at
          the whim of the sender.  From, Sender, Return-Path, and
          Message-ID can all contain whatever information the sender

          For example, if you inject a message through sendmail or
          qmail-inject or SMTP, you can simply type in a From field.
          In fact, qmail-inject lets you set up MAILUSER, MAILHOST,
          and MAILNAME environment variables to produce your desired
          From field on every message.

          Like postal mail, electronic mail is postmarked when it is
          sent.  Each machine that receives an electronic mail message
          adds a Received line to the top.

          A modern Received line contains quite a bit of information.
          In conjunction with the machine's logs, it lets a competent
          system administrator determine where the machine received
          the message from, as long as the sender did not break low-
          level TCP/IP security or security on that machine.

          Large multi-user machines often come with inadequate logging
          software.  Fortunately, a system administrator can easily
          obtain a copy of a 931/1413/Ident/TAP server, such as
          pidentd.  Unfortunately, some system administrators fail to
          do this, and are thus unable to figure out which local user
          was responsible for generating a message.

          If all intermediate system administrators are competent, and
          the sender did not break machine security or low-level
          TCP/IP security, it is possible to trace a message
          backwards.  Unfortunately, some traces are stymied by
          intermediate system administrators who are uncooperative or

          The sender of a mail message may place his message into a
          cryptographic envelope stamped with his seal.  Strong
          cryptography guarantees that any two messages with the same
          seal were sent by the same cryptographic entity:  perhaps a
          single person, perhaps a group of cooperating people, but in
          any case somebody who knows a secret originally held only by
          the creator of the seal.  The seal is called a public key.

          Unfortunately, the creator of the seal is often an insecure
          machine, or an untrustworthy central agency, but most of the
          time seals are kept secure.

          One popular cryptographic program is pgp.

          pgp(1), identd(8), qmail-header(8)

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