104 lines
2.9 KiB
Groff
104 lines
2.9 KiB
Groff
.TH forgeries 7
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.SH "NAME"
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forgeries \- how easy it is to forge mail
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.SH "SUMMARY"
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An electronic mail message can easily be forged.
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Almost everything in it,
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including the return address,
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is completely under the control of the sender.
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An electronic mail message can be manually traced to its origin
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if (1) all system administrators of intermediate machines
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are both cooperative and competent,
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(2) the sender did not break low-level TCP/IP security,
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and
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(3) all intermediate machines are secure.
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Users of
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.I cryptography
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can automatically ensure the integrity and secrecy
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of their mail messages, as long as
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the sending and receiving machines are secure.
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.SH "FORGERIES"
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Like postal mail,
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electronic mail can be created entirely at the whim of the sender.
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.BR From ,
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.BR Sender ,
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.BR Return-Path ,
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and
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.BR Message-ID
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can all contain whatever information the sender wants.
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For example, if you inject a message through
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.B sendmail
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or
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.B qmail-inject
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or
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.BR SMTP ,
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you can simply type in a
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.B From
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field.
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In fact,
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.B qmail-inject
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lets you set up
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.BR MAILUSER ,
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.BR MAILHOST ,
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and
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.B MAILNAME
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environment variables
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to produce your desired
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.B From
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field on every message.
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.SH "TRACING FORGERIES"
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Like postal mail,
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electronic mail is postmarked when it is sent.
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Each machine that receives an electronic mail message
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adds a
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.B Received
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line to the top.
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A modern
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.B Received
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line contains quite a bit of information.
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In conjunction with the machine's logs,
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it lets a competent system administrator
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determine where the machine received the message from,
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as long as the sender did not break low-level TCP/IP security
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or security on that machine.
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Large multi-user machines often come with inadequate logging software.
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Fortunately, a system administrator can easily obtain a copy of a
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931/1413/Ident/TAP server, such as
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.BR pidentd .
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Unfortunately,
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many incompetent system administrators fail to do this,
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and are thus unable to figure out which local user
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was responsible for generating a message.
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If all intermediate system administrators are competent,
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and the sender did not break machine security or low-level TCP/IP security,
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it is possible to trace a message backwards.
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Unfortunately, some traces are stymied by intermediate system
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administrators who are uncooperative or untrustworthy.
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.SH "CRYPTOGRAPHY"
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The sender of a mail message may place his message into a
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.I cryptographic
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envelope stamped with his seal.
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Strong cryptography guarantees that any two messages with the same seal
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were sent by the same cryptographic entity:
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perhaps a single person, perhaps a group of cooperating people,
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but in any case somebody who knows a secret originally held
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only by the creator of the seal.
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The seal is called a
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.I public key\fR.
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Unfortunately, the creator of the seal is often an insecure machine,
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or an untrustworthy central agency,
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but most of the time seals are kept secure.
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One popular cryptographic program is
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.BR pgp .
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.SH "SEE ALSO"
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pgp(1),
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identd(8),
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qmail-header(8)
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