1 .TH forgeries 7

     2 .SH "NAME"

     3 forgeries \- how easy it is to forge mail

     4 .SH "SUMMARY"

     5 An electronic mail message can easily be forged.

     6 Almost everything in it,

     7 including the return address,

     8 is completely under the control of the sender.

     9 

    10 An electronic mail message can be manually traced to its origin

    11 if (1) all system administrators of intermediate machines

    12 are both cooperative and competent,

    13 (2) the sender did not break low-level TCP/IP security,

    14 and

    15 (3) all intermediate machines are secure.

    16 

    17 Users of

    18 .I cryptography

    19 can automatically ensure the integrity and secrecy

    20 of their mail messages, as long as

    21 the sending and receiving machines are secure.

    22 .SH "FORGERIES"

    23 Like postal mail,

    24 electronic mail can be created entirely at the whim of the sender.

    25 .BR From ,

    26 .BR Sender ,

    27 .BR Return-Path ,

    28 and

    29 .BR Message-ID

    30 can all contain whatever information the sender wants.

    31 

    32 For example, if you inject a message through

    33 .B sendmail

    34 or

    35 .B qmail-inject

    36 or

    37 .BR SMTP ,

    38 you can simply type in a

    39 .B From

    40 field.

    41 In fact,

    42 .B qmail-inject

    43 lets you set up

    44 .BR MAILUSER ,

    45 .BR MAILHOST ,

    46 and

    47 .B MAILNAME

    48 environment variables

    49 to produce your desired

    50 .B From

    51 field on every message.

    52 .SH "TRACING FORGERIES"

    53 Like postal mail,

    54 electronic mail is postmarked when it is sent.

    55 Each machine that receives an electronic mail message

    56 adds a

    57 .B Received

    58 line to the top.

    59 

    60 A modern

    61 .B Received

    62 line contains quite a bit of information.

    63 In conjunction with the machine's logs,

    64 it lets a competent system administrator

    65 determine where the machine received the message from,

    66 as long as the sender did not break low-level TCP/IP security

    67 or security on that machine.

    68 

    69 Large multi-user machines often come with inadequate logging software.

    70 Fortunately, a system administrator can easily obtain a copy of a

    71 931/1413/Ident/TAP server, such as

    72 .BR pidentd .

    73 Unfortunately,

    74 some system administrators fail to do this,

    75 and are thus unable to figure out which local user

    76 was responsible for generating a message.

    77 

    78 If all intermediate system administrators are competent,

    79 and the sender did not break machine security or low-level TCP/IP security,

    80 it is possible to trace a message backwards.

    81 Unfortunately, some traces are stymied by intermediate system

    82 administrators who are uncooperative or untrustworthy.

    83 .SH "CRYPTOGRAPHY"

    84 The sender of a mail message may place his message into a

    85 .I cryptographic

    86 envelope stamped with his seal.

    87 Strong cryptography guarantees that any two messages with the same seal

    88 were sent by the same cryptographic entity:

    89 perhaps a single person, perhaps a group of cooperating people,

    90 but in any case somebody who knows a secret originally held

    91 only by the creator of the seal.

    92 The seal is called a

    93 .I public key\fR.

    94 

    95 Unfortunately, the creator of the seal is often an insecure machine,

    96 or an untrustworthy central agency,

    97 but most of the time seals are kept secure.

    98 

    99 One popular cryptographic program is

   100 .BR pgp .

   101 .SH "SEE ALSO"

   102 pgp(1),

   103 identd(8),

   104 qmail-header(8)