3 forgeries \- how easy it is to forge mail
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.
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,
15 (3) all intermediate machines are secure.
19 can automatically ensure the integrity and secrecy
20 of their mail messages, as long as
21 the sending and receiving machines are secure.
24 electronic mail can be created entirely at the whim of the sender.
30 can all contain whatever information the sender wants.
32 For example, if you inject a message through
38 you can simply type in a
49 to produce your desired
51 field on every message.
52 .SH "TRACING FORGERIES"
54 electronic mail is postmarked when it is sent.
55 Each machine that receives an electronic mail message
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.
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
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.
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.
84 The sender of a mail message may place his message into a
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.
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.
99 One popular cryptographic program is