forgeries(7) forgeries(7) NNAAMMEE forgeries - how easy it is to forge mail SSUUMMMMAARRYY 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 _c_r_y_p_t_o_g_r_a_p_h_y can automatically ensure the integrity and secrecy of their mail messages, as long as the sending and receiving machines are secure. FFOORRGGEERRIIEESS Like postal mail, electronic mail can be created entirely at the whim of the sender. FFrroomm, SSeennddeerr, RReettuurrnn--PPaatthh, and MMeessssaaggee--IIDD can all con- tain whatever information the sender wants. For example, if you inject a message through sseennddmmaaiill or qqmmaaiill--iinnjjeecctt or SSMMTTPP, you can simply type in a FFrroomm field. In fact, qqmmaaiill--iinnjjeecctt lets you set up MMAAIILLUUSSEERR, MMAAIILLHHOOSSTT, and MMAAIILLNNAAMMEE environment variables to produce your desired FFrroomm field on every message. TTRRAACCIINNGG FFOORRGGEERRIIEESS Like postal mail, electronic mail is postmarked when it is sent. Each machine that receives an electronic mail message adds a RReecceeiivveedd line to the top. A modern RReecceeiivveedd line contains quite a bit of information. In con- junction with the machine’s logs, it lets a competent system adminis- trator 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 ppiiddeennttdd. 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 pos- sible to trace a message backwards. Unfortunately, some traces are stymied by intermediate system administrators who are uncooperative or untrustworthy. CCRRYYPPTTOOGGRRAAPPHHYY The sender of a mail message may place his message into a _c_r_y_p_t_o_g_r_a_p_h_i_c 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 _p_u_b_l_i_c _k_e_y. 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 ppggpp. SSEEEE AALLSSOO pgp(1), identd(8), qmail-header(8) forgeries(7)