Computer Security and Cryptography

4.3 GILBERT S. VERNAM

Gilbert S. Vernam was an engineer for The American Telephone and Telegraph Company. He was asked in 1917 to develop a teletypewriter to perform on-line encipherment/decipherment. Alphanumeric plaintext was first coded into 0's and 1's using the Baudot code², in which each character in a small alphabet is represented by a 5-bit sequence, as shown in Table 4.1. The key in Vernam's implementation of a rediscovered Vigenère polyalphabetic system was written on a paper tape as a sequence of five 0's and 1's and the Baudot-coded plaintext was XOR-ed with the key (Fig. 4.1). Vernam glued the ends of the paper tape into a loop, yielding additive encipherment with a periodic running key. Realizing that the strength of the encipherment would increase with the key length, Vernam combined several tapes with periods {r_i} (Fig. 4.2). If the periods are properly chosen, a key formed from a total of Σ_i r_i independently chosen key values could generate a key with period as large as R = Π_i r_i. Unfortunately, this way of making a large period R is not equivalent to a tape of length R [Tuckerman, 1970].

TABLE 4.1 Baudot Coding Table

Figure 4.1 Vernam's Teletypewriter Polyalphabetic Encipherment System (Courtesy of NSA).

Figure 4.2 Vernam's multitape polyalphabetic teletypewriter ...