In substitution cipher, these original text-letters are replaced with substitutes, or cipher-symbols, and these symbols are arranged in the same order as their originals. There may, of course, be combinations of types, or combinations of several forms belonging to a single type.
The aristocrat of the cipher family is code. This is a form of the substitution cipher which requires the preparation, in advance, of a code book. A series of terms likely to be used in future correspondence (that is, words, phrases, and even sentences) is first gathered into a vocabulary, or “dictionary”; and beside each of these terms is placed a substitute known as a code group, or code word. These substitutes may be groups of letters, or groups of digits, or actual words selected from ordinary language. Very common words or expressions are usually provided with more than one substitute; and nearly always there are substitutes provided for syllables and single letters, so as to take care of all words not originally included in the vocabulary.
No code presents any real security unless the code symbols have been assigned in a thoroughly haphazard manner. This means that any really good code would have to be printed in two separate sections. In one of these, the vocabulary terms would be arranged in alphabetical order, so that they could be readily found when enciphering (encoding) messages; but the code groups would be in mixed order and hard to find. In the other section, the code groups would be rearranged in straight alphabetical (or numerical) order, so as to be readily found when deciphering (decoding), and the vocabulary terms would be in mixed order. Just what is meant can be seen in Fig. 1, showing fragments from an imaginary code book.
| Figure 1 ENCIPHERMENT SECTION DECIPHERMENT SECTION Vocabulary Term Code Symbol Code Symbol Vocabulary Term A 9001, 2114, 3000* 1120 Assenting to your Aachen 8463 1121 Horse About 1119, 0034* 1122 Meet me About time for 5434 1123* Come; Paris Armored car 1125 1124 Th- Assenting to your 1120 1125 Armored car *) When a plaintext term has more than one symbol, these are called homophones. Polyphones are symbols which may have more than one meaning. The terms encoding, decoding are usually preferred to enciphering, deciphering. |
A code of this kind, with symbols assigned absolutely at random, provided it is carefully used (never without re-encipherment) and a close guard kept over the code books, represents perhaps the maximum of security to be attained in cryptographic correspondence; and security, of course, is of prime importance in the selection of a cipher for any practical purpose.
But in considering the relative merits of the various ciphers, it is always necessary to take into account many factors other than security, each cipher being evaluated in connection with the purpose for which it is wanted: Under what conditions must the encipherment and decipherment take place? How must the cryptograms be transmitted? How much of the enciphered correspondence is likely to be intercepted? What degree of security, after all, is absolutely imperative?
A commercial, or other, firm, having a permanent base of operations, and in little danger of being blown to bits by an enemy shell, would not consider the first of these questions from the same angle as the War Department, and the War Department, though considering all of them from several different angles of its own, would still not consider them from the same viewpoint as the State Department.
If messages are to be sent by mail, or by hand, or by telephone, or pasted on a billboard, it is conceivable that a cipher which doubles or trebles their length could still be a practical cipher. For transmission by telephone, the presumption is that the cryptogram must be pronounceable, or, certainly, audible. For written communication, individual purposes have been served by means of pictures.
But when the cryptograms are to be sent by wire or radio, it must be possible to convert them into Morse symbols, either letters or figures, but not intermingled letters and figures. Here, length must be considered, involving questions of time, expense, and the current telegraphic regulations. Moreover, it is conceded that a meaningless text will not be transmitted with absolute accuracy, and a cryptogram which is to be sent by this means must not be of such a nature that ordinary errors of transmission will render it unintelligible at the receiving office.
A factor of particularly grave importance in the selection of a cipher to fit a given purpose is the probable amount of enciphered material which is going to fall into the possession of unauthorized persons. A criminal, who has had to send but one brief cryptogram in a lifetime, might reasonably expect that it will remain forever unread, no matter how weak the cipher. A commercial firm, transmitting thousands of words over the air, is more vulnerable; and the diplomatic office, or the newspaper office, which makes the mistake of publishing almost verbatim the translations of cryptograms which have been transmitted by radio, and thus has surely furnished the cipher expert with a cryptogram and its translation, might just as well have presented him with a copy of its code book.