Communication Theory of Secrecy Systems - Network Research Lab

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two information sources—a message source and a key source. The key source produces a particular key from among those which are possible in the system. This key is transmitted by some means, supposedly not interceptible, for example by messenger, to the receiving end. The message source produces a message (the “clear”) which is enciphered and the resulting cryptogram sent to the receiving end by a possibly interceptible means, for example radio. At the receiving end the cryptogram and key are combined in the decipherer to recover the message. MESSAGE SOURCE MESSAGE ENCIPHERER CRYPTOGRAM M TK E KEY K KEY SOURCE ENEMY CRYPTANALYST KEY K Fig. 1. Schematic of a general secrecy system E E DECIPHERER T−1 K MESSAGE M Evidently the encipherer performs a functional operation. If M is the message, K the key, and E the enciphered message, or cryptogram, we have E = f(M, K) that is E is function of M and K. It is preferable to think of this, however, not as a function of two variables but as a (one parameter) family of operations or transformations, and to write it E = TiM. The transformation Ti applied to message M produces cryptogram E. The index i corresponds to the particular key being used. We will assume, in general, that there are only a finite number of possible keys, and that each has an associated probability pi. Thus the key source is represented by a statistical process or device which chooses one from the set of transformations T1, T2, · · ·, Tm with the respective probabilities p1, p2, · · ·, pm. Similarly we will generally assume a finite number of possible messages M1, M2, · · ·, Mn with associate a priori probabilities q1, q2, · · ·, qn. The possible messages, for example, might be the possible sequences of English letters all of length N, and the associated probabilities are then the relative frequencies of occurrence of these sequences in normal English text. 661
At the receiving end it must be possible to recover M, knowing E and K. Thus the transformations Ti in the family must have unique inverses T −1 i such that TiT −1 i = I, the identity transformation. Thus: M = T −1 i E. At any rate this inverse must exist uniquely for every E which can be obtained from an M with key i. Hence we arrive at the definition: A secrecy system is a family of uniquely reversible transformations Ti of a set of possible messages into a set of cryptograms, the transformation Ti having an associated probability pi. Conversely any set of entities of this type will be called a “secrecy system”. The set of possible messages will be called, for convenience, the “message space” and the set of possible cryptograms the “cryptogram space”. Two secrecy systems will be the same if they consist of the same set of transformations Ti, with the same messages and cryptogram space (range and domain) and the same probabilities for the keys. A secrecy system can be visualized mechanically as a machine with one or more controls on it. A sequence of letters, the message, is fed into the input of the machine and a second series emerges at the output. The particular setting of the controls corresponds to the particular key being used. Some statistical method must be prescribed for choosing the key from all the possible ones. To make the problem mathematically tractable we shall assume that the enemy knows the system being used. That is, he knows the family of transformations Ti, and the probabilities of choosing various keys. It might be objected that this assumption is unrealistic, in that the cryptanalyst often does not know what system was used or the probabilities in question. There are two answers to this objection: 1. The restriction is much weaker than appears at first, due to our broad definition of what constitutes a secrecy system. Suppose a cryptographer intercepts a message and does not know whether a substitution transposition, or Vigenère type cipher was used. He can consider the message as being enciphered by a system in which part of the key is the specification of which of these types was used, the next part being the particular key for that type. These three different possibilities are assigned probabilities according to his best estimates of the a priori probabilities of the encipherer using the respective types of cipher. 2. The assumption is actually the one ordinary used in cryptographic studies. It is pessimistic and hence safe, but in the long run realistic, since one must expect his system to be found out eventually. Thus, even when an entirely new system is devised, so that the enemy cannot assign any a 662
Page 1 and 2: Communication Theory of Secrecy Sys
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Page 9 and 10: is that the decipherer knows the pa
Page 11 and 12: 4.2 Transposition (Fixed Period d)
Page 13 and 14: The matrix aij is the key, and deci
Page 15 and 16: 5.2 Size of Key The key must be tra
Page 17 and 18: It may be noted that multiplication
Page 19 and 20: the same as the M space, i.e., that
Page 21 and 22: (4) Each message in Ci can be encip
Page 23 and 24: The columns are then cut apart and
Page 25 and 26: of the values of these. In this cas
Page 27 and 28: In a secrecy system there are two s
Page 29 and 30: text has been started at a random p
Page 31 and 32: in which E, M and K are the cryptog
Page 33 and 34: If we have a product system S = T R
Page 35 and 36: 13 EQUIVOCATION FOR SIMPLE SUBSTITU
Page 37 and 38: keys from high probability messages
Page 39 and 40: 1. We assumed for the random cipher
Page 41 and 42: Fig. 9. Equivocation for simple sub
Page 43 and 44: simple substitution will be approxi
Page 45 and 46: Theorem 12. A necessary and suffici
Page 47 and 48: On the other hand, if the language
Page 49 and 50: much higher) work characteristic. A
Page 51 and 52: the number of trials) only 88 trial
Page 53 and 54: serves to limit the possible keys t
Page 55 and 56: and all the fi involve all the ki.
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egion F R is equal to the measure o
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27 INCOMPATIBILITY OF THE CRITERIA
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