In , during a debate over the Falkland Islands War , a member of Parliament, in a now-famous gaffe, revealed that the British were reading Argentine diplomatic ciphers with as much ease as Argentine code clerks. While cryptography is clearly a science with well-established analytic and synthetic principles, cryptanalysis in the past was as much an art as it was a science.
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The reason is that success in cryptanalyzing a cipher is as often as not a product of flashes of inspiration, gamelike intuition , and, most important, recognition by the cryptanalyst of pattern or structure, at almost the subliminal level, in the cipher. It is easy to state and demonstrate the principles on which the scientific part of cryptanalysis depends, but it is nearly impossible to convey an appreciation of the art with which the principles are applied.
In present-day cryptanalysis, however, mathematics and enormous amounts of computing power are the mainstays. Cryptanalysis of single-key cryptosystems described in the section Cryptography: Key cryptosystems depends on one simple fact—namely, that traces of structure or pattern in the plaintext may survive encryption and be discernible in the ciphertext.
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Take, for example, the following: in a monoalphabetic substitution cipher in which each letter is simply replaced by another letter , the frequency with which letters occur in the plaintext alphabet and in the ciphertext alphabet is identical. The cryptanalyst can use this fact in two ways: first, to recognize that he is faced with a monoalphabetic substitution cipher and, second, to aid him in selecting the likeliest equivalences of letters to be tried.
The table shows the number of occurrences of each letter in the text of this article, which approximates the raw frequency distribution for most technical material. The following cipher is an encryption of the first sentence of this paragraph minus the parenthetical clause using a monoalphabetic substitution:. W occurs 21 times in the cipher, H occurs 18, and so on. Even the rankest amateur, using the frequency data in the table, should have no difficulty in recovering the plaintext and all but four symbols of the key in this case.
It is possible to conceal information about raw frequency of occurrence by providing multiple cipher symbols for each plaintext letter in proportion to the relative frequency of occurrence of the letter—i. The collection of cipher symbols representing a given plaintext letter are called homophones.
If the homophones are chosen randomly and with uniform probability when used, the cipher symbols will all occur on average equally often in the ciphertext. The great German mathematician Carl Friedrich Gauss — believed that he had devised an unbreakable cipher by introducing homophones.
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Unfortunately for Gauss and other cryptographers, such is not the case, since there are many other persistent patterns in the plaintext that may partially or wholly survive encryption. Digraphs, for example, show a strong frequency distribution: TH occurring most often, about 20 times as frequently as HT, and so forth. With the use of tables of digraph frequencies that partially survive even homophonic substitution, it is still an easy matter to cryptanalyze a random substitution cipher, though the amount of ciphertext needed grows to a few hundred instead of a few tens of letters.
In the discussion of the preceding paragraphs, the cryptanalyst knows only the ciphertext and general structural information about the plaintext.
Of course, for single-key cryptography there is no distinction between chosen plaintext and chosen ciphertext, but in two-key cryptography it is possible for one of the encryption or decryption functions to be secure against chosen input while the other is vulnerable. One measure of the security of a cryptosystem is its resistance to standard cryptanalysis; another is its work function—i.
The first can be thought of as an attempt to find an overlooked back door into the system, the other as a brute-force frontal attack. Assume that the analyst has only ciphertext available and, with no loss of generality, that it is a block cipher described in the section Cryptography: Block and stream ciphers.
- Chosen plaintext attack.
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He could systematically begin decrypting a block of the cipher with one key after another until a block of meaningful text was output although it would not necessarily be a block of the original plaintext. If the cryptanalyst has the time and resources to try every key, he will eventually find the right one.
Clearly, no cryptosystem can be more secure than its work function. Informally known as " the Crypt ," the personnel referred to themselves as lignyots , a term which no one else deciphered the meaning of.
Cryptanalysis cultivated a reputation for the bizarre; they would send scandocs with simple encryption codes that would produce unflattering holos of the recipient if the code was not broken fast enough. They would also break communications security and forge inflammatory messages between branches. They had a habit of moving the entrance to their offices, or relocating entirely, leaving only a scandoc with clues to their new location. Other branches continually complained about Crypt's antics to the Ubiqtorate , but the lignyots defended their behavior by claiming that the time-sensitive nature of their work—cracking codes and ciphers discovered by Media and Signal —entitled them to be a little eccentric.