10:45 Cryptanalysis of the cipher Vigenere | |
| First of all, let us assume that the enemy was convinced that the ciphertext was obtained either by monoalfavitnoy substitution, or with the help ¬ schyu cipher Vigenere. To find out which of these two methods was used, it is possible to conduct a simple test. If you used monoalfa ¬ Whitney permutation statistics ciphertext will not differ from those in the language of the written text of the open ¬ th. If the analysis is only one report, an exact match ¬ skih statistical indicators can not get it. But if the statistics accurately repeats the usual statistics of the plaintext, we can assume that we used monoalfavitnaya substitution. If, on the contrary, everything points to the fact that the code has been applied Vigenere, then, as we shall see later, the success of further analysis of the text depends on its ability to determine whether the length of the keyword. The solution to this problem wasps ¬ Nova next feature of this cipher: if the initial characters of the two identical sequences of plaintext are from each other at a distance, the multiple key length, these sequences will be presented identical sequences in the ciphertext. For example, suppose that in clear text, there are two identical sequences of characters (a word or a combination thereof), then if they are encrypted by using the same fragment of the key, we get the same sequence of characters of ciphertext. The analyst, having at its disposal only the encrypted text, will find a repeating sequence of characters with a shift in K (a multiple of the key length) characters. Further analysis based on other features of this Cipher. If the keyword has length N, the cipher, in fact, consists of N monoalfavitnyh substitution ciphers. For example, using the keyword deceptive letters that are on the 1 st, 10 th, 19 th, etc. positions, is encrypted by the same monoalfavitnym cipher. This allows the use of the known characteristics of frequency distributions of letters of the plaintext to crack the cipher monoalfavitnogo each separately. Periodicity in the key string can be avoided by using a key line non-periodic sequence of the same length as the original community of ¬. Vigenere proposed approach, called systems with automatic ¬ Skim selection key when the key sequence string obtained by concatenation of the keyword with the very clear text. For example, consider ¬ under consideration, we obtain the following. Key: deceptivewearediscoveredsav plaintext: wearediscoveredsaveyourself ciphertext: ZICVTWQNGKZEIIGASXSTSLVVWLA However, this scheme is vulnerable. As in the key string in clear text value of the frequency distribution of letters are the same, statistical methods can be applied in this case. Then, for example, the symbol of a cipher with a key symbol b, will occur with a frequency equal to the product of the frequencies of these characters. It was such laws can be successful in the analysis of the ciphertext. The best defense against such methods of cryptanalysis is the choice of keyword, length equal to the length of the plaintext, but differs from the plain language of statistical indicators. Such a system was proposed as an engineer at AT & T Varnamo Gilbert (Gilbert Vernam) in 1918 His system operates with no letters, and binary numbers. Briefly it can be expressed by the formula: Thus, the ciphertext is generated by bitwise ¬ fulfillment of the XOR operation to the plaintext and the key. Due to the properties of this operation to decipher enough to perform this operation: The essence of this technology is a method for choosing the key. Verna suggested the use of loopback tape, which means a cyclic repetition of key words ¬-beam, so that its system actually involves working though and with very long, but still a duplicate key. Despite the fact that such a scheme by virtue of a very large key lengths greatly complicates the task of cryptanalysis, the scheme, however, can be hacked, with quite a large piece of ciphertext, known or probably known pieces of plain text or something, and both. Officer Corps communications Moborn Joseph (Joseph Mauborgne) suggested that such improvements in encryption schemes Varnamo who made the scheme very reliable. Moborn offered to give up reps, and randomly generate a key length equal to the length of the message. This scheme, known as single-use tapes (or circuits with one-time pad), cracking can not be. As a result of its use of the output is a random sequence that has no statistical relationship with the plaintext. Since in this case the ciphertext gives no information about the plaintext, and there is no way to crack the code. The complexity of the practical application of this method lies in the fact that both the sender and receiver must have the same random key and be able to protect him from strangers. Therefore, despite all the advantages over other Varnamo cipher cipher in practice resort to it often. | |
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