# Specific Examples

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## Techniques

• ### Cryptanalysis of the Affine Cipher

The affine cipher has only 311 possible keys, so it presents few problems to brute force attacks. We use quadgram statistics to rank each key, the final solution will be the key that produces ciphertext that is most similar to english.

• ### Cryptanalysis of the Autokey Cipher

The Autokey cipher is similar the Vigenere cipher in operation, however the key is not treated in the same way. These differences make the Autokey cipher stronger than Vigenere, but still fairly easy to break.

• ### Cryptanalysis of the Bifid cipher

Bifid is a fractionating cipher which uses a key square along with a period as the key.

• ### Cryptanalysis of the Caesar Cipher

The Caesar Cipher is one of the simplest ciphers, as a result cryptanalysis is fairly simple. This page introduces the use of quadgram statistics for text characterisation, which is also used extensively to break more complicated ciphers.

• ### Cryptanalysis of the Columnar Transposition Cipher

The Columnar Transposition Cipher is a very simple cipher which can offer a great deal of security if long key words are used (keys > 20 characters). If short key words are used, however, or if dictionary words are used as keys the cipher becomes much easier to break.

• ### Cryptanalysis of the Foursquare Cipher

This page will describe the breaking of the Foursquare cipher using Simulated Annealing. Foursquare is a digraphic cipher which uses two complete alphabets as the key, both of which we have to find for a decryption.

• ### Cryptanalysis of the Hill Cipher

The Hill Cipher is a linear digraphic substitution cipher. Its key is a matrix of numbers. On the whole, it is not very secure for small matrices. For matrices of around 5 by 5 or larger it becomes fairly secure, but there is a lot of key material for larger matrices.

• ### Cryptanalysis of the Playfair cipher

Playfair is a digraphic substitution cipher, more difficult than the simple substitution cipher to crack. This page will deal with automatic cryptanalysis of playfair ciphers using Simulated Annealing.

• ### Cryptanalysis of the Simple Substitution Cipher

The simple substitution cipher is fairly easy to break, simple enough that it can usually be broken by hand in a few minutes. This page describes an automated technique for breaking substitution ciphers using a hill climbing algorithm.

• ### Cryptanalysis of the Straddle Checkerboard

The straddling checkerboard is a substitution cipher, except that the substitutions are of variable length. It has formed a component of several impotant field ciphers, the most notable being the VIC cipher used by russian spies during the cold war.

• ### Cryptanalysis of the Vigenere Cipher

The Vigenere cipher was though to be completely unbreakable for hundreds of years, and indeed, if very long keys are used the vigenere cipher can be unbreakable. But if short keys are used, or if we have a lot of ciphertext compared to the key length, the vigenere cipher is quite solvable.

• ### Cryptanalysis of the Vigenere Cipher, Part 2

In part 1, we employ techniques that are used more often when trying to crack the Vigenere cipher by hand, including using the Index of Coincidence to identify the key length and the Chi-squared test to determine the key. In this example we will use local search techniques to find good key candidates. This is fast enough that we can simply try it for all possible key lengths.

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