ClassicCrypto

The ciphertext is y k mod 26 chosen ciphertext you get

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Unformatted text preview: (mod 26) Chosen plaintext: You get to choose the plaintext x. For simplicity, take x = ‘a’ = 0. The ciphertext is y = k (mod 26) Chosen Ciphertext: You get to choose the ciphertext y. Choose y=‘A’, then plaintext is x = -k (mod 26). x = - k (mod 26) What about Known Ciphertext? This is the toughest one. We have the least information. Ciphertext Only on Shift Cipher Ciphertext If all Eve gets is the ciphertext, she may try one of two strategies: – She could try all choices for k. There are only 26 and most likely (if message is long enough) only one key will produce something intelligent. – She could do a frequency counting: Use knowledge of the underlying language. For example, ‘e’ occurs the most often in English. Whatever letter occurs the most in the ciphertext probably corresponds to the plaintext ‘e’. (If you have a long enough message). The key is probably the value needed to shift ‘e’ to the most frequent ciphertext letter. If the most frequent fails, try the next most frequent… Affine Cipher Affine The affine...
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