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A repeating-key Vigenère cipher leaves a periodic trace even when no repeated word is obvious. One way to look for that trace is to compare the ciphertext with shifted copies of itself and count how often the letters line up exactly. If a shift realigns the same key positions, the coincidence rate c

Finding the repeating period is one of the most important steps in ciphertext-only Vigenère cryptanalysis. It is also one of the easiest places to become overconfident. A repeated sequence may point to several factors. An Index of Coincidence peak may occur at a multiple of the real period. Autocorr

There is no universal minimum amount of ciphertext that guarantees a Vigenère key can be recovered. The amount of text needed depends on the key period, the plaintext language, how typical that text is, whether the period is already known, and the scoring method used by the cracker. A 100-letter cip

Reading a Vigenère example is not the same as being able to work one correctly. The mistakes usually appear when the key repeats, arithmetic wraps around the alphabet, a negative decryption result has to be reduced modulo 26, or the text contains characters that do not consume key positions. This pr

A Vigenère implementation can produce readable-looking output and still be wrong. The most common failures are small: treating A as shift 1 instead of 0, advancing the key across spaces in one part of the program but not another, mishandling wraparound at Z , repeating the key from the wrong positio

An unknown block of letters can look like Vigenère without actually being Vigenère. A flattened frequency distribution, an Index of Coincidence near the random-text range, or a few repeated trigrams can all be useful clues. None of them proves the cipher type on its own. A stronger identification co

Cipher security comparison Vigenère Cipher vs One-Time Pad: Key Differences The Vigenère cipher and a one-time pad can perform almost the same letter-by-letter arithmetic and still have radically different security. With an A–Z alphabet, both can represent letters as numbers from 0 to 25 and combine plaintext with a key using addition modulo 26: C…

Known-plaintext cryptanalysis Known-Plaintext Attacks on the Vigenère Cipher A known-plaintext attack begins with something a ciphertext-only attack does not have: part of the original message and the ciphertext produced from that same part of the message. Against the Vigenère cipher, that information is especially powerful because the key shift used at each known position can…

Vigenere key search Brute-Force and Dictionary Attacks on Vigenère Two programs can attack the same Vigenère ciphertext by trying possible keys, yet perform fundamentally different searches. One program might test: AAAAA AAAAB AAAAC AAAAD … ZZZZZ until every possible five-letter A–Z key has been considered. That is a brute-force attack. Another program might try: APPLE…

Vigenere key recovery Vigenère Cipher Frequency Analysis Explained Frequency analysis becomes useful against a repeating-key Vigenère cipher after the period is known or strongly suspected. Before that point, several shifted alphabets are mixed together and the frequency distribution of the ciphertext as a whole is difficult to interpret. Once the period is known, the situation…