Work out vigenere cipher key instantly with clear inputs, formula shown and shareable results.
The Vigenere cipher applies a repeating keyword as a series of Caesar shifts, which flattens simple letter frequencies and held out against casual cryptanalysis for centuries. Its weakness is the repetition: Kasiski examination finds the key length from repeated ciphertext sequences, after which the ciphertext splits into that many independent Caesar ciphers, each trivially solved.
Vigenere
c_i = (p_i + k_(i mod m)) mod 26; decrypt with (c_i - k_(i mod m)) mod 26; keyspace = 26^m
It increases the number of independent Caesar ciphers and the amount of ciphertext needed for frequency analysis on each. A key as long as the message and never reused becomes a one-time pad.
Kasiski examination on repeated sequences, or the index of coincidence, which peaks when the ciphertext is split into the correct number of columns.