CIT 2640 · Network Security · Cipher tool
The Vigenère cipher shifts each letter of the plaintext by an amount set by a repeating keyword, so the same plaintext letter can encrypt to a different ciphertext letter every time it appears. That defeated simple frequency analysis for nearly three centuries — it was called le chiffre indéchiffrable.
Polyalphabetic substitutionEach row of the square is the alphabet shifted by that row's letter — exactly a Caesar cipher. To encrypt a letter, find its column under the plaintext letter, follow it down to the row of the current key letter, and read off the cell where they meet. To decrypt, go along the key row until you find the ciphertext letter; its column header is the plaintext. The highlight tracks the last letter processed above.
The Kasiski examination (1863) looks for repeated sequences of three or more letters in the ciphertext. If the same short stretch of plaintext lines up with the same stretch of key twice, it produces identical ciphertext — and the distance between the repeats is a multiple of the key length. Enough repeats narrow the key length down to one likely value.
Once the key length n is known, the ciphertext splits into n interleaved streams,
each encrypted with a single, constant shift — a plain Caesar cipher. Frequency analysis (E, T, A are the
most common English letters) cracks each stream on its own, and the key falls out letter by letter.