Vigenere Tableau — Bench tool
Published 11 Sep 2026 ·
The Vigenere cipher was called le chiffre indéchiffrable — the indecipherable cipher — for roughly three hundred years, and the name was only half bravado. It is the first cipher on this loom whose key space is genuinely out of reach of brute force, and the first whose break required a completely new idea rather than more patience.
- Era
- Renaissance
- Type
- Polyalphabetic
- Key space
- 26^k
- Level
- Field
IWhat this tool does
The Vigenere is a Caesar cipher whose shift changes on every letter, driven by a repeating keyword.
Plaintext: LOOM NEVER STOPS
Key: BRASSBRASSBRA
Convert both to numbers with A = 0. Add them position by position, take the result modulo 26, convert back.
| Pos | Plain | Plain # | Key | Key # | Sum | Cipher |
|---|---|---|---|---|---|---|
| 1 | L | 11 | B | 1 | 12 | M |
| 2 | O | 14 | R | 17 | 31 → 5 | F |
| 3 | O | 14 | A | 0 | 14 | O |
| 4 | M | 12 | S | 18 | 30 → 4 | E |
| 5 | N | 13 | S | 18 | 31 → 5 | F |
| 6 | E | 4 | B | 1 | 5 | F |
| 7 | V | 21 | R | 17 | 38 → 12 | M |
| 8 | E | 4 | A | 0 | 4 | E |
| 9 | R | 17 | S | 18 | 35 → 9 | J |
The same plaintext letter produces a different ciphertext letter at every position. The letter E appeared at positions 6 and 8 and left as F and E. That single fact is what defeats frequency counting, and it is the whole point of the design.
IIThe key
For a keyword of length k the number of distinct keys is 26^k. That sounds enormous, and for k = 12 it is: 9.5 x 10^16, comfortably beyond manual search and beyond 1930s machinery.
The gap between the arithmetic and the reality is the key length. 26^k assumes the attacker does not know k. In practice cryptanalysts find k first, using an observation published in 1863 by Friedrich Kasiski: repeated sequences in the ciphertext occur at distances that are multiples of the key length.
Once k is known, the cipher collapses. Take every k-th letter, and you are holding k separate Caesar ciphers, each with 25 keys, each readable by frequency analysis on its own. A 12-letter key reduces to 12 small homework problems.
The Vigenere is not weak because 26^12 is small. It is weak because the key repeats, and repetition is measurable.
IIIWhat it will not protect
Historical ciphers are for study and recreation and are not suitable for protecting real information. The full analysis is in the cipher entry.
Read the full entry, with deciphering steps and weakness analysis →