Ciphers · VigenèreVigenère cipher — encode, decode and crack KEY
The Vigenère cipher shifts every letter by its own amount, following the letters of a keyword: “attack at dawn” with the key “lemon” becomes “lxfopv ef rnhr”. Encode and decode with English or Russian letters, follow the step table for a lesson, and if there is no key we find it from the text.
Everything runs in your browser — the text goes nowhere
Encode
A one-letter key is the Caesar cipher · Open Caesar →
Step by step
for a lesson: text letter, key letter, shift, resultKey length
index of coincidence and repeated trigrams (Kasiski)Turn on “I don’t know the key — find it” on the Decode tab — likely key lengths will appear here.
The search relies on letter frequencies and a dictionary — reliable from 100 letters, it may be wrong on short texts.
Vigenère square
row — key letter, column — text letter, intersection — cipherHow it works
Shift by the key
a + l → l · t + e → xEach key letter is its own shift: “a” is 0, “l” is 11. The key repeats over the letters of the text; spaces and punctuation do not move it.
Gronsfeld and autokey
2015 · key + textIn Gronsfeld the key is digits, shifts from 0 to 9. In autokey, after the keyword the shifts come from the letters of the plaintext itself.
How it is cracked
length → columns → frequenciesFind the key length by the index of coincidence and repeated trigrams, cut the text into columns and pick each shift by letter frequencies.
Frequently asked questions
Updated