Calculator
No keyword needed: Break mode searches for one. The field shows the best keyword found.
A one-letter Vigenère key is exactly a Caesar Cipher: is the shift. Open in Caesar Cipher
Advanced hints
Known letters, with ? for each unknown one. Its length is the key length.
A word or phrase the plaintext contains, anywhere. Every position it could occupy is tried.
Background searching is not available here, most likely because this page was opened as a file instead of from a web server. Searches still work, but the page pauses while one runs, shows no progress, and cannot be stopped; Maximum effort can take a while.
Vigenère key-length analysis is unreliable on very short ciphertext. More text usually produces better results. Short ciphertext detected. Dictionary and constrained-key searches may be more useful than frequency analysis.
No key fits these hints. Check the partial key, the key length and the expected plaintext.
Key-length evidence
| Length | Combined | Avg IC | Kasiski | Autocorr. | Friedman | Notes |
|---|
Kasiski examination
| Sequence | Positions | Distances | Likely factors |
|---|
Autocorrelation
Column analysis
| Position | Letters | Best shift | Alternatives |
|---|
Current likely key from the columns alone:
Search details
Several attacks propose keys: every short key is tried outright, each column of a likely key length is solved as a Caesar cipher and the columns' best shifts are combined, dictionary words are tried as keys, and promising keys are polished one letter at a time. Each candidate's plaintext is then scored, mainly by how English its letter sequences are, with listed words and the plainness of the key as further evidence. Select a candidate to decrypt with it. The top one is marked Likely English only when it reads as English, stands clear of the next reading, and rests on enough ciphertext or strong hints; Low confidence on short or un-English text; and Best guess otherwise. These are heuristics, not probabilities: no search can promise the right key, and a short ciphertext can have several equally good readings.
See the repeating key
Current Caesar shift
How your message is transformed
| Position | Input | Key | Values | Calculation (mod 26) | Output |
|---|
Position counts letters only; spaces, digits, and punctuation are not transformed, do not use a keyword letter, and are left out of the table.
Why does the keyword repeat?
A keyword of m letters supplies m Caesar shifts, one per letter. The message is usually longer than the keyword, so after the last key letter has been used the key starts again from the beginning. Spaces and punctuation that are preserved do not advance the keyword.
How is this related to the Caesar Cipher?
The Caesar Cipher uses one fixed shift for every letter, Ci = Pi + k. Vigenère uses a sequence of shifts, Ci = Pi + Ki: each keyword letter stands for one Caesar shift, and the strip above shows the one in force at the cursor. A one-letter keyword is exactly a Caesar cipher.
Why is Vigenère harder to attack with simple frequency analysis?
In the Caesar and affine ciphers one plaintext letter always becomes the same ciphertext letter, so the most common ciphertext letter gives the key away. In Vigenère the mapping depends on the position in the keyword, so the frequencies of the message are spread over several substitution alphabets and simple letter counting no longer works. The cipher is still not secure by modern standards: once the key length is known, each column is a Caesar cipher again, which is what Break mode exploits.
What is the Index of Coincidence?
It measures how often two letters picked at random from a text are the same: IC = Σ fi(fi − 1) / [N(N − 1)], where fi counts each letter and N is the number of letters. English-like text scores higher than uniformly random letters, because a few letters are very common. When a Vigenère ciphertext is split into columns using the correct key length, each column is Caesar-encrypted English and its IC tends to rise; with a wrong length the columns stay mixed and the IC stays low.
Related topics
The Caesar Cipher is Vigenère with a one-letter keyword.
The Affine Cipher is monoalphabetic: for a fixed key, a plaintext letter always maps to the same ciphertext letter. Vigenère is polyalphabetic: the same plaintext letter can become different ciphertext letters, depending on its position under the repeating keyword.