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Vigenere Cipher Key Calculator

Work out vigenere cipher key instantly with clear inputs, formula shown and shareable results.

About Vigenere Cipher Key Calculator

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.

Formula

Vigenere

c_i = (p_i + k_(i mod m)) mod 26; decrypt with (c_i - k_(i mod m)) mod 26; keyspace = 26^m

m
Key length

Worked example

With the default values below, Vigenere Cipher Key Calculator returns Transformed text of Lxfopv ef rnhr. Change any input above and every figure here is recalculated in your browser.

Inputs used

Text
Attack at dawn
Keyword
LEMON
Operationone of Encrypt, Decrypt
Encrypt

Results

Transformed text
Lxfopv ef rnhr
Key length
5
Keyspace
23.5 bits
Cryptanalysis exposure
Repeated ciphertext sequences at distances that are multiples of 5 reveal the key length to Kasiski examination

Step by step

  1. 1Values used
  2. 2Vigenere
  3. 3Transformed text= Lxfopv ef rnhr
  4. 4Key length= 5
  5. 5Keyspace= 23.5 bits
  6. 6Cryptanalysis exposure= Repeated ciphertext sequences at distances that are multiples of 5 reveal the key length to Kasiski examination

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is a longer keyword stronger?

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.

How is the key length found?

Kasiski examination on repeated sequences, or the index of coincidence, which peaks when the ciphertext is split into the correct number of columns.

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