Applet 12. Affine Cipher Calculator
This calculator enciphers and deciphers text using an affine
cipher (such as the Caesar cipher) in which letters are encoded
using the formula C=aP+b (mod 26) where a and b are
whole numbers between 0 and 25 and a is relatively prime to 26.
The calculator also supports general substitution ciphers in which each letter is
replaced by a another letter in a given permutation.
- To encipher, enter plain text in the upper text box, and click on the
- To decipher, enter cipher text in the lower text box, and click on the
- Before you can encipher or decipher text, you need to set a cipher. The
two columns of letters on the left show the plaintext letters (blue)
and their cipher substitutions (white).
- To set an affine cipher, click "Affine", and enter values for the parameters
a and b. The default values (a=1 and b=3) give
the Caesar cipher. After you click "OK", notice how the pattern of letters
in right hand colums of the main display change.
- To set a general substitution cipher, you need to choose letter-by-letter
which substitutions to make. In the table on the right, click on a plaintext letter
(blue background) to select a value, and drop that letter in a ciphertext position
by clicking on a cell with a white background. Repeat until you have set up
the desired substitutions.
- If you're trying to crack a cipher and you think you know a few of the
substitutions, it often helps to "empty" the cipher and fill in only the
letters you think you know. When you decipher the message with a partially-completed
cipher, letters that can't be decoded are left blank (like Wheel of Fortune).
- Use this tool in conjunction with the
Letter Frequency Analysis Calculator
to experiment with cracking codes. The letter frequency analysis method illustrates
the flaws of letter-substitution ciphers and, more generally, shows how hidden
clues withing ciphertext can reveal weaknesses that expose it to cryptanalysis