Affine Cipher

Encrypt text with E(x) = (ax + b) mod 26 using configurable keys and formatting options.

Learn how the Affine cipher works

Keys

Key a

Multiplier key. It must be coprime with 26 so the cipher can be reversed.

Allowed values: 1, 3, 5, 7, 9, 11, 15, 17, 19, 21, 23, 25

Key b

Shift key. Any integer works here, and the effective value wraps around modulo 26.

The chosen b value is reduced modulo 26 before encryption or decryption.

Normalised a: 5 Normalised b: 8 Inverse of a: 21 Plain A -> Cipher I Plain B -> Cipher N

Options

Preserve case

Keep punctuation & numbers

Remove whitespace

Group letters

Affine encryption uses (ax + b) mod 26. Decryption uses a^-1(y - b) mod 26.

Output stats

Mode: Encrypt Effective key: a=5, b=8 Letters processed: 0 Input chars: 0 Output chars: 0 Blocks: 0

Input

Output

Using This Tool: Guide & Notes Show guide

The Affine cipher transforms each letter number with E(x) = (ax + b) mod 26.

How to use it

Choose Encrypt or Decrypt.
Select a valid multiplier key a and set the shift key b.
Paste text into the input box and check the output plus the key summary.
Use Reset defaults if the keys or formatting get confusing.

Options and settings

Key a is the multiplier. It must be coprime with 26 so the cipher can be reversed.
Key b is the shift. Any integer is accepted and then reduced modulo 26.
Preserve case and keep punctuation control how the visible text is carried through.
Group letters and block size format the output without changing the underlying cipher.

Notes

Valid a values are limited because not every number has a modular inverse mod 26.
Affine is stronger-looking than Caesar, but still easy to attack with enough text.

Affine Cipher: Where Substitution Becomes Algebra

How multiplication, modular arithmetic, and a two-number key turned simple alphabet substitution into one of classical cryptography's cleanest mathematical teaching tools.

Read the article

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A note from the team

We build these tools because we love cryptography, encoding, and making difficult ideas easier to explore. If they've helped you, even a small coffee means the world to the project. Thank you!

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