Hill Cipher

Encrypt letter blocks with square key matrices and modular arithmetic.

Key matrix

Hill encrypts fixed-size letter blocks by multiplying a plaintext vector by a square key matrix: C = K x P mod 26. Decryption needs the matrix inverse modulo 26.

Type: Polygraphic substitution Arithmetic: Mod 26 Block size: 3 letters Padding: X

Matrix size

Any integer is accepted. Values are reduced modulo 26, so 27 behaves like 1 and -3 behaves like 23.

Options

Preserve case

Keep punctuation & numbers

Remove whitespace

Group letters

Strip trailing X padding after decrypt

Encryption pads the final letter block with X when needed. Decryption expects a ciphertext letter count that is a multiple of the matrix size.

Output stats

Mode: Encrypt Matrix: 3x3 det(K) mod 26: 0 Invertible: No Input letters: 0 Input chars: 0 Output chars: 0 Matrix blocks: 0 Padding added: 0

Example: with the classic 3x3 Hill key, ACT encrypts to POH.

Input

Output

Using This Tool: Guide & Notes Show guide

The Hill cipher encrypts letter blocks by multiplying plaintext vectors by a key matrix modulo 26.

How to use it

Choose the matrix size, then enter or adjust the key matrix values.
Check that the matrix is valid before relying on decryption.
Paste plaintext or ciphertext into the input box.
Use the output and matrix status to confirm how blocks are being processed.

Options and settings

Matrix size controls how many letters are processed together in each block.
Key values define the matrix. The determinant must allow an inverse modulo 26.
Padding fills incomplete final blocks so the matrix operation can run cleanly.
Grouping formats the output for readability but does not change the mathematics.

Notes

A non-invertible key matrix may encrypt but cannot decrypt reliably.
Hill is useful for seeing linear algebra inside classical cryptography.

Hill Cipher: The Day Classical Cryptography Became Mathematical

How Lester S. Hill's 1929 matrix cipher moved secret writing beyond alphabet tricks and toward the logic of modern cryptography.

Read the article

Signal acquired

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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