One-Time Pad

Encrypt with a pad that is never repeated

A one-time pad is simple but strict: the pad must be random, at least as long as the message, and used once. This tool enforces the length rule instead of repeating a short key like a Vigenère or repeating-XOR cipher would. If the pad is too short, it stops with an error that names the shortfall.

Choose one of three alphabets:

• Letters — adds pad letters to message letters with C = (P + K) mod 26. Case, spaces, punctuation, and numbers are preserved and do not consume pad.
• Digits — the same idea over 0..9 with C = (P + K) mod 10. Non-digits pass through unchanged.
• XOR bytes — XORs the UTF-8 bytes of the message with random pad bytes, with pad and ciphertext encoded as hex or Base64.

Leave Pad empty while encrypting to generate a fresh pad sized to the message. Use Generate pad when you only need pad material. Random pads come from the platform cryptographic RNG; generated letter and digit pads use rejection sampling so the alphabet is uniform.

Worked example

For the classic letter example:

• Message: HELLO
• Pad: XMCKA
• Mode: encrypt
• Cipher: letters

The result is EQNVO because each letter is added modulo 26: H + X = E, E + M = Q, L + C = N, L + K = V, O + A = O. Switch to Decrypt with message EQNVO and the same pad XMCKA to recover HELLO.

Limits and edge cases

• Maximum message/pad work is 16,384 letters, digits, or bytes per run.
• Group output every N characters is for display only. 0 keeps the original layout; 5 creates classic five-character groups; 20 is the maximum.
• Letter and digit modes preserve characters outside their alphabet. Those characters are not encrypted and do not consume pad.
• XOR decrypt expects the ciphertext in the selected encoding and returns UTF-8 text. If the pad or encoding is wrong, decrypted bytes may not be valid UTF-8.
• Generated pads are returned so you can store or share them out of band. Never reuse the same pad material for another message.

FAQ