One-Time Pad Generator

A one-time pad encrypts each symbol of a message with a corresponding symbol from a truly random key of equal or greater length. Because every possible plaintext is equally likely given the ciphertext (when the key is random and used only once), an attacker learns nothing about the original message—even with infinite computing power. This property is called perfect secrecy and was proved by Claude Shannon.

No. All generation, encryption, and decryption happen entirely in your browser using the Web Cryptography API. Nothing is uploaded or logged.

Pads are produced with crypto.getRandomValues(), the standard browser interface to the operating system’s cryptographically secure random number generator. It draws from hardware entropy sources and is suitable for cryptographic keys.

Modular mode operates only on the 26 English letters (A–Z). It adds or subtracts key letters modulo 26 and leaves spaces and punctuation untouched. XOR mode works on the raw UTF-8 bytes of any text, including Unicode characters, and produces binary-safe output that can be shown as hex or base64.

If the key is shorter and is repeated or padded, the construction ceases to be a one-time pad and becomes a stream cipher that can be broken (the classic “two-time pad” attack). The tool therefore rejects keys that are too short for the chosen mode.

Never. Reusing even a single key segment against two different plaintexts allows an attacker to recover information by XORing the two ciphertexts. Always generate a fresh pad (or use a fresh unused segment) for every message and destroy used material on both ends.

Share the pad through a separate, secure channel (ideally in person or via a previously established trusted medium). Store unused pads offline in a physically protected location. After use, securely wipe both the electronic and any paper copies.

Traditional letter-based one-time pads only define operations on an alphabet of 26 symbols. Spaces, punctuation, and digits are therefore left in place so the resulting text remains readable for manual checking. Only alphabetic characters consume key material.

For alphabet sizes that are not powers of two, reducing a random byte modulo the alphabet size introduces a tiny statistical bias. For educational use and short messages this is negligible. Production systems that require perfect uniformity typically use rejection sampling; this tool prioritizes simplicity and speed while still drawing from a CSPRNG.

The mathematics are sound, but practical security also depends on key generation, distribution, storage, and destruction discipline. Most real-world systems prefer authenticated encryption (e.g., AES-GCM) because key management for large one-time pads is difficult. Treat this tool as an excellent learning and demonstration environment rather than a drop-in replacement for modern cryptographic protocols.