One-Time Pad (OTP) Cipher Tool
The only provably unbreakable cipher — if used correctly.
About this tool
The one-time pad is the only cipher proven unbreakable — Claude Shannon showed in 1949 that with a truly random key, at least as long as the message and never reused, the ciphertext reveals nothing about the plaintext. Each letter is shifted by the corresponding key letter (mod 26).
Those conditions are also why it's rarely practical: distributing enormous amounts of random key material securely is hard, and any reuse destroys the guarantee. This tool enforces that the key is at least as long as the message. Everything runs in your browser.
Like everything on encrypt.tools, this runs entirely in your browser — nothing you enter is transmitted or stored.
Frequently asked questions about One-Time Pad
Why is the one-time pad unbreakable?+
With a truly random key as long as the message, every possible plaintext is equally consistent with the ciphertext, so there is no information to attack — a property called perfect secrecy, proven by Claude Shannon. No amount of computing power helps.
What breaks the one-time pad's security?+
Reusing key material (the 'two-time pad' mistake), using a key shorter than the message, or using a non-random key. Any of these leaks information and has broken real systems, such as the Soviet messages exposed by the VENONA project.
Why isn't the one-time pad used everywhere?+
The key must be as long as all the data you'll ever send, truly random, shared secretly in advance, and never reused. Distributing that much key material securely is usually harder than the encryption problem itself, so modern systems use shorter keys with computational security instead.
Related tools
Caesar Cipher
Shift cipher with instant all-25 bruteforce
VIGENÈRE · POLYVigenère Cipher
The polyalphabetic cipher that resisted breaking for 300 years
ATBASH · AFFINEAtbash & Affine
Mirror-alphabet and modular substitution ciphers
RAIL FENCE · ZIGZAGRail Fence Cipher
Zigzag transposition across configurable rails