It is not sufficient for OTP keys to pass statistical randomness tests as such tests cannot measure entropy, and the number of bits of entropy must be at least equal to the number of bits in the plaintext. The resulting ciphertext will be impossible to decrypt or break if the following four conditions are met:

Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the pad using modular addition. In this technique, a plaintext is paired with a random secret key (also referred to as a one-time pad). In cryptography, the one-time pad ( OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent. The table on the right is an aid for converting between plaintext and ciphertext using the characters at left as the key.

National Security Agency, code named DIANA. A format of one-time pad used by the U.S.