One-Time Pad


One-Time Pad is an encryption Scheme that is Information-theoretic Secure requires the use of a random one-time Pre-Shared Key (this is the one-time pad) the same size as, or longer than, the message being sent.

One-Time Pad is Information-theoretic Secure. However, practical implementation problems have prevented One-Time Pad from being widely used.

One-Time Pad a plaintext is paired with a random one-time Pre-Shared Key. Then, each bit or character of the plaintext is encrypted by combining it with the corresponding bit or character from the One-Time Pad. (Typically using modular addition)


  • the One-Time Pad is truly random
  • the One-Time Pad at least as long as the plaintext
  • the One-Time Pad is never reused in whole or in part
  • the One-Time Padis kept completely secret
then Decryption of the resulting ciphertext will be impossible.

Suppose Alice wishes to send the message "HELLO" to Bob without Eve decoding the message.

Assume two pads of paper containing identical random sequences of letters were somehow previously produced and securely issued to Alice And Bob and NOT Eve.

Alice chooses the appropriate unused page from the pad. The way to do this is normally arranged for in advance, as for instance 'use the 12th sheet on 1 May', or 'use the next available sheet for the next message'.

The material on the selected sheet is the random one-time Pre-Shared Key (or One-Time Pad) for this message. Each letter from the One-Time Pad will be combined in a predetermined way with one letter of the message.

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