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Security Fundamentals & Crypto · Intermediate

One-Time Pad (OTP)

Cipher using a truly random key as long as the message, used only once, offering information-theoretic security.

Definition

A one-time pad XORs plaintext with a random pad of equal length. If the pad is truly random, kept secret, and never reused, the ciphertext is unbreakable. Its key distribution and management requirements make it impractical for most uses.

Example

The Venona project decrypted Soviet messages because some one-time pads were reused.

Exam tip

Requirements: truly random, at least as long as the message, used once, kept secret. Don't confuse with one-time password.

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