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Chapter 13. Cryptography > One-Time Pads

One-Time Pads

Is there a perfect cryptosystem? Surprisingly, the answer is yes. It is called the one-time pad. The idea of the one-time pad is to have a completely random key that is the same length as the message. The key is never reused, and only the sender and the receiver have copies. To send, for example, a 100-bit message, the message is exclusive-ORed[1] with 100 bits of the key. That portion of the key is crossed off, never to be used again. The receiver reverses the process, exclusive-ORing the ciphertext with her copy of the key to reveal the message. If the one-time pad key contains truly random bits, this scheme is absolutely secure. The attacker does not know what is on the pad and must guess—but there is no way to know when he is right. By changing the guess, the attacker can decode the ciphertext into any message, be it “attack at dawn” or “negotiate surrender.”

[1] The exclusive-OR operation produces a “1” if exactly one of the two input bits is a “1.”

The one-time pad offers perfect security and is indeed used when perfect security is needed, but the system has many disadvantages.


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