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Cryptography — Learning Library

The one-time pad

masiarek/cryptography-learning-library

Cryptography — Learning Library

masiarek/cryptography-learning-library

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What Cryptography Is
What Cryptography Is
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  What a key is
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  Kerckhoffs's principle
Classical Ciphers
Classical Ciphers
- Breaking a cipher by counting
  Breaking a cipher by counting
- The one-time pad
  The one-time pad
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  random is not secrets
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  A hash is not encryption
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The one-time pad¶

Level: 201 · working knowledge

Stub — an outline, not a lesson. There is no runnable example behind this page yet, so nothing on it has been through the check that backs every other claim in this library. The bullets below are the questions the finished page has to answer.

One line: The only cipher with a proof of perfect secrecy — and the reason that proof does not make it useful.

The questions this page has to answer¶

What does perfect secrecy mean precisely, and why is it a statement about probability rather than about effort?
Why does the key have to be as long as the message, truly random, and used exactly once?
What happens when a pad is reused twice? (The two-time pad, and why XORing two ciphertexts removes the key entirely.)
VENONA: what the Soviets' partial pad reuse cost them, and over how many years.
Why is 'we XOR with a long key' in a product almost always this mistake?

See also¶

02_Classical_Ciphers — the chapter this belongs to
ROADMAP.md — where this sits in the queue