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Security Fundamentals & Crypto · Foundational

Substitution Cipher

Cipher that replaces each plaintext unit with another symbol according to a fixed or keyed system.

Definition

Monoalphabetic substitution ciphers such as Caesar and ROT13 are broken easily with frequency analysis. Polyalphabetic ciphers like Vigenere use multiple substitution alphabets to resist this. Substitution remains a building block (S-boxes) of modern ciphers.

Example

ROT13 shifts every letter 13 places, so 'HELLO' becomes 'URYYB'.

Exam tip

Frequency analysis breaks simple substitution ciphers.

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