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Security Fundamentals & Crypto · Foundational

Stream Cipher

A symmetric cipher that encrypts data one bit or byte at a time by combining it with a pseudorandom keystream.

Definition

Stream ciphers XOR plaintext with a keystream generated from a key and nonce. They are efficient for continuous data and low-latency applications. Reusing a key and nonce pair is catastrophic because XORing two ciphertexts reveals the XOR of the plaintexts.

Example

ChaCha20 generates a keystream that is XORed with each byte of a TLS record.

Exam tip

RC4 is a deprecated stream cipher; ChaCha20 is the modern choice. Never reuse a nonce.

Don't confuse it with

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