ChaCha

As explained in Real-World Cryptography

ChaCha20 is a stream cipher that generates a pseudorandom keystream from a key, nonce, and counter.

ChaCha20 is a software-friendly stream cipher. It generates a keystream from a 256-bit key, 12-byte nonce, and counter, then XORs that keystream with plaintext to produce equal-length ciphertext. section 4.5.3

Encryption follows a short sequence:

• Call the block function repeatedly, increasing the counter each time; each call produces 64 keystream bytes. section 4.5.3
• Truncate the keystream to the plaintext length and XOR the two byte sequences. section 4.5.3
• Decrypt by generating the same keystream and XORing it with the ciphertext. section 4.5.3

ChaCha20 is commonly paired with Poly1305 as ChaCha20-Poly1305: ChaCha20 encrypts and derives keys, while Poly1305 authenticates ciphertext and associated data. Never reuse a nonce with a key; doing so exposes relationships between plaintexts. section 4.5.3

Figure 4.18 ChaCha20 works by taking a symmetric key and a unique nonce. It then generates a keystream that is XORed with the plaintext (or ciphertext) to produce the ciphertext (or plaintext). The encryption is length-preserving as the ciphertext and the plaintext are of the same length.

Trace the key-and-nonce input, keystream generation, XOR operation, and length-preserving output. section 4.5.3

How Real-World Cryptography covers it

How deep it goes

Explains keystream production, block inputs, XOR flows, limits, and nonce-reuse consequences, but not the quarter-round internals in detail.

Examples

Uses encryption diagrams, 64-byte block generation, and deployments in OpenSSH, TLS, Noise, and Android.

What you can do after

Evaluate ChaCha20 encryption flows, distinguish encryption from authentication, and recognize nonce-reuse failures.

How it connects

Connects stream ciphers, Salsa20, XOR, Poly1305, authenticated encryption, and AES-GCM.

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Related concepts

• ChaCha20-Poly1305used by · ChaCha20-Poly1305 is an authenticated-encryption-with-associated-data construction combining the ChaCha20 stream cipher with the Poly1305 message authentication code.
• stream cipherkind of · A stream cipher is an encryption algorithm that combines plaintext sequentially with a generated keystream, typically using XOR.
• pseudo-random number generatorused by · A pseudorandom number generator is a deterministic computational process that uses an initial seed or internal state to produce a sequence with random-like statistical properties.

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.