kryptos - v1.6.0

kryptos/stream

Stream cipher implementations.

ChaCha20 (RFC 8439) and XChaCha20 (draft-irtf-cfrg-xchacha) with a caller-chosen starting block counter.

IMPORTANT SECURITY WARNING: These ciphers do NOT provide authentication. An attacker can modify ciphertext without detection. For most applications, you should use ChaCha20-Poly1305 or XChaCha20-Poly1305 from the kryptos/aead module instead.

Use these ciphers only when:

Nonce reuse is catastrophic - NEVER reuse a nonce with the same key.

Example

import kryptos/crypto
import kryptos/stream

let key = crypto.random_bytes(32)
let nonce = crypto.random_bytes(24)
let assert Ok(cipher) = stream.xchacha20(key, nonce:, counter: 0)
let assert Ok(ciphertext) = stream.xor(cipher, <<"secret":utf8>>)
let assert Ok(plaintext) = stream.xor(cipher, ciphertext)
// plaintext == <<"secret":utf8>>

Types

StreamCipher

opaque </>

A stream cipher with its key, nonce, and starting block counter.

pub opaque type StreamCipher

Values

chacha20

</> 
pub fn chacha20(
  key: BitArray,
  nonce nonce: BitArray,
  counter counter: Int,
) -> Result(StreamCipher, Nil)

Creates a ChaCha20 cipher (RFC 8439).

The key must be exactly 32 bytes, the nonce exactly 12 bytes, and the counter between 0 and 2^32 - 1.

xchacha20

</> 
pub fn xchacha20(
  key: BitArray,
  nonce nonce: BitArray,
  counter counter: Int,
) -> Result(StreamCipher, Nil)

Creates an XChaCha20 cipher (draft-irtf-cfrg-xchacha).

The key must be exactly 32 bytes, the nonce exactly 24 bytes, and the counter between 0 and 2^32 - 1. The extended nonce is large enough to be generated randomly.

xor

</> 
pub fn xor(
  cipher: StreamCipher,
  data: BitArray,
) -> Result(BitArray, Nil)

XORs data with the keystream. The same call encrypts plaintext and decrypts ciphertext, and the output is the same size as the input.

Every call starts at the cipher’s counter, so XORing two messages with the same cipher reuses the keystream. To continue a stream, create a new cipher with the counter advanced by the number of 64-byte blocks already used.

Returns an error if the data would need a block counter above 2^32 - 1.

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