ChaCha20

ChaCha20 is a stream cipher developed by Daniel J. Bernstein. Its original design expands a 256-bit key into 2^64 randomly accessible streams, each containing 2^64 randomly accessible 64-byte (512 bits) blocks. It is a variant of Salsa20 with better diffusion.

ChaCha20 doesn’t require any lookup tables and avoids the possibility of timing attacks.

Internally, ChaCha20 works like a block cipher used in counter mode. It includes an internal block counter to avoid incrementing the nonce after each block.

Two variants of the ChaCha20 cipher are implemented in libsodium:

• The original ChaCha20 cipher with a 64-bit nonce and a 64-bit counter, allowing a practically unlimited amount of data to be encrypted with the same (key, nonce) pair.
• The IETF variant increases the nonce size to 96 bits, but reduces the counter size down to 32 bits, allowing only up to 256 GB of data to be safely encrypted with a given (key, nonce) pair.

These primitives should only be used to implement protocols that specifically require them. For all other applications, it is recommended to use the high-level crypto_stream API (XSalsa20) or the ChaCha20-based construction with an extended nonce, XChaCha20 (crypto_stream_xchacha20).

Usage (original construction)

int crypto_stream_chacha20(unsigned char *c, unsigned long long clen,
                           const unsigned char *n, const unsigned char *k);

The crypto_stream_chacha20() function stores clen pseudo random bytes into c using a nonce n (crypto_stream_chacha20_NONCEBYTES bytes) and a secret key k (crypto_stream_chacha20_KEYBYTES bytes).

int crypto_stream_chacha20_xor(unsigned char *c, const unsigned char *m,
                               unsigned long long mlen, const unsigned char *n,
                               const unsigned char *k);

The crypto_stream_chacha20_xor() function encrypts a message m of length mlen using a nonce n (crypto_stream_chacha20_NONCEBYTES bytes) and a secret key k (crypto_stream_chacha20_KEYBYTES bytes).

The ciphertext is put into c. The ciphertext is the message combined with the output of the stream cipher using the XOR operation, and doesn’t include any authentication tag.

m and c can point to the same address (in-place encryption/decryption). If they don’t, the regions should not overlap.

int crypto_stream_chacha20_xor_ic(unsigned char *c, const unsigned char *m,
                                  unsigned long long mlen,
                                  const unsigned char *n, uint64_t ic,
                                  const unsigned char *k);

The crypto_stream_chacha20_xor_ic() function is similar to crypto_stream_chacha20_xor() but adds the ability to set the initial value of the block counter to a non-zero value, ic.

This permits direct access to any block without having to compute the previous ones.

m and c can point to the same address (in-place encryption/decryption). If they don’t, the regions should not overlap.

Usage (IETF variant, message length limited to 256 GB)

int crypto_stream_chacha20_ietf(unsigned char *c, unsigned long long clen,
                               const unsigned char *n, const unsigned char *k);

The crypto_stream_chacha20_ietf() function stores clen pseudo random bytes into c using a nonce n (crypto_stream_chacha20_ietf_NONCEBYTES bytes) and a secret key k (crypto_stream_chacha20_ietf_KEYBYTES bytes).

int crypto_stream_chacha20_ietf_xor(unsigned char *c, const unsigned char *m,
                                    unsigned long long mlen, const unsigned char *n,
                                    const unsigned char *k);

The crypto_stream_chacha20_ietf_xor() function encrypts a message m of length mlen using a nonce n (crypto_stream_chacha20_ietf_NONCEBYTES bytes) and a secret key k (crypto_stream_chacha20_ietf_KEYBYTES bytes).

The ciphertext is put into c. The ciphertext is the message combined with the output of the stream cipher using the XOR operation, and doesn’t include any authentication tag.

m and c can point to the same address (in-place encryption/decryption). If they don’t, the regions should not overlap.

int crypto_stream_chacha20_ietf_xor_ic(unsigned char *c, const unsigned char *m,
                                       unsigned long long mlen,
                                       const unsigned char *n, uint32_t ic,
                                       const unsigned char *k);

The crypto_stream_chacha20_ietf_xor_ic() function is similar to crypto_stream_chacha20_ietf_xor() but adds the ability to set the initial value of the block counter to a non-zero value, ic.

This permits direct access to any block without having to compute the previous ones.

m and c can point to the same address (in-place encryption/decryption). If they don’t, the regions should not overlap.

Constants

• crypto_stream_chacha20_KEYBYTES
• crypto_stream_chacha20_NONCEBYTES
• crypto_stream_chacha20_ietf_KEYBYTES
• crypto_stream_chacha20_ietf_NONCEBYTES

Notes

The nonce is short. In order to prevent nonce reuse, if a key is being reused, it is recommended to increment the previous nonce instead of generating a random nonce every time a new stream is required.

With the IETF variant, up to 256 GB can be produced from the a (key, nonce) pair. The original design doesn’t have this limitation.