CRYPTO_CHACHA20(3MONOCYPHER) | 3MONOCYPHER | CRYPTO_CHACHA20(3MONOCYPHER) |
NAME
ChaCha20 and XChaCha20 encryption functions#include
<monocypher.h>
void
crypto_chacha20
(uint8_t
*cipher_text, const uint8_t *plain_text,
size_t text_size, const uint8_t
key[32], const uint8_t nonce[8]);
void
crypto_xchacha20
(uint8_t
*cipher_text, const uint8_t *plain_text,
size_t text_size, const uint8_t
key[32], const uint8_t nonce[24]);
uint64_t
crypto_chacha20_ctr
(uint8_t
*cipher_text, const uint8_t *plain_text,
size_t text_size, const uint8_t
key[32], const uint8_t nonce[8],
uint64_t ctr);
uint64_t
crypto_xchacha20_ctr
(uint8_t
*cipher_text, const uint8_t *plain_text,
size_t text_size, const uint8_t
key[32], const uint8_t nonce[24],
uint64_t ctr);
DESCRIPTION
These functions provide an interface for the ChaCha20 encryption primitive.
ChaCha20 is a low-level primitive. Consider using authenticated encryption, implemented by crypto_lock().
The arguments are:
- key
- A 32-byte secret key.
- nonce
- An 8-byte or 24-byte number, used only once with any given key. It does not need to be secret or random, but it does have to be unique. Repeating a nonce with the same key reveals the XOR of two different messages, which allows decryption. 24-byte nonces can be selected at random. 8-byte nonces cannot because they are too small and the same nonce may be selected twice by accident. See intro() for advice about generating random numbers (use the operating system's random number generator).
- plain_text
- The message to encrypt. It is allowed to be
NULL
, in which case it will be interpreted as an all-zero input. cipher_text will then contain the raw ChaCha20 stream. - cipher_text
- The encrypted message.
- text_size
- Length of both plain_text and cipher_text, in bytes.
- ctr
- The number of 64-byte blocks since the beginning of the stream.
The key and nonce buffers may overlap. plain_text and cipher_text must either be the same buffer (for in-place encryption) or non-overlapping.
crypto_chacha20
()
performs a ChaCha20 operation. It uses an 8-byte nonce, which is too small
to be selected at random. Use a message counter as a nonce instead.
crypto_xchacha20
()
performs an XChaCha20 operation. It uses a 24-byte nonce, which is large
enough to be selected at random.
crypto_xchacha20
()
is recommended over crypto_chacha20
(). The ability
to use random nonces makes it easier to use securely, and the performance
hit is often negligible in practice.
The
crypto_chacha20
()
and crypto_xchacha20
() encrypt
plain_text by XORing it with a pseudo-random stream of
numbers, seeded by the provided key and
nonce.
Since XOR is its own inverse, decryption is the same operation as encryption. To decrypt the cipher text, “encrypt” it again with the same key and nonce. You will likely want to wipe the key when you are done with encryption or decryption. Use crypto_wipe() to wipe them.
The plain_text pointer is allowed
to be NULL
, in which case it will be interpreted as
an all-zero input. This is useful as a user space random number generator.
While this should not be
used as a random number generator for secrets, for which the
operating system random number generator should be preferred, it can be
handy outside of a security context. Deterministic procedural generation and
reproducible property-based tests come to mind. Additionally, it
can be used
to generate large amounts of random-looking data quickly – for
example to generate padding.
The
crypto_chacha20_ctr
()
and
crypto_xchacha20_ctr
()
perform ChaCha20 or XChaCha20 encryption, starting the stream at the block
ctr (which is the byte ‘ctr
× 64
’). This can be used to encrypt (or decrypt) part
of a long message or to implement some AEAD constructions such as the one
described in RFC 8439. When using this, be careful not to accidentally reuse
parts of the random stream as that would destroy confidentiality.
RETURN VALUES
crypto_chacha20
() and
crypto_xchacha20
() return nothing.
crypto_chacha20_ctr
() and
crypto_xchacha20_ctr
() functions return the next
ctr to use with the same key and nonce values; this is
always text_size divided by 64, plus one if there was
a remainder.
EXAMPLES
The following examples assume the existence of
arc4random_buf
(), which fills the given buffer with
cryptographically secure random bytes. If
arc4random_buf
() does not exist on your system, see
intro() for advice about how to
generate cryptographically secure random bytes.
Simple encryption:
uint8_t key [ 32]; /* Secret random key */ uint8_t nonce [ 24]; /* Unique nonce (possibly random) */ uint8_t plain_text [500] = {1}; /* Secret message */ uint8_t cipher_text[500]; /* Encrypted message */ arc4random_buf(key, 32); arc4random_buf(nonce, 24); crypto_xchacha20(cipher_text, plain_text, 500, key, nonce); /* Wipe secrets if they are no longer needed */ crypto_wipe(key, 32); crypto_wipe(plain_text, 500);
To decrypt the above:
uint8_t key [ 32]; /* Same key as above */ const uint8_t nonce [ 24]; /* Same nonce as above */ uint8_t plain_text [500]; /* Message to decrypt */ uint8_t cipher_text[500]; /* Secret message */ crypto_xchacha20(cipher_text, plain_text, 500, key, nonce); /* Wipe secrets if they are no longer needed */ crypto_wipe(key, 32); /* The plaintext likely needs to be processed before you wipe it */ crypto_wipe(plain_text, 12);
Incremental encryption (in blocks of 64 bytes):
uint8_t key [ 32]; /* Secret random key */ uint8_t nonce [ 24]; /* Unique nonce (possibly random) */ uint8_t plain_text [500]; /* Secret message */ uint8_t cipher_text[500]; /* Encrypted message */ uint64_t ctr = 0; /* Block counter */ unsigned int i; arc4random_buf(key, 32); arc4random_buf(nonce, 24); for(i = 0; i < 500; i += 64) { ctr = crypto_xchacha20_ctr(cipher_text+i, plain_text+i, 64, key, nonce, ctr); } /* Process data that didn't fit into 64-byte pieces */ crypto_xchacha20_ctr(cipher_text+500-(i-64), plain_text+500-(i-64), 500-(i-64), key, nonce, ctr); /* Wipe secrets if they are no longer needed */ crypto_wipe(key, 32); crypto_wipe(plain_text, 500);
Encryption by jumping around (do not do this, this is only meant
to show how crypto_xchacha20_ctr
() works):
uint8_t key [ 32]; /* Secret random key */ uint8_t nonce [ 24]; /* Unique nonce (possibly random) */ uint8_t plain_text [500] = {1}; /* Message to be encrypted */ uint8_t cipher_text[500]; /* Will be the encrypted message */ arc4random_buf(key, 32); arc4random_buf(nonce, 24); /* Encrypt the second part of the message first... */ crypto_xchacha20_ctr(cipher_text + (3 * 64), plain_text + (3 * 64), 500 - (3 * 64), key, nonce, 3); /* ...then encrypt the first part */ crypto_xchacha20_ctr(cipher_text, plain_text, 3 * 64, key, nonce, 0); /* Wipe secrets if they are no longer needed */ crypto_wipe(key, 32); crypto_wipe(plain_text, 500);
SEE ALSO
crypto_ietf_chacha20(), crypto_lock(), crypto_wipe(), intro()
STANDARDS
These functions implement ChaCha20 and XChaCha20. ChaCha20 is described in: Daniel J. Bernstein, ChaCha, a variant of Salsa20, SASC 2008 – The State of the Art of Stream Ciphers, pp. 273–278. The nonce and counter sizes were modified in RFC 8439. XChaCha20 derives from ChaCha20 the same way XSalsa20 derives from Salsa20 and benefits from the same security reduction (proven secure as long as ChaCha20 itself is secure).
HISTORY
crypto_chacha20
(),
crypto_chacha20_ctr
(),
crypto_xchacha20
(), and
crypto_xchacha20_ctr
() were added in Monocypher
3.0.0. They replace crypto_chacha20_encrypt
(),
crypto_chacha20_init
(),
crypto_chacha20_stream
(),
crypto_chacha20_x_init
(), and
crypto_chacha20_set_ctr
() that were deprecated in
Monocypher 3.0.0.
SECURITY CONSIDERATIONS
Encrypted does not mean secure
ChaCha20 only protects against eavesdropping, not forgeries. Most applications need protection against forgeries to be properly secure. To ensure the integrity of a message, use BLAKE2b in keyed mode or authenticated encryption; see crypto_blake2b() and crypto_lock().
Nonce reuse
Repeating a nonce with the same key exposes the XOR of two or more plaintext messages, effectively destroying confidentiality.
For the same reason,
do not select small nonces at
random. The crypto_chacha20
() nonce spans
only 64 bits, which is small enough to trigger accidental reuses. A message
counter should be used instead. If multiple parties send out messages, each
can start with an initial nonce of 0, 1, 2 (...) n-1 respectively, and
increment them by n for each new message. Make sure the counters never wrap
around.
Secure random number generation
Do not use these functions as a cryptographic random number generator. Always use the operating system's random number generator for cryptographic purposes; see intro().
Protection against side channels
Secrets should not dwell in memory longer than needed. Use crypto_wipe() to erase secrets you no longer need. For ChaCha20, this means the key and in some cases the plaintext itself.
June 11, 2021 | Debian |