IETF ChaCha20-Poly1305 construction

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The IETF variant of the ChaCha20-Poly1305 construction can safely encrypt a practically unlimited number of messages, but individual messages cannot exceed 64*(2^32)-64 bytes (approximatively 256 GiB).
Example (combined mode)
#define MESSAGE (const unsigned char *) "test"
#define MESSAGE_LEN 4
#define ADDITIONAL_DATA (const unsigned char *) "123456"
#define ADDITIONAL_DATA_LEN 6
​
unsigned char nonce[crypto_aead_chacha20poly1305_IETF_NPUBBYTES];
unsigned char key[crypto_aead_chacha20poly1305_IETF_KEYBYTES];
unsigned char ciphertext[MESSAGE_LEN + crypto_aead_chacha20poly1305_IETF_ABYTES];
unsigned long long ciphertext_len;
​
crypto_aead_chacha20poly1305_ietf_keygen(key);
randombytes_buf(nonce, sizeof nonce);
​
crypto_aead_chacha20poly1305_ietf_encrypt(ciphertext, &ciphertext_len,
                                          MESSAGE, MESSAGE_LEN,
                                          ADDITIONAL_DATA, ADDITIONAL_DATA_LEN,
                                          NULL, nonce, key);
​
unsigned char decrypted[MESSAGE_LEN];
unsigned long long decrypted_len;
if (crypto_aead_chacha20poly1305_ietf_decrypt(decrypted, &decrypted_len,
                                              NULL,
                                              ciphertext, ciphertext_len,
                                              ADDITIONAL_DATA,
                                              ADDITIONAL_DATA_LEN,
                                              nonce, key) != 0) {
    /* message forged! */
}
Combined mode
In combined mode, the authentication tag is directly appended to the encrypted message. This is usually what you want.
int crypto_aead_chacha20poly1305_ietf_encrypt(unsigned char *c,
                                              unsigned long long *clen_p,
                                              const unsigned char *m,
                                              unsigned long long mlen,
                                              const unsigned char *ad,
                                              unsigned long long adlen,
                                              const unsigned char *nsec,
                                              const unsigned char *npub,
                                              const unsigned char *k);
The crypto_aead_chacha20poly1305_ietf_encrypt() function encrypts a message m whose length is mlen bytes using a secret key k (crypto_aead_chacha20poly1305_IETF_KEYBYTES bytes) and public nonce npub (crypto_aead_chacha20poly1305_IETF_NPUBBYTES bytes).
The encrypted message, as well as a tag authenticating both the confidential message m and adlen bytes of non-confidential data ad, are put into c.
ad can be a NULL pointer with adlen equal to 0 if no additional data are required.
At most mlen + crypto_aead_chacha20poly1305_IETF_ABYTES bytes are put into c, and the actual number of bytes is stored into clen unless clen is a NULL pointer.
nsec is not used by this particular construction and should always be NULL.
The public nonce npub should never ever be reused with the same key. The recommended way to generate it is to use randombytes_buf() for the first message, and increment it for each subsequent message using the same key.
int crypto_aead_chacha20poly1305_ietf_decrypt(unsigned char *m,
                                              unsigned long long *mlen_p,
                                              unsigned char *nsec,
                                              const unsigned char *c,
                                              unsigned long long clen,
                                              const unsigned char *ad,
                                              unsigned long long adlen,
                                              const unsigned char *npub,
                                              const unsigned char *k);
The crypto_aead_chacha20poly1305_ietf_decrypt() function verifies that the ciphertext c (as produced by crypto_aead_chacha20poly1305_ietf_encrypt()) includes a valid tag using a secret key k, a public nonce npub, and additional data ad (adlen bytes).
ad can be a NULL pointer with adlen equal to 0 if no additional data are required.
nsec is not used by this particular construction and should always be NULL.
The function returns -1 if the verification fails.
If the verification succeeds, the function returns 0, puts the decrypted message into m and stores its actual number of bytes into mlen if mlen is not a NULL pointer.
At most clen - crypto_aead_chacha20poly1305_IETF_ABYTES bytes will be put into m.
Detached mode
Some applications may need to store the authentication tag and the encrypted message at different locations.
For this specific use case, "detached" variants of the functions above are available.
int crypto_aead_chacha20poly1305_ietf_encrypt_detached(unsigned char *c,
                                                       unsigned char *mac,
                                                       unsigned long long *maclen_p,
                                                       const unsigned char *m,
                                                       unsigned long long mlen,
                                                       const unsigned char *ad,
                                                       unsigned long long adlen,
                                                       const unsigned char *nsec,
                                                       const unsigned char *npub,
                                                       const unsigned char *k);
The crypto_aead_chacha20poly1305_ietf_encrypt_detached() function encrypts a message m with a key k and a nonce npub. It puts the resulting ciphertext, whose length is equal to the message, into c.
It also computes a tag that authenticates the ciphertext as well as optional, additional data ad of length adlen. This tag is put into mac, and its length is crypto_aead_chacha20poly1305_IETF_ABYTES bytes.
nsec is not used by this particular construction and should always be NULL.
int crypto_aead_chacha20poly1305_ietf_decrypt_detached(unsigned char *m,
                                                       unsigned char *nsec,
                                                       const unsigned char *c,
                                                       unsigned long long clen,
                                                       const unsigned char *mac,
                                                       const unsigned char *ad,
                                                       unsigned long long adlen,
                                                       const unsigned char *npub,
                                                       const unsigned char *k);
The crypto_aead_chacha20poly1305_ietf_decrypt_detached() function verifies that the authentication tag mac is valid for the ciphertext c of length clen bytes, the key k , the nonce npub and optional, additional data ad of length adlen bytes.
If the tag is not valid, the function returns -1 and doesn't do any further processing.
If the tag is valid, the ciphertext is decrypted and the plaintext is put into m. The length is equal to the length of the ciphertext.
nsec is not used by this particular construction and should always be NULL.
void crypto_aead_chacha20poly1305_ietf_keygen(unsigned char k[crypto_aead_chacha20poly1305_ietf_KEYBYTES]);
This helper function introduced in libsodium 1.0.12 creates a random key k.
It is equivalent to calling randombytes_buf() but improves code clarity and can prevent misuse by ensuring that the provided key length is always be correct.
Constants
crypto_aead_chacha20poly1305_IETF_ABYTES
crypto_aead_chacha20poly1305_IETF_KEYBYTES
crypto_aead_chacha20poly1305_IETF_NPUBBYTES
On earlier versions, use crypto_aead_chacha20poly1305_KEYBYTES and crypto_aead_chacha20poly1305_NPUBBYTES - The nonce size is the only constant that differs between the original variant and the IETF variant.
Algorithm details
Encryption: ChaCha20 stream cipher
Authentication: Poly1305 MAC
Notes
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 for each message.
To prevent nonce reuse in a client-server protocol, either use different keys for each direction, or make sure that a bit is masked in one direction, and set in the other.
See also
​ChaCha20 and Poly1305 for IETF protocols - Specification of the IETF variant
Original ChaCha20-Poly1305 construction
XChaCha20-Poly1305 construction
Last modified 9mo ago
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Outline
Example (combined mode)