Authenticated encryption

Last updated 2 months ago

Example

#define MESSAGE ((const unsigned char *) "test")
#define MESSAGE_LEN 4
#define CIPHERTEXT_LEN (crypto_secretbox_MACBYTES + MESSAGE_LEN)
unsigned char key[crypto_secretbox_KEYBYTES];
unsigned char nonce[crypto_secretbox_NONCEBYTES];
unsigned char ciphertext[CIPHERTEXT_LEN];
crypto_secretbox_keygen(key);
randombytes_buf(nonce, sizeof nonce);
crypto_secretbox_easy(ciphertext, MESSAGE, MESSAGE_LEN, nonce, key);
unsigned char decrypted[MESSAGE_LEN];
if (crypto_secretbox_open_easy(decrypted, ciphertext, CIPHERTEXT_LEN, nonce, key) != 0) {
/* message forged! */
}

Purpose

This operation:

  • Encrypts a message with a key and a nonce to keep it confidential

  • Computes an authentication tag. This tag is used to make sure that the message

    hasn't been tampered with before decrypting it.

A single key is used both to encrypt/sign and verify/decrypt messages. For this reason, it is critical to keep the key confidential.

The nonce doesn't have to be confidential, but it should never ever be reused with the same key. The easiest way to generate a nonce is to use randombytes_buf().

Messages encrypted are assumed to be independent. If multiple messages are sent using this API and random nonces, there will be no way to detect if a message has been received twice, or if messages have been reordered. If this is a requirement, see the encrypting a sequence of a set of related messages section.

Combined mode

In combined mode, the authentication tag and the encrypted message are stored together. This is usually what you want.

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

The crypto_secretbox_easy() function encrypts a message m whose length is mlen bytes, with a key k and a nonce n.

k should be crypto_secretbox_KEYBYTES bytes and n should be crypto_secretbox_NONCEBYTES bytes.

c should be at least crypto_secretbox_MACBYTES + mlen bytes long.

This function writes the authentication tag, whose length is crypto_secretbox_MACBYTES bytes, in c, immediately followed by the encrypted message, whose length is the same as the plaintext: mlen.

c and m can overlap, making in-place encryption possible. However do not forget that crypto_secretbox_MACBYTES extra bytes are required to prepend the tag.

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

The crypto_secretbox_open_easy() function verifies and decrypts a ciphertext produced by crypto_secretbox_easy().

c is a pointer to an authentication tag + encrypted message combination, as produced by crypto_secretbox_easy(). clen is the length of this authentication tag + encrypted message combination. Put differently, clen is the number of bytes written by crypto_secretbox_easy(), which is crypto_secretbox_MACBYTES + the length of the message.

The nonce n and the key k have to match the used to encrypt and authenticate the message.

The function returns -1 if the verification fails, and 0 on success. On success, the decrypted message is stored into m.

m and c can overlap, making in-place decryption possible.

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_secretbox_detached(unsigned char *c, unsigned char *mac,
const unsigned char *m,
unsigned long long mlen,
const unsigned char *n,
const unsigned char *k);

This function encrypts a message m of length mlen with a key k and a nonce n, and puts the encrypted message into c. Exactly mlen bytes will be put into c, since this function does not prepend the authentication tag. The tag, whose size is crypto_secretbox_MACBYTES bytes, will be put into mac.

int crypto_secretbox_open_detached(unsigned char *m,
const unsigned char *c,
const unsigned char *mac,
unsigned long long clen,
const unsigned char *n,
const unsigned char *k);

The crypto_secretbox_open_detached() function verifies and decrypts an encrypted message c whose length is clen. clen doesn't include the tag, so this length is the same as the plaintext.

The plaintext is put into m after verifying that mac is a valid authentication tag for this ciphertext, with the given nonce n and key k.

The function returns -1 if the verification fails, or 0 on success.

void crypto_secretbox_keygen(unsigned char k[crypto_secretbox_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_secretbox_KEYBYTES

  • crypto_secretbox_MACBYTES

  • crypto_secretbox_NONCEBYTES

Algorithm details

  • Encryption: XSalsa20 stream cipher

  • Authentication: Poly1305 MAC

Notes

Internally, crypto_secretbox calls crypto_stream_xor() to encrypt the message. As a result, a secret key used with the former should not be reused with the later. But as a general rule, a key should not be reused for different purposes.

The original NaCl crypto_secretbox API is also supported, albeit not recommended.

crypto_secretbox() takes a pointer to 32 bytes before the message, and stores the ciphertext 16 bytes after the destination pointer, the first 16 bytes being overwritten with zeros. crypto_secretbox_open() takes a pointer to 16 bytes before the ciphertext and stores the message 32 bytes after the destination pointer, overwriting the first 32 bytes with zeros.

The _easy and _detached APIs are faster and improve usability by not requiring padding, copying or tricky pointer arithmetic.