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Generic hashing

Single-part example without a key

#define MESSAGE ((const unsigned char *) "Arbitrary data to hash")
#define MESSAGE_LEN 22

unsigned char hash[crypto_generichash_BYTES];

crypto_generichash(hash, sizeof hash,
                   MESSAGE, MESSAGE_LEN,
                   NULL, 0);

Single-part example with a key

#define MESSAGE ((const unsigned char *) "Arbitrary data to hash")
#define MESSAGE_LEN 22

unsigned char hash[crypto_generichash_BYTES];
unsigned char key[crypto_generichash_KEYBYTES];

randombytes_buf(key, sizeof key);

crypto_generichash(hash, sizeof hash,
                   MESSAGE, MESSAGE_LEN,
                   key, sizeof key);

Multi-part example with a key

#define MESSAGE_PART1 \
    ((const unsigned char *) "Arbitrary data to hash")
#define MESSAGE_PART1_LEN 22

#define MESSAGE_PART2 \
    ((const unsigned char *) "is longer than expected")
#define MESSAGE_PART2_LEN 23

unsigned char hash[crypto_generichash_BYTES];
unsigned char key[crypto_generichash_KEYBYTES];
crypto_generichash_state state;

randombytes_buf(key, sizeof key);

crypto_generichash_init(&state, key, sizeof key, sizeof hash);

crypto_generichash_update(&state, MESSAGE_PART1, MESSAGE_PART1_LEN);
crypto_generichash_update(&state, MESSAGE_PART2, MESSAGE_PART2_LEN);

crypto_generichash_final(&state, hash, sizeof hash);

Purpose

This API computes a fixed-length fingerprint for an arbitrarily long message.

Sample use cases:

  • File integrity checking
  • Creating unique identifiers to index arbitrarily long data

Usage

int crypto_generichash(unsigned char *out, size_t outlen,
                       const unsigned char *in, unsigned long long inlen,
                       const unsigned char *key, size_t keylen);

The crypto_generichash() function puts a fingerprint of the message in whose length is inlen bytes into out. The output size can be chosen by the application.

The minimum recommended output size is crypto_generichash_BYTES. This size makes it practically impossible for two messages to produce the same fingerprint.

However, for specific use cases, the size can be any value between crypto_generichash_BYTES_MIN (included) and crypto_generichash_BYTES_MAX (included).

key can be NULL and keylen can be 0. In this case, a message will always have the same fingerprint, like the MD5 or SHA-1 functions for which crypto_generichash() is a faster and more secure alternative.

But a key can also be specified. A message will always have the same fingerprint for a given key, but different keys used to hash the same message are very likely to produce distinct fingerprints.

In particular, the key can be used to make sure that different applications generate different fingerprints even if they process the same data.

The recommended key size is crypto_generichash_KEYBYTES bytes.

However, the key size can be any value between 0 (included) and crypto_generichash_KEYBYTES_MAX (included). If the key is meant to be secret, the recommended minimum length is crypto_generichash_KEYBYTES_MIN.

int crypto_generichash_init(crypto_generichash_state *state,
                            const unsigned char *key,
                            const size_t keylen, const size_t outlen);

int crypto_generichash_update(crypto_generichash_state *state,
                              const unsigned char *in,
                              unsigned long long inlen);

int crypto_generichash_final(crypto_generichash_state *state,
                             unsigned char *out, const size_t outlen);

The message doesn’t have to be provided as a single chunk. The generichash operation also supports a streaming API.

The crypto_generichash_init() function initializes a state state with a key key (that can be NULL) of length keylen bytes to eventually produce outlen bytes of output.

Each chunk of the complete message can then be sequentially processed by calling crypto_generichash_update(), providing the previously initialized state state, a pointer to the chunk in, and the length of the chunk in bytes, inlen.

The crypto_generichash_final() function completes the operation and puts the final fingerprint into out as outlen bytes.

After crypto_generichash_final() returns, the state should not be used any more, unless it is reinitialized using crypto_generichash_init().

This alternative API is especially useful to process very large files and data streams.

void crypto_generichash_keygen(unsigned char k[crypto_generichash_KEYBYTES]);

The crypto_generichash_keygen() function creates a key k of the recommended length crypto_generichash_KEYBYTES.

Constants

  • crypto_generichash_BYTES
  • crypto_generichash_BYTES_MIN
  • crypto_generichash_BYTES_MAX
  • crypto_generichash_KEYBYTES
  • crypto_generichash_KEYBYTES_MIN
  • crypto_generichash_KEYBYTES_MAX

Data types

  • crypto_generichash_state

Algorithm details

BLAKE2b

Notes

The crypto_generichash_* function set is implemented using BLAKE2b, a simple, standardized (RFC 7693), and secure hash function that is as strong as SHA-3 but faster than MD5 and SHA-1.

Unlike MD5, SHA-1, and SHA-256, this function is safe against hash length extension attacks.

With a key, the function can be used as a PRF.

BLAKE2b’s salt and personalization parameters are accessible through the lower-level functions whose prototypes are defined in crypto_generichash_blake2b.h.

BLAKE2b is not suitable for hashing passwords. For this purpose, use the crypto_pwhash API documented in the Password Hashing section.