Short-input hashing

Example

#define SHORT_DATA ((const unsigned char *) "Sparkling water")
#define SHORT_DATA_LEN 15

unsigned char hash[crypto_shorthash_BYTES];
unsigned char key[crypto_shorthash_KEYBYTES];

crypto_shorthash_keygen(key);
crypto_shorthash(hash, SHORT_DATA, SHORT_DATA_LEN, key);

Purpose

Many applications and programming language implementations were recently found to be vulnerable to denial-of-service attacks when a hash function with weak security guarantees, such as Murmurhash 3, was used to construct a hash table.

In order to address this, Sodium provides the crypto_shorthash() function, which outputs short but unpredictable (without knowing the secret key) values suitable for picking a list in a hash table for a given key.

This function is optimized for short inputs.

The output of this function is only 64 bits. Therefore, it should not be considered collision-resistant.

Use cases:

• Hash tables
• Probabilistic data structures such as Bloom filters
• Integrity checking in interactive protocols

Usage

int crypto_shorthash(unsigned char *out, const unsigned char *in,
                     unsigned long long inlen, const unsigned char *k);

Compute a fixed-size (crypto_shorthash_BYTES bytes) fingerprint for the message in whose length is inlen bytes, using the key k.

The k is crypto_shorthash_KEYBYTES bytes and can be created using crypto_shorthash_keygen().

The same message hashed with the same key will always produce the same output.

Constants

• crypto_shorthash_BYTES
• crypto_shorthash_KEYBYTES

Algorithm details

SipHash-2-4

Notes

• The key has to remain secret. This function will not provide any mitigations against DoS attacks if the key is known from attackers.
• When building hash tables, it is recommended to use a prime number for the table size. This ensures that all bits from the output of the hash function are being used. Mapping the range of the hash function to [0..N) can be done efficiently without modulo reduction.
• libsodium >= 1.0.12 also implements a variant of SipHash with the same key size but a 128-bit output, accessible as crypto_shorthash_siphashx24().