NAME

crypto_verify16, crypto_verify32, crypto_verify64 — timing-safe data comparison

SYNOPSIS

#include <monocypher.h>

int
crypto_verify16(const uint8_t a[16], const uint8_t b[16]);

int
crypto_verify32(const uint8_t a[32], const uint8_t b[32]);

int
crypto_verify64(const uint8_t a[64], const uint8_t b[64]);

DESCRIPTION

Cryptographic operations often require comparison of secrets or values derived from secrets. Standard comparison functions like memcmp(3) tend to exit when they find the first difference, leaking information through timing differences.

As an example, say a message authentication code (MAC) is sent over the network along with a message, but the correct MAC is secret. If the attacker attempts a forgery, one does not want to reveal “your MAC is wrong, and it took 384 microseconds to tell”. If the next attempt takes 462 microseconds instead, it tells the attacker that they just guessed a byte correctly. That way, an attacker can derive the correct MAC byte by byte and successfully forge a message. This has led to practical attacks in the past.

To avoid such catastrophic failure, crypto_verify16(), crypto_verify32(), and crypto_verify64() provide comparison functions whose timing is independent from the content of their input. They compare the first 16, 32, or 64 bytes of the two byte arrays a and b.

When in doubt, prefer these functions over memcmp(3).

RETURN VALUES

These functions return 0 if the two memory chunks are the same and -1 otherwise.

SEE ALSO

intro(3monocypher)

HISTORY

The crypto_verify16(), crypto_verify32(), crypto_verify64(), functions first appeared in Monocypher 1.1.0. They replaced the crypto_memcmp() and crypto_zerocmp() functions that were present until Monocypher 1.0.1.

CAVEATS

Monocypher does not perform any input validation. Any deviation from the specified input and output length ranges results in undefined behaviour. Make sure your inputs are correct.