crypto_verify16(const uint8_t a, const uint8_t b);
crypto_verify32(const uint8_t a, const uint8_t b);
crypto_verify64(const uint8_t a, const uint8_t b);
Cryptographic operations often require comparison of secrets or values derived from secrets. Standard comparison functions like
memcmp() 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 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 lead to practical attacks in the past.
To avoid such catastrophic failure,
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
These functions return 0 if the two memory chunks are the same, -1 otherwise.
crypto_verify64() functions first appeared in Monocypher 1.1.0. They replaced the
crypto_zerocmp() functions that were present until Monocypher 1.0.1.
The man pages crypto_verify32.3monocypher(3) and crypto_verify64.3monocypher(3) are aliases of crypto_verify16.3monocypher(3).