This is a small and portable implementation of the HMAC algorithm written in C.
Description from Wikipedia (emphasis added):
In cryptography, an HMAC (sometimes expanded as either keyed-hash message authentication code or hash-based message authentication code) is a specific type of message authentication code (MAC) involving a cryptographic hash function and a secret cryptographic key. As with any MAC, it may be used to simultaneously verify both the data integrity and the authenticity of a message.
HMAC can provide message authentication using a shared secret instead of using digital signatures with asymmetric cryptography. It trades off the need for a complex public key infrastructure by delegating the key exchange to the communicating parties, who are responsible for establishing and using a trusted channel to agree on the key prior to communication.
The API looks like this (I am using C99 <stdint.h>-style annotated types):
#define HMAC_SHA1_HASH_SIZE 20
/***********************************************************************'
* HMAC(K,m) : HMAC SHA1
* @param key : secret key
* @param keysize : key-length in bytes
* @param msg : msg to calculate HMAC over
* @param msgsize : msg-length in bytes
* @param output : writeable buffer with at least 20 bytes available
*/
void hmac_sha1(const uint8_t* key,
const uint32_t keysize,
const uint8_t* msg,
const uint32_t msgsize,
uint8_t* output);