Generic implementation of Hash-based Message Authentication Code (HMAC).

To use it you'll need a cryptographic hash function implementation from RustCrypto project. You can either import specific crate (e.g. sha2), or meta-crate crypto-hashes which reexport all related crates.

Usage

Let us demonstrate how to use HMAC using SHA256 as an example.

To get the authentication code:

extern crate hmac;
extern crate sha2;

use sha2::Sha256;
use hmac::{Hmac, Mac};

# fn main() {
// Create `Mac` trait implementation, namely HMAC-SHA256
let mut mac = Hmac::<Sha256>::new(b"my secret and secure key").unwrap();
mac.input(b"input message");

// `result` has type `MacResult` which is a thin wrapper around array of
// bytes for providing constant time equality check
let result = mac.result();
// To get underlying array use `code` method, but be carefull, since
// incorrect use of the code value may permit timing attacks which defeat
// the security provided by the `MacResult`
let code_bytes = result.code();
# }

To verify the message:

# extern crate hmac;
# extern crate sha2;
# use sha2::Sha256;
# use hmac::{Hmac, Mac};
# fn main() {
let mut mac = Hmac::<Sha256>::new(b"my secret and secure key").unwrap();

mac.input(b"input message");

# let mac2 = mac.clone();
# let code_bytes = mac2.result().code();
// `verify` will return `Ok(())` if code is correct, `Err(MacError)` otherwise
mac.verify(&code_bytes).unwrap();
# }

Block and input sizes

Usually it is assumed that block size is larger than output size, due to the generic nature of the implementation this edge case must be handled as well to remove potential panic scenario. This is done by truncating hash output to the hash block size if needed.