private void
init_hmac(String macAlgorithm, SecretKey key) {
	try {
		hmac = Mac.getInstance(macAlgorithm);
		hmac.init(key);
	}
	catch (GeneralSecurityException ex) {
		throw new IllegalArgumentException("Caught security " +
						   "exception setting up " +
						   "HMAC.");
	}
}
Verifies the data (computes the secure hash and compares it to the input)
@param mac The HMAC generator
@param signature The signature to compare against
@param truncation_ok If true, the signature may be truncated; only the
number of bytes in the provided signature are compared.
@return true if the signature matches, false otherwise
src/main/java/core/org/xbill/DNS/TSIG.java