RFC 2104 HMAC February 1997 5.Truncated output A well-known practice with message authentication codes is to truncate the output of the MAC and output only part of the bits (e.g., MM, ANSI).Preneel and van Oorschot show some analytical advantages of truncating the output of hash-based MAC functions.The results in this area are not absolute as for the overall security advantages of truncation. Di..