HMAC consists of 3 parts: compute kipad and kopad, mhsh=hash(kipad+msg), hash(kopad+msh). kipad and kopad are derived from the input key. When the length of key is greater than hash’s block size, K=hash(key). kipad is K XOR kip while kopad is K XOR kop, in which kip is a constant consisting of repeated bytes valued 0x36 block size times and kop is repeating 0x5c.
