> It has to do with maintaining the integrity of the communication This requires secrecy, or else any adversary can do exactly what you did and fake a message.

> It has to do with maintaining the integrity of the communication This requires secrecy, or else any adversary can do exactly what you did and fake a message. Note that it doesn't require secrecy of the method, but it does require secrecy of at least some of the input (a key). Therefore whatever method you use should preserve the secrecy of that input to be suitable for cryptography.