[sleepingeights]

I am a bit lost. How is cryptography about secrecy of the text through some operation, like a function, while at the same time being a system that defines itself?

Cryptography has nothing to do with secrecy at all. It has to do with maintaining the integrity of the communication (or specifically, the message) given highly active and adversarial entities.

Anyone who thinks they can have security through secrecy (i.e. 'cryptography') is a fool, imo.

[irishsultan]

> 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.