When you want to enter a place and you have to say a code word to be let in, it's "santo y seña".
-Para entrar al cuartel hay que dar santo y seña.
-Si no dan santo y seña, no dejes pasar a nadie.
As far as I know, this came from the army. The secret words were the name of the saint of the day and some other word or expression. That way a group of soldiers recognized each other.
Side note: In Mexico, we also use "santo y seña" to express that someone told you a story with every little detail.
-Me contó santo y seña de la boda.