Don'tcha just know it bugs the dickens out of governments at how God encrypted the human brain beyond their reach?
Others cite First Amendment rights to speech to protect encryption. I don't see how the right to speak to those who want to listen is related to the right to not have one's speech heard by those one does not wish.
Then there is a bit of Fifth Amendment about not being required to self-incriminate. That doesn't work either else the signature on a check paying a bribe would not be acceptable evidence. Can't be forced to speak against oneself, but past speech is fair game. Encryption would be past speech.
If I was a justice on the SCOTUS I'd say the right to keep and bear arms in the Second Amendment has the most bearing. Recognizes citizens' right to own the means to defend against all comers, including the right to own tools which could be used against the government's wishes.
I'll upgrade my iPhone 5 when Apple announces the theoretically-impossible secure encrypted model (as opposed to the current theoretically breakable just hasn't been done).