The actual text of the Constitution is "Congress shall make no law ... abridging the freedom of speech", so I guess a first amendment argument comes down to the

The actual text of the Constitution is "Congress shall make no law ... abridging the freedom of speech", so I guess a first amendment argument comes down to the interpretation of "abridge" -- I would certainly think outlawing speech of a certain nature, i.e. encrypted speech, or authenticated speech, would be abridging that freedom. But I really don't really know how far removed the courts have taken the law from the original text.