ACLU v Ashcroft and ACLU v Reno are really interesting to read, if you haven't already.

Part of the conclusion of the court at that time was (at least regarding the CDA):

In order to deny minors access to potentially harmful speech, the CDA effectively suppresses a large amount of speech that adults have a constitutional right to receive and to address to one another. That burden on adult speech is unacceptable if less restrictive alternatives would be at least as effective in achieving the legitimate purpose that the statute was enacted to serve

In ACLU vs. Ashcroft, the court ruled that less restrictive measures like Internet filters should be used, rather than the law in question (COPA).

I kind of think an argument exists that a system like what you mentioned with cryptographic keys could be a "less restrictive measures" given today's technology. But I think we should still be careful, and keep in mind that nearly all pornography (with the exception of obscenity -- a very narrowly defined category) is speech that enjoys strong protections under the First Amendment. So any decisions around restricting this free speech, regardless of our good intentions in protecting our children, can have unintended negative consequences around first amendment speech in general.

I assume the goal is not actually to keep kids from watching porn but rather to have a chilling effect on it

Probably a safe assumption. It's difficult to tend towards other conclusions when the state of Utah has declared pornography a public health crisis, for example. Children are often just a means to an end in laws and public conversation. But don't forget that most of these kinds of "protect the children" laws are often rooted in some sort of good intentions, so I can't completely ascribe malice to the actions of these lawmakers. Evil is often wrapped in good intentions.

By the way, part of the Free Speech Coalition's arguments in Utah was around the impossibility of actually implementing age verification as no system actually exists in Utah to enforce that. Utah's law essentially ducks the first amendment by outsourcing enforcement to private action rather than government action. Scary stuff.