[nickpsecurity]

" if Tor made users untrackable by US intelligence, would US intelligence really keep funding it?"

On top of dragonwriter's answer, I'll point out that "U.S. intelligence" doesn't exist as one entity in the way you ask that question. There are a number of groups that cooperate in some ways and compete or just diverge in others. The NSA and FBI want Tor cracked the most to find their targets. Whereas, the State Dept and/or the CIA that back Tor's funding want to protect both dissidents and assets overseas from state-level agencies monitoring communications. They need it to be unbreakable for some set of nation-state attackers.

Now, that doesn't mean that it needs to be unbreakable for the NSA, etc. The original guidance I read on Tor even warned that global adversaries would probably break it. The Many Eyes collaborations have visibility into a lot of the network. They're probably also honeypotting it with high-bandwidth links. It's also written in a tricky protocol in unsafe language on OS's done similarly running untrustworthy apps. They'll probably always have attacks on it for at least worthwhile targets even if State and CIA don't want that. It will still be valuable in many threat models, including NSA if combined with other methods. Especially if about delaying rather than permanently denying them info.

[emodendroket]

Of course it is true that US intelligence is far from monolithic, but I think it's a useful-enough abstraction for the purpose of thinking about the issue.