A Vulnerability in Libsodium (opens in new tab)

I wrote a C++ implementation of the Framework for Integrated Test (FIT) called CeeFIT, and I was really proud of the way it registered fixtures at compile time.

Anyhow, I was surprised that more than one user was using CeeFIT as a sort of batch runner for C++ code, feeding in rows tabular data and executing it against their code. There were a couple bugs I had to fix to support their use cases.

I was just happy to have users.

>That is interesting to see the common fallacy of what we think users want versus what they really want.

Or a fallacy of what users think they want versus what they really want.

Non-cryptographers shouldn't use cryptographic primitives directly in security critical coffee paths. Libsodium tried to protect users from themselves in that regard. I think that's a worthy goal - library should try to make it impossible to use it incorrectly, which means high level primitives.

See also one of my favorite cryptographic essays, "If You're Typing The Letters A-E-S Into Your Code, You're Doing It Wrong" https://people.eecs.berkeley.edu/~daw/teaching/cs261-f12/mis...

> The important point is to be able to recognize that and not coerce users into using your project only how you envisioned it and only like that. Some projects are failure on that count having switched on dictatorial direction on that aspect.

There is certainly a balance there. If every function inside your code is now considered part of your API contract, almost anything is a breaking change and you can basically forget about ever meaningfully refactoring that codebase.

Many times making things private or marking them as internal-only is the right call.

I'm not really intimate enough with libsodium to judge if they made the right cut there or not in hindsight.

I found several libraries that simply didn't implement the check, but none that implemented in incorrectly in the same way as the vulnerability discussed above.

If you didn't receive an email from me, either your implementation isn't listed on https://ianix.com/pub/ed25519-deployment.html, I somehow missed it, or you're safe.

Thank you for your work on open source.

I work for a big company (Apple) but I have no idea who Frank is, nor how to sponsor them; and even if I knew them and how to sponsor them, the money would come directly from my pocket instead of Apple’s banking account.

You have a public repo of this?

Do note thought that X25519 and Ed25519 were designed so they wouldn’t need those checks at all. It’s only when you’re trying to design fancier protocols on top of Curve25519 or Edwards25519 that you can run into subgroup issues.

And for those use cases, I personally try my best to just reproject everything back into the prime order subgroup whenever possible. Monocypher has a number of such fancy functions:

  crypto_x25519_dirty_fast()
  crypto_x25519_dirty_small()
  crypto_elligator_map()
  crypto_elligator_rev()
  crypto_elligator_key_pair()

The dirty functions explicitly produce public keys that cover the entire curve, so that random such keys are truly indistinguishable from random when converted with `crypto_elligator_rev()`. But instead of just removing the clamp operation, I instead add random low-order point, so that when we later use the point in an X25519 key exchange, the shared secret is exactly the same as it would have been for a genuine X255119 key.

That’s where I thank DJB for designing a key exchange protocol that project the shared secret to the prime order subgroup, even when the public key it processes is not. The original intent may have been to make checks easier (low order keys all end up yielding zero), but a nice side effect is how it enabled a nice API for Mike Hamburg’s Elligator2.

> Accepting points outside the prime-order subgroup can quietly undermine higher-level assumptions, even if no immediate exploit is obvious.

If on the other hand we can prove that all computed results are low-order-component-independent (as is the case for X25519), then we know for sure we’re safe. In the end, Ristretto is only really needed when we can’t tweak the protocol to safely reproject to the prime order subgroup.

Don’t get me wrong, having a prime order group abstraction does help. But if someone is qualified to design a protocol that may require this, they’re qualified to try and make it work with a non-trivial cofactor as well — that, or prove it cannot be done.