Hi Ard, Thanks for taking the initiative on this. When I first discussed with DaveM porting WireGuard to the crypto API and doing Zinc later yesterday, I thought to myself, “I wonder if Ard might want to work on this with me…” and sent you a message on IRC. It didn’t occur to me that you were the one who had pushed this endeavor! I must admit, though, I’m a bit surprised to see how it’s appearing. When I wrote [1], I had really imagined postponing the goals of Zinc entirely, and instead writing a small shim that calls into the existing crypto API machinery. I imagined the series to look like this: 1. Add blake2s generic as a crypto API shash object. 2. Add blake2s x86_64 as a crypto API shash object. 3. Add curve25519 generic as a crypto API dh object. 4. Add curve25519 x86_64 as a crypto API dh object. 5. Add curve25519 arm as a crypto API dh object. 6. The unmodified WireGuard commit. 7. A “cryptoapi.c” file for WireGuard that provides definitions of the “just functions” approach of Zinc, but does so in terms of the crypto API’s infrastructure, with global per-cpu lists and a few locks to handle quick buffer and tfm reuse. I wouldn’t expect (7) to be pretty, for the various reasons that most people dislike the crypto API, but at least it would somewhat “work”, not affect the general integrity of WireGuard, and provide a clear path forward in an evolutionary manner for gradually, piecemeal, swapping out pieces of that for a Zinc-like thing, however that winds up appearing. Instead what we’ve wound up with in this series is a Frankenstein’s monster of Zinc, which appears to have basically the same goal as Zinc, and even much of the same implementation just moved to a different directory, but then skimps on making it actually work well and introduces problems. (I’ll elucidate on some specific issues later in this email so that we can get on the same page with regards to security requirements for WireGuard.) I surmise from this Zinc-but-not series is that what actually is going on here is mostly some kind of power or leadership situation, which is what you’ve described to me also at various other points and in person. I also recognize that I am at least part way to blame for whatever dynamic there has stagnated this process; let me try to rectify that: A principle objection you’ve had is that Zinc moves to its own directory, with its own name, and tries to segment itself off from the rest of the crypto API’s infrastructure. You’ve always felt this should be mixed in with the rest of the crypto API’s infrastructure and directory structures in one way or another. Let’s do both of those things – put this in a directory structure you find appropriate and hook this into the rest of the crypto API’s infrastructure in a way you find appropriate. I might disagree, which is why Zinc does things the way it does, but I’m open to compromise and doing things more your way. Another objection you’ve had is that Zinc replaces many existing implementations with its own. Martin wasn’t happy about that either. So let’s not do that, and we’ll have some wholesale replacement of implementations in future patchsets at future dates discussed and benched and bikeshedded independently from this. Finally, perhaps most importantly, Zinc’s been my design rather than our design. Let’s do this together instead of me git-send-email(1)-ing a v37. If the process of doing that together will be fraught with difficulty, I’m still open to the “7 patch series” with the ugly cryptoapi.c approach, as described at the top. But I think if we start with Zinc and whittle it down in accordance with the above, we’ll get something mutually acceptable, and somewhat similar to this series, with a few important exceptions, which illustrate some of the issues I see in this RFC: Issue 1) No fast implementations for the “it’s just functions” interface. This is a deal breaker. I know you disagree here and perhaps think all dynamic dispatch should be by loadable modules configured with userspace policy and lots of function pointers and dynamically composable DSL strings, as the current crypto API does it. But I think a lot of other people agree with me here (and they’ve chimed in before) that the branch predictor does things better, doesn’t have Spectre issues, and is very simple to read and understand. For reference, here’s what that kind of thing looks like: [2]. In this case, the relevance is that the handshake in WireGuard is extremely performance sensitive, in order to fend off DoS. One of the big design gambits in WireGuard is – can we make it 1-RTT to reduce the complexity of the state machine, but keep the crypto efficient enough that this is still safe to do from a DoS perspective. The protocol succeeds at this goal, but in many ways, just by a hair when at scale, and so I’m really quite loathe to decrease handshake performance. Here’s where that matters specifically: - Curve25519 does indeed always appear to be taking tiny 32 byte stack inputs in WireGuard. However, your statement, “the fact that they operate on small, fixed size buffers means that there is really no point in providing alternative, SIMD based implementations of these, and we can limit ourselves to generic C library version,” is just plain wrong in this case. Curve25519 only ever operates on 32 byte inputs, because these represent curve scalars and points. It’s not like a block cipher where parallelism helps with larger inputs or something. In this case, there are some pretty massive speed improvements between the generic C implementations and the optimized ones. Like huge. On both ARM and on Intel. And Curve25519 is the most expensive operation in WireGuard, and each handshake message invokes a few of them. (Aside - Something to look forward to: I’m in the process of getting a formally verified x86_64 ADX implementation ready for kernel usage, to replace our existing heavily-fuzzed one, which will be cool.) - Blake2s actually does benefit from the optimized code even for relatively short inputs. While you might have been focused on the super-super small inputs in noise.c, there are slightly larger ones in cookie.c, and these are the most sensitive computations to make in terms of DoS resistance; they’re on the “front lines” of the battle, if you will. (Aside - Arguably WireGuard may have benefited from using siphash with 128-bit outputs here, or calculated some security metrics for DoS resistance in the face of forged 64-bit outputs or something, or a different custom MAC, but hindsight is 20/20.) - While 25519 and Blake2s are already in use, the optimized versions of ChaPoly wind up being faster as well, even if it’s just hitting the boring SSE code. - On MIPS, the optimized versions of ChaPoly are a necessity. They’re boring integer/scalar code, but they do things that the compiler simply cannot do on the platform and we benefit immensely from it. Taken together, we simply can’t skimp on the implementations available on the handshake layer, so we’ll need to add some form of implementation selection, whether it’s the method Zinc uses ([2]), or something else we cook up together. Issue 2) Linus’ objection to the async API invasion is more correct than he realizes. I could re-enumerate my objections to the API there, but I think we all get it. It’s horrendous looking. Even the introduction of the ivpad member (what on earth?) in the skb cb made me shutter. But there’s actually another issue at play: wg_noise_handshake_begin_session→derive_keys→symmetric_key_init is all part of the handshake. We cannot afford to allocate a brand new crypto object, parse the DSL string, connect all those function pointers, etc. The allocations involved here aren’t really okay at all in that path. That’s why the cryptoapi.c idea above involves just using a pool of pre-allocated objects if we’re going to be using that API at all. Also keep in mind that WireGuard instances sometimes have hundreds of thousands of peers. I’d recommend leaving this synchronous as it exists now, as Linus suggested, and we can revisit that later down the road. There are a number of improvements to the async API we could make down the line that could make this viable in WireGuard. For example, I could imagine decoupling the creation of the cipher object from its keys and intermediate buffers, so that we could in fact allocate the cipher objects with their DSLs globally in a safe way, while allowing the keys and working buffers to come from elsewhere. This is deep plumbing into the async API, but I think we could get there in time. There’s also a degree of practicality: right now there is zero ChaPoly async acceleration hardware anywhere that would fit into the crypto API. At some point, it might come to exist and have incredible performance, and then we’ll both feel very motivated to make this work for WireGuard. But it might also not come to be (AES seems to have won over most of the industry), in which case, why hassle? Issue 3) WireGuard patch is out of date. This is my fault, because I haven’t posted in a long time. There are some important changes in the main WireGuard repo. I’ll roll another patch soon for this so we have something recent to work off of. Sorry about that. Issue 4) FPU register batching? When I introduced the simd_get/simd_put/simd_relax thing, people seemed to think it was a good idea. My benchmarks of it showed significant throughput improvements. Your patchset doesn’t have anything similar to this. But on the other hand, last I spoke with the x86 FPU guys, I thought they might actually be in the process of making simd_get/put obsolete with some internal plumbing to make restoration lazier. I’ll see tglx later today and will poke him about this, as this might already be a non-issue. So given the above, how would you like to proceed? My personal preference would be to see you start with the Zinc patchset and rename things and change the infrastructure to something that fits your preferences, and we can see what that looks like. Less appealing would be to do several iterations of you reworking Zinc from scratch and going through the exercises all over again, but if you prefer that I guess I could cope. Alternatively, maybe this is a lot to chew on, and we should just throw caution into the wind, implement cryptoapi.c for WireGuard (as described at the top), and add C functions to the crypto API sometime later? This is what I had envisioned in [1]. And for the avoidance of doubt, or in case any of the above message belied something different, I really am happy and relieved to have an opportunity to work on this _with you_, and I am much more open than before to compromise and finding practical solutions to the past political issues. Also, if you’re into chat, we can always spec some of the nitty-gritty aspects out over IRC or even the old-fashioned telephone. Thanks again for pushing this forward. Regards, Jason [1] https://lore.kernel.org/wireguard/CAHmME9pmfZAp5zd9BDLFc2fWUhtzZcjYZc2atTPTyNFFmEdHLg@xxxxxxxxxxxxxx/ [2] https://git.kernel.org/pub/scm/linux/kernel/git/zx2c4/linux.git/tree/lib/zinc/chacha20/chacha20-x86_64-glue.c?h=jd/wireguard#n54
