Hello. My review is the summary of a few discussions I've had with Peter and Christian regarding the API design being employed for hash2. I'm aware hash2 is inspired by one of Hinnat's proposals to standardisation that didn't come to fruition. I would like first to commend the authors for their work on this project. Having good libraries for this type of task is important, especially when the big names like OpenSSL provide not-so-great interfaces. Having said that, I have a few thoughts on what this library may be missing. *Spanification* There's a significant change in mood with regards to safe library design taking place in the industry. With clang, we now have -Wunsafe-buffer-usage, and this warning has been continuously improved recently. For reference https://clang.llvm.org/docs/SafeBuffers.html When I was going through this library's code I was left with a frustrating thought that boost should be at the forefront of this type of innovation: safe, and intuitive library design. Each function that takes a pair of void*/size_t should be a span<uint_8>. The model of how to approach these areas is a solved issue at this point. A robust span type with intuitive extensions would have matching constructors for all sorts of contiguous ranges, including native arrays, which would make using this library much simpler. It would also afford the necessary compile-time validations that you get with static extents in a span. At the current state, hash2 relies on a lot of pinky-promise guarantees that we tend to live with in C++, but that in fact can be avoided without much (or any) overhead. *Digest type* Due to limitations to how much of std::array is constexpr in different C++ versions, this library offers its own digest type. However, this just wraps a stack array. I feel that rather than returning a digest that is stack allocated, it would have been better for algorithms to take a fixed size span where the digest can be written to. I say this because in many cases a stack allocated array will be fine. However a mutable fixed span would provide more flexibility about the storage, for instance, a vector. There's also the matter of hardening (https://libcxx.llvm.org/Hardening.html) which one loses by using this particular digest type, rather than storage provided by a standard type. But I would also like to mention that sha3 has a variable digest size, so again, another reason why an out param mutable span would be a more flexible choice on the whole. *Boost Identity* I have been reluctant to write a review because it seems unfair to demand of hash2 to provide robust spanified interfaces, when no other library before was required to do so. However, at the same time, considering that this type of code tends to be used in safety-sensitive contexts, I fear this library may fail to achieve its potential, when a lot of its interfaces are unsafe. if I had to bring this library to any project I'm working on, I would have to wrap it into safe spanified interfaces, to hide away all these pointer/length pairs around, before I could use it in my code. However, I understand that to produce a spanified design, boost would have first to introduce a mature span implementation + extensions that can be leveraged for this type of code design. *Security* I think for things like HMAC, there is a question of allocators that zero-out memory, of secure obfuscated memory content in case of crash dumps, etc, that this library has not been attentive to, so I thought it was worth mentioning. *In conclusion* I'm here to support the authors. I don't feel like voting either accept or reject, because I think Boost as a whole has not provided the mission critical answer to that. I want this library to succeed. I just wish it was a bit more ambitious in its goal on the library-design space. Claudio.