How to use BLAKE3 with HKDF?

[jbis9051]

I attempted to use the following from https://github.com/RustCrypto/KDFs/

Hkdf::<CoreWrapper<CtVariableCoreWrapper<blake3::Hasher, U32>>>

however I am getting

error[E0277]: the trait bound `blake3::Hasher: digest::core_api::VariableOutputCore` is not satisfied
  --> peer/src/services/wpskka_client_handler.rs:65:27
   |
65 |         /*             code          */
   |                           ^^^^ the trait `digest::core_api::VariableOutputCore` is not implemented for `blake3::Hasher`
   |
   = note: required because of the requirements on the impl of `crypto_common::BlockSizeUser` for `digest::core_api::ct_variable::CtVariableCoreWrapper<blake3::Hasher, typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UTerm, typenum::bit::B1>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>>`
   = note: required because of the requirements on the impl of `digest::core_api::wrapper::CoreProxy` for `digest::core_api::wrapper::CoreWrapper<digest::core_api::ct_variable::CtVariableCoreWrapper<blake3::Hasher, typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UInt<typenum::uint::UTerm, typenum::bit::B1>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>, typenum::bit::B0>>>`

[oconnor663]

The hmac crate provides a SimpleHmac API for hashes that haven't integrated themselves with the new UpdateCore trait, but it looks like hkdf doesn't have something similar? I haven't put in the time yet to understand these new traits or how much work it would be to integrate this crate with them. See also #212 and RustCrypto/KDFs#54.

For what it's worth, BLAKE3 provides a derive_key, which supports many of the same use cases as HKDF. That could be an alternative.

[jbis9051]

@oconnor663 Thanks for the suggestion, unfortunately the spec calls for HMAC so I can't use derive_key.

---

Possibly a different issue, but I'm having trouble using SimpleHmac as well

use hmac::{Mac, SimpleHmac};
use blake3::{Hasher};

fn main() {
   let mut x = SimpleHmac::<Hasher>::new_from_slice(b"test");
   x.update(b"test2");
   println!("{:?}", x.finalize().into_bytes());
}

error[E0599]: the function or associated item `new_from_slice` exists for struct `SimpleHmac<blake3::Hasher>`, but its trait bounds were not satisfied
  --> src/main.rs:5:38
   |
5  |    let mut x = SimpleHmac::<Hasher>::new_from_slice(b"test");
   |                                      ^^^^^^^^^^^^^^ function or associated item cannot be called on `SimpleHmac<blake3::Hasher>` due to unsatisfied trait bounds
   |
  ::: /Users/jbis/.cargo/registry/src/github.com-1ecc6299db9ec823/hmac-0.12.0/src/simple.rs:14:1
   |
14 | pub struct SimpleHmac<D: Digest + BlockSizeUser> {
   | ------------------------------------------------
   | |
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: FixedOutput`
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: KeyInit`
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: MacMarker`
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: Mac`
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: Sized`
   | doesn't satisfy `SimpleHmac<blake3::Hasher>: Update`
   |
   = note: the following trait bounds were not satisfied:
           `SimpleHmac<blake3::Hasher>: Sized`
           which is required by `SimpleHmac<blake3::Hasher>: Mac`
           `SimpleHmac<blake3::Hasher>: KeyInit`
           which is required by `SimpleHmac<blake3::Hasher>: Mac`
           `SimpleHmac<blake3::Hasher>: Update`
           which is required by `SimpleHmac<blake3::Hasher>: Mac`
           `SimpleHmac<blake3::Hasher>: FixedOutput`
           which is required by `SimpleHmac<blake3::Hasher>: Mac`
           `SimpleHmac<blake3::Hasher>: MacMarker`
           which is required by `SimpleHmac<blake3::Hasher>: Mac`
           `&SimpleHmac<blake3::Hasher>: KeyInit`
           which is required by `&SimpleHmac<blake3::Hasher>: Mac`
           `&SimpleHmac<blake3::Hasher>: Update`
           which is required by `&SimpleHmac<blake3::Hasher>: Mac`
           `&SimpleHmac<blake3::Hasher>: FixedOutput`
           which is required by `&SimpleHmac<blake3::Hasher>: Mac`
           `&SimpleHmac<blake3::Hasher>: MacMarker`
           which is required by `&SimpleHmac<blake3::Hasher>: Mac`
           `&mut SimpleHmac<blake3::Hasher>: KeyInit`
           which is required by `&mut SimpleHmac<blake3::Hasher>: Mac`
           `&mut SimpleHmac<blake3::Hasher>: Update`
           which is required by `&mut SimpleHmac<blake3::Hasher>: Mac`
           `&mut SimpleHmac<blake3::Hasher>: FixedOutput`
           which is required by `&mut SimpleHmac<blake3::Hasher>: Mac`
           `&mut SimpleHmac<blake3::Hasher>: MacMarker`
           which is required by `&mut SimpleHmac<blake3::Hasher>: Mac`

This is with

[dependencies]
hmac = "0.12.0"
blake3 = { version = "1.3.0", features = ["traits-preview"] }

[tarcieri]

cc @newpavlov

[jbis9051]

Closed in favor of #224. HKDF uses Hmac. BLAKE3 isn't compatible with Hmac. However, it should be compatible with SimpleHmac (see the other issue). Once it works for SimpleHmac and RustCrypto/KDFs#56 is merged, blake3 will work with HKDF.