Ability to construct hmac::Algorithm

[littledivy]

Hi 👋
I'm working on Deno's WebCrypto implementation. Webcrypto allows modifying length of the key, which is not possible since hmac::Algorithm digest field is private.

ring/src/hmac.rs

Line 116 in 1c36465

pub struct Algorithm(&'static digest::Algorithm);

Would you be interested in making this public? or are there other ways to do this? 🙂 Thanks!

[briansmith]

hash A DOMString representing the name of the digest function to use. You can pass any of SHA-1, SHA-256, SHA-384, or SHA-512 here.

Your project probably has a function that maps from strings to &;static digest::Algorithm. Instead, change that map to be from strings to hmac::Algorithm or hkdf::Algorithm. You can get the underlying hmac::Algorithm from the hkdf::Algorithm and you can get the underlying digest::Algorithm from an hmac::Algorithm. We do this in Rustls and it works great. Check out rustls/rustls#457.

Alternatively, less ideally, you can easily write a function that returns an HMAC algorithm from a digest algorithm. Rustls use to do this before I refactored the code. Again, check out the above PR, this time looking at the old code, to see how to do it.

I intentionally make the hmac::Algorithm and hkdf::Algorithm types opaque because in the near future (or maybe even presently; I forget) there will be optimizations that rely on each hmac::Algorithm knowing more details about the digest algorithm than are available from a digest::Algorithm. So I don't intend to make an API to construct hmac::Algorithm from a digest::Algorithm.

[littledivy]

Yes, I directly map to hmac::Algorithm (code).

Here's a small example of my use case:

// Behaves as per WebCrypto HmacKeyGenParams. 
// https://developer.mozilla.org/en-US/docs/Web/API/HmacKeyGenParams#properties
fn generate_hmac(hash: String, length: Option<usize>) {
  let hmac_algorithm: ring::hmac::Algorithm = match hash { ... };
  // Ability to modify length (?)
  if let Some(length) = length {
    let mut alg = hmac_algorithm.digest_algorithm();
    alg.output_len = length;
    alg.chaining_len = length;
    hmac_algorithm = ring::hmac::Algorithm(alg);
  }
  // ...
  // generate key
}

Modifying the digest's output_len is what I'm looking for. Also, Thanks for the reply!

[briansmith]

Notice in the WebCrypto API documentation they recommend that the user uses the default length unless they have a good reason to use a different length. ring's API encourages that too, by having the generate function always use the default length.

Currently, if you need to generate a key with a different length, then you need to use ring::rand to generate a slice of random bytes, and then use hmac::Key::new to construct the key from those random bytes.

If you'd like, you can submit a PR that adds a generate_with_length function to hmac::Key that accepts the length of the key.

[littledivy]

Opened #1218