Validating HMACs with incremental computation

[semicoleon]

As far as I can tell, it's currently not possible to create a MAC from data with swift-crypto. The only way to validate an existing MAC that wasn't computed by swift-crypto in the current process (e.g. a MAC that was sent as part of an HTTP request, or a MAC that was saved to disk between process launches) with the HMAC API is to use isValidAuthenticationCode, which requires that the data you're validating fits in contiguous memory.

Motivation:

Most of the HMAC API is oriented around allowing computing HMACs from incremental data. Not being able to validate a MAC with this mode of operation directly seems like an oversight in the API. I can compare the MAC bytes manually, but swift-crypto doesn't expose its safeCompare function so it wouldn't be constant time any more.

Importance:

Low

[semicoleon]

I knew there had to be something I was missing MessageAuthenticationCode has an equatable conformance which allows comparing a code to a type conforming to DataProtocol which allows comparing a computed code to one computed in another process