Using HMAC as a generic - rust

Question

I am having trouble updating the crates hmac and digest. I have a function defined that takes a generic type of a HMAC function, and computes the HMAC over a given input. I have a function working with the version of hmac and digest being 0.7 and 0.8 respectively. However, I'm getting blocked when trying to get the same logic running for the latest versions 0.10 and 0.9 respectively.
In my machine, I use rustc 1.48.0 (7eac88abb 2020-11-16).
The working example has the following Cargo.toml dependencies
[dependencies]
hmac = "0.7"
sha2 = "0.8"
digest = "0.8"
The minimal working example is the following:
use sha2::{Sha256};
use hmac::{Mac, Hmac};
type HmacSha256 = Hmac<Sha256>;
use digest::generic_array::typenum::{U32};
pub struct Key([u8; 2]);
impl Key {
pub fn print_hmac<D>(&self, message: &[u8])
where
D: Mac<OutputSize = U32>,
{
let mut mac = D::new_varkey(self.0.as_ref()).unwrap();
mac.input(message);
let result = mac.result();
let code_bytes = result.code();
println!("{:?}", code_bytes)
}
}
pub fn main() {
let verif_key = Key([12u8, 33u8]);
verif_key.print_hmac::<HmacSha256>(&[83u8, 123u8]);
}
The above code works well, and compiles. However, when I try to upgrade the dependencies to the latests versions, everything breaks.
Updated Cargo.toml:
[dependencies]
hmac = "0.10"
sha2 = "0.9"
digest = "0.9"
With the updates, we have some changes in the nomenclature:
.input() -> .update()
.result() -> .finalize()
.code() -> .into_bytes()
When I try to run it, I get the following error
no function or associated item named 'new_varkey' found for type parameter 'D' in the current scope
So I tried to define the generic type to be NewMac (for that need to change the second line to use hmac::{Mac, Hmac, NewMac};). However, now the error is in the functions .update() and .finalize().
I've also tried to pass the Digest generic type, rather than the Hmac, as follows:
pub fn print_hmac<D>(&self, message: &[u8])
where
D: Digest,
{
let mut mac = Hmac::<D>::new_varkey(self.0.as_ref()).unwrap();
mac.update(message);
let result = mac.finalise();
let code_bytes = result.into_bytes();
println!("{:?}", code_bytes)
}
But still not working.
How should I handle the generic Hmac function for the updated crates?
Sorry for the long post, and I hope I made my problem clear. Thanks community!

Answer 1

It's good to look at example code which is updated across version updates, and luckily hmac has some tests in its repository.
Those tests use the new_test macro defined here in the crypto-mac crate. In particular, there is one line similar to yours...
let mut mac = <$mac as NewMac>::new_varkey(key).unwrap();
...which suggests that D should be cast to a NewMac implementor in your code as well.
After implementing the nomenclature updates you've already identified, your code works with the additional as NewMac cast from above, as well as the corresponding new + NewMac trait bound on D:
use sha2::{Sha256};
use hmac::{NewMac, Mac, Hmac};
type HmacSha256 = Hmac<Sha256>;
use digest::generic_array::typenum::{U32};
pub struct Key([u8; 2]);
impl Key {
pub fn print_hmac<D>(&self, message: &[u8])
where
D: Mac<OutputSize = U32> + NewMac, // `+ NewMac` input trait bound
{
let mut mac = <D as NewMac>::new_varkey(self.0.as_ref()).unwrap(); // `as NewMac` cast
mac.update(message);
let result = mac.finalize();
let code_bytes = result.into_bytes();
println!("{:?}", code_bytes)
}
}
pub fn main() {
let verif_key = Key([12u8, 33u8]);
verif_key.print_hmac::<HmacSha256>(&[83u8, 123u8]);
}