darkfi/script/research/x3dh/src/hmac.rs

//! HMAC simplementation.
//! https://en.wikipedia.org/wiki/Hmac
use digest::{
    core_api::Block, crypto_common::BlockSizeUser, Digest, FixedOutput, Output, OutputSizeUser,
    Update,
};

const IPAD: u8 = 0x36;
const OPAD: u8 = 0x5C;

fn get_der_key<D: Digest + BlockSizeUser + Clone>(key: &[u8]) -> Block<D> {
    let mut der_key = Block::<D>::default();
    // The key that HMAC processes must be the same as the block size
    // of the underlying hash function. If the provided key is smaller
    // than that, we just pad it with zeroes. If it's larger, we hash
    // it and then pad it with zeroes.
    if key.len() <= der_key.len() {
        der_key[..key.len()].copy_from_slice(key);
        return der_key
    }

    let hash = D::digest(key);
    // All commonly used hash functions have block size bigger than
    // output hash size, but to be extra rigorous we handle the
    // potential uncommon cases as well. The condition is calculated
    // at compile time, so this branch gets removed from final binary.
    if hash.len() <= der_key.len() {
        der_key[..hash.len()].copy_from_slice(&hash);
    } else {
        let n = der_key.len();
        der_key.copy_from_slice(&hash[..n]);
    }

    der_key
}

/// HMAC for arbitrary hash functions that implement `Digest`
/// and `BlockSizeUser` traits.
#[derive(Clone)]
pub struct Hmac<D: Digest + BlockSizeUser + Clone> {
    digest: D,
    opad_key: Block<D>,
}

impl<D: Digest + BlockSizeUser + Clone> Hmac<D> {
    /// Initialize a new `Hmac` with the given key.
    #[inline]
    pub fn new_from_slice(key: &[u8]) -> Self {
        let der_key = get_der_key::<D>(key);

        let mut ipad_key = der_key.clone();
        for b in ipad_key.iter_mut() {
            *b ^= IPAD;
        }

        let mut digest = D::new();
        digest.update(&ipad_key);

        let mut opad_key = der_key;
        for b in opad_key.iter_mut() {
            *b ^= OPAD;
        }

        Self { digest, opad_key }
    }

    /// Finalize the HMAC
    pub fn finalize(self) -> Output<D> {
        Output::<D>::clone_from_slice(&self.finalize_fixed())
    }
}

impl<D: Digest + BlockSizeUser + Clone> FixedOutput for Hmac<D> {
    fn finalize_into(self, out: &mut Output<Self>) {
        let mut h = D::new();
        h.update(&self.opad_key);
        h.update(&self.digest.finalize());
        h.finalize_into(out);
    }
}

impl<D: Digest + BlockSizeUser + Clone> OutputSizeUser for Hmac<D> {
    type OutputSize = D::OutputSize;
}

impl<D: Digest + BlockSizeUser + Clone> Update for Hmac<D> {
    /// Update the HMAC with the given data.
    fn update(&mut self, data: &[u8]) {
        self.digest.update(data);
    }
}