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use elliptic_curve::{
digest::{BlockInput, FixedOutput, Reset, Update},
ff::PrimeField,
generic_array::GenericArray,
ops::Invert,
scalar::{NonZeroScalar, Scalar},
zeroize::{Zeroize, Zeroizing},
FieldBytes, FromDigest, ProjectiveArithmetic,
};
use hmac::{Hmac, Mac, NewMac};
pub fn generate_k<C, D>(
secret_scalar: &NonZeroScalar<C>,
msg_digest: D,
additional_data: &[u8],
) -> Zeroizing<NonZeroScalar<C>>
where
C: ProjectiveArithmetic,
D: FixedOutput<OutputSize = C::FieldSize> + BlockInput + Clone + Default + Reset + Update,
FieldBytes<C>: From<Scalar<C>> + for<'r> From<&'r Scalar<C>>,
Scalar<C>:
PrimeField<Repr = FieldBytes<C>> + FromDigest<C> + Invert<Output = Scalar<C>> + Zeroize,
{
let mut x = secret_scalar.to_repr();
let h1 = Scalar::<C>::from_digest(msg_digest).to_repr();
let mut hmac_drbg = HmacDrbg::<D>::new(&x, &h1, additional_data);
x.zeroize();
loop {
if let Some(k) = NonZeroScalar::from_repr(hmac_drbg.next()) {
return Zeroizing::new(k);
}
}
}
struct HmacDrbg<D>
where
D: BlockInput + FixedOutput + Clone + Default + Reset + Update,
{
k: Hmac<D>,
v: GenericArray<u8, D::OutputSize>,
}
impl<D> HmacDrbg<D>
where
D: BlockInput + FixedOutput + Clone + Default + Reset + Update,
{
pub fn new(entropy_input: &[u8], nonce: &[u8], additional_data: &[u8]) -> Self {
let mut k = Hmac::new(&Default::default());
let mut v = GenericArray::default();
for b in &mut v {
*b = 0x01;
}
for i in 0..=1 {
k.update(&v);
k.update(&[i]);
k.update(entropy_input);
k.update(nonce);
k.update(additional_data);
k = Hmac::new_varkey(&k.finalize().into_bytes()).unwrap();
k.update(&v);
v = k.finalize_reset().into_bytes();
}
Self { k, v }
}
pub fn next(&mut self) -> GenericArray<u8, D::OutputSize> {
self.k.update(&self.v);
let t = self.k.finalize_reset().into_bytes();
self.k.update(&t);
self.k.update(&[0x00]);
self.k = Hmac::new_varkey(&self.k.finalize_reset().into_bytes()).unwrap();
self.k.update(&t);
self.v = self.k.finalize_reset().into_bytes();
t
}
}
#[cfg(test)]
mod tests {
use super::generate_k;
use elliptic_curve::{dev::NonZeroScalar, ff::PrimeField};
use hex_literal::hex;
use sha2::{Digest, Sha256};
#[test]
fn appendix_2_5_test_vector() {
let x = NonZeroScalar::from_repr(
hex!("c9afa9d845ba75166b5c215767b1d6934e50c3db36e89b127b8a622b120f6721").into(),
)
.unwrap();
let digest = Sha256::new().chain("sample");
let k = generate_k(&x, digest, &[]);
assert_eq!(
k.to_repr().as_slice(),
&hex!("a6e3c57dd01abe90086538398355dd4c3b17aa873382b0f24d6129493d8aad60")[..]
);
}
}