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ecdsa: reduce z mod q when performing RFC6979 #777

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Jan 17, 2024
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ecdsa: reduce z mod q when performing RFC6979 #777

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12 changes: 10 additions & 2 deletions ecdsa/src/hazmat.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -86,7 +86,7 @@ where
/// described in [RFC6979] for computing ECDSA ephemeral scalar `k`.
///
/// Accepts the following parameters:
/// - `z`: message digest to be signed.
/// - `z`: message digest to be signed, i.e. `H(m)`. Does not have to be reduced in advance.
/// - `ad`: optional additional data, e.g. added entropy from an RNG
///
/// [RFC6979]: https://datatracker.ietf.org/doc/html/rfc6979
Expand All @@ -100,10 +100,18 @@ where
Self: From<ScalarPrimitive<C>> + Invert<Output = CtOption<Self>>,
D: Digest + BlockSizeUser + FixedOutput + FixedOutputReset,
{
// From RFC6979 § 2.4:
//
// H(m) is transformed into an integer modulo q using the bits2int
// transform and an extra modular reduction:
//
// h = bits2int(H(m)) mod q
let z2 = <Scalar<C> as Reduce<C::Uint>>::reduce_bytes(z);

let k = Scalar::<C>::from_repr(rfc6979::generate_k::<D, _>(
&self.to_repr(),
&C::ORDER.encode_field_bytes(),
z,
&z2.to_repr(),
ad,
))
.unwrap();
Expand Down