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//! Support for verifying SM2DSA signatures.
//!
//! ## Algorithm
//!
//! ```text
//! B1: verify whether r' in [1,n-1], verification failed if not
//! B2: verify whether s' in [1,n-1], verification failed if not
//! B3: set M'~=ZA || M'
//! B4: calculate e'=Hv(M'~)
//! B5: calculate t = (r' + s') modn, verification failed if t=0
//! B6: calculate the point (x1', y1')=[s']G + [t]PA
//! B7: calculate R=(e'+x1') modn, verification pass if yes, otherwise failed
//! ```
use super::Signature;
use crate::{
distid::hash_z, AffinePoint, DistId, EncodedPoint, FieldBytes, Hash, ProjectivePoint,
PublicKey, Scalar, Sm2,
};
use elliptic_curve::{
generic_array::typenum::Unsigned,
ops::{LinearCombination, Reduce},
point::AffineCoordinates,
sec1::ToEncodedPoint,
Curve, Group,
};
use signature::{hazmat::PrehashVerifier, Error, Result, Verifier};
use sm3::{digest::Digest, Sm3};
#[cfg(feature = "alloc")]
use alloc::{boxed::Box, string::String};
/// SM2DSA public key used for verifying signatures are valid for a given
/// message.
///
/// ## Usage
///
/// The [`signature`] crate defines the following traits which are the
/// primary API for verifying:
///
/// - [`Verifier`]: verify a message against a provided key and signature
/// - [`PrehashVerifier`]: verify the low-level raw output bytes of a message digest
///
/// # `serde` support
///
/// When the `serde` feature of this crate is enabled, it provides support for
/// serializing and deserializing ECDSA signatures using the `Serialize` and
/// `Deserialize` traits.
///
/// The serialization leverages the encoding used by the [`PublicKey`] type,
/// which is a binary-oriented ASN.1 DER encoding.
#[derive(Clone, Debug)]
pub struct VerifyingKey {
/// Signer's public key.
public_key: PublicKey,
/// Signer's user information hash `Z`.
identity_hash: Hash,
/// Distinguishing identifier used to compute `Z`.
#[cfg(feature = "alloc")]
distid: String,
}
impl VerifyingKey {
/// Initialize [`VerifyingKey`] from a signer's distinguishing identifier
/// and public key.
pub fn new(distid: &DistId, public_key: PublicKey) -> Result<Self> {
let identity_hash = hash_z(distid, &public_key).map_err(|_| Error::new())?;
Ok(Self {
identity_hash,
public_key,
#[cfg(feature = "alloc")]
distid: distid.into(),
})
}
/// Initialize [`VerifyingKey`] from a SEC1-encoded public key.
pub fn from_sec1_bytes(distid: &DistId, bytes: &[u8]) -> Result<Self> {
let public_key = PublicKey::from_sec1_bytes(bytes).map_err(|_| Error::new())?;
Self::new(distid, public_key)
}
/// Initialize [`VerifyingKey`] from an affine point.
///
/// Returns an [`Error`] if the given affine point is the additive identity
/// (a.k.a. point at infinity).
pub fn from_affine(distid: &DistId, affine: AffinePoint) -> Result<Self> {
let public_key = PublicKey::from_affine(affine).map_err(|_| Error::new())?;
Self::new(distid, public_key)
}
/// Borrow the inner [`AffinePoint`] for this public key.
pub fn as_affine(&self) -> &AffinePoint {
self.public_key.as_affine()
}
/// Get the distinguishing identifier for this key.
#[cfg(feature = "alloc")]
pub fn distid(&self) -> &DistId {
self.distid.as_str()
}
/// Convert this [`VerifyingKey`] into the
/// `Elliptic-Curve-Point-to-Octet-String` encoding described in
/// SEC 1: Elliptic Curve Cryptography (Version 2.0) section 2.3.3
/// (page 10).
///
/// <http://www.secg.org/sec1-v2.pdf>
#[cfg(feature = "alloc")]
pub fn to_sec1_bytes(&self) -> Box<[u8]> {
self.public_key.to_sec1_bytes()
}
/// Compute message hash `e` according to [draft-shen-sm2-ecdsa § 5.2.1]
///
/// [draft-shen-sm2-ecdsa § 5.2.1]: https://datatracker.ietf.org/doc/html/draft-shen-sm2-ecdsa-02#section-5.2.1
pub(crate) fn hash_msg(&self, msg: &[u8]) -> Hash {
Sm3::new_with_prefix(self.identity_hash)
.chain_update(msg)
.finalize()
}
}
//
// `*Verifier` trait impls
//
impl PrehashVerifier<Signature> for VerifyingKey {
fn verify_prehash(&self, prehash: &[u8], signature: &Signature) -> Result<()> {
if prehash.len() != <Sm2 as Curve>::FieldBytesSize::USIZE {
return Err(Error::new());
}
// B1: verify whether r' in [1,n-1], verification failed if not
let r = signature.r(); // NonZeroScalar checked at signature parse time
// B2: verify whether s' in [1,n-1], verification failed if not
let s = signature.s(); // NonZeroScalar checked at signature parse time
// B4: calculate e'=Hv(M'~)
let e = Scalar::reduce_bytes(FieldBytes::from_slice(prehash));
// B5: calculate t = (r' + s') modn, verification failed if t=0
let t = *r + *s;
if t.is_zero().into() {
return Err(Error::new());
}
// B6: calculate the point (x1', y1')=[s']G + [t]PA
let x = ProjectivePoint::lincomb(
&ProjectivePoint::generator(),
&s,
&ProjectivePoint::from(&self.public_key),
&t,
)
.to_affine()
.x();
// B7: calculate R=(e'+x1') modn, verification pass if yes, otherwise failed
if *r == e + Scalar::reduce_bytes(&x) {
Ok(())
} else {
Err(Error::new())
}
}
}
impl Verifier<Signature> for VerifyingKey {
fn verify(&self, msg: &[u8], signature: &Signature) -> Result<()> {
// B3: set M'~=ZA || M'
let hash = self.hash_msg(msg);
self.verify_prehash(&hash, signature)
}
}
//
// Other trait impls
//
impl AsRef<AffinePoint> for VerifyingKey {
fn as_ref(&self) -> &AffinePoint {
self.as_affine()
}
}
impl From<VerifyingKey> for PublicKey {
fn from(verifying_key: VerifyingKey) -> PublicKey {
verifying_key.public_key
}
}
impl From<&VerifyingKey> for PublicKey {
fn from(verifying_key: &VerifyingKey) -> PublicKey {
verifying_key.public_key
}
}
impl ToEncodedPoint<Sm2> for VerifyingKey {
fn to_encoded_point(&self, compress: bool) -> EncodedPoint {
self.as_affine().to_encoded_point(compress)
}
}