ed25519-zebra/src/verification_key.rs

use core::convert::{TryFrom, TryInto};

use curve25519_dalek::{
    digest::Update,
    edwards::{CompressedEdwardsY, EdwardsPoint},
    scalar::Scalar,
    traits::IsIdentity,
};
use sha2::Sha512;
use zeroize::DefaultIsZeroes;

use crate::{Error, Signature};

/// A refinement type for `[u8; 32]` indicating that the bytes represent an
/// encoding of an Ed25519 verification key.
///
/// This is useful for representing an encoded verification key, while the
/// [`VerificationKey`] type in this library caches other decoded state used in
/// signature verification.
///
/// A `VerificationKeyBytes` can be used to verify a single signature using the
/// following idiom:
/// ```
/// use core::convert::TryFrom;
/// # use rand::thread_rng;
/// # use ed25519_zebra::*;
/// # let msg = b"Zcash";
/// # let sk = SigningKey::new(thread_rng());
/// # let sig = sk.sign(msg);
/// # let vk_bytes = VerificationKeyBytes::from(&sk);
/// VerificationKey::try_from(vk_bytes)
///     .and_then(|vk| vk.verify(&sig, msg));
/// ```
#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct VerificationKeyBytes(pub(crate) [u8; 32]);

impl core::fmt::Debug for VerificationKeyBytes {
    fn fmt(&self, fmt: &mut core::fmt::Formatter) -> core::fmt::Result {
        fmt.debug_tuple("VerificationKeyBytes")
            .field(&hex::encode(&self.0))
            .finish()
    }
}

impl AsRef<[u8]> for VerificationKeyBytes {
    fn as_ref(&self) -> &[u8] {
        &self.0[..]
    }
}

impl TryFrom<&[u8]> for VerificationKeyBytes {
    type Error = Error;
    fn try_from(slice: &[u8]) -> Result<VerificationKeyBytes, Error> {
        if slice.len() == 32 {
            let mut bytes = [0u8; 32];
            bytes[..].copy_from_slice(slice);
            Ok(bytes.into())
        } else {
            Err(Error::InvalidSliceLength)
        }
    }
}

impl From<[u8; 32]> for VerificationKeyBytes {
    fn from(bytes: [u8; 32]) -> VerificationKeyBytes {
        VerificationKeyBytes(bytes)
    }
}

impl From<VerificationKeyBytes> for [u8; 32] {
    fn from(refined: VerificationKeyBytes) -> [u8; 32] {
        refined.0
    }
}

/// A valid Ed25519 verification key.
///
/// This is also called a public key by other implementations.
///
/// This type holds decompressed state used in signature verification; if the
/// verification key may not be used immediately, it is probably better to use
/// [`VerificationKeyBytes`], which is a refinement type for `[u8; 32]`.
///
/// ## Zcash-specific consensus properties
///
/// Ed25519 checks are described in [§5.4.5][ps] of the Zcash protocol specification and in
/// [ZIP 215].  The verification criteria for an (encoded) verification key `A_bytes` are:
///
/// * `A_bytes` MUST be an encoding of a point `A` on the twisted Edwards form of
///   Curve25519, and non-canonical encodings MUST be accepted;
///
/// [ps]: https://zips.z.cash/protocol/protocol.pdf#concreteed25519
#[derive(Copy, Clone, Debug)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
#[cfg_attr(feature = "serde", serde(try_from = "VerificationKeyBytes"))]
#[cfg_attr(feature = "serde", serde(into = "VerificationKeyBytes"))]
#[allow(non_snake_case)]
pub struct VerificationKey {
    pub(crate) A_bytes: VerificationKeyBytes,
    pub(crate) minus_A: EdwardsPoint,
}

impl From<VerificationKey> for VerificationKeyBytes {
    fn from(vk: VerificationKey) -> VerificationKeyBytes {
        vk.A_bytes
    }
}

impl AsRef<[u8]> for VerificationKey {
    fn as_ref(&self) -> &[u8] {
        &self.A_bytes.0[..]
    }
}

impl Default for VerificationKey {
    fn default() -> VerificationKey {
        let identity: EdwardsPoint = Default::default();
        let identity_bytes = identity.compress().to_bytes();

        VerificationKey {
            A_bytes: VerificationKeyBytes::from(identity_bytes),
            minus_A: -identity,
        }
    }
}

impl DefaultIsZeroes for VerificationKey {}

impl From<VerificationKey> for [u8; 32] {
    fn from(vk: VerificationKey) -> [u8; 32] {
        vk.A_bytes.0
    }
}

impl TryFrom<VerificationKeyBytes> for VerificationKey {
    type Error = Error;
    #[allow(non_snake_case)]
    fn try_from(bytes: VerificationKeyBytes) -> Result<Self, Self::Error> {
        // * `A_bytes` and `R_bytes` MUST be encodings of points `A` and `R` respectively on the
        //   twisted Edwards form of Curve25519, and non-canonical encodings MUST be accepted;
        let A = CompressedEdwardsY(bytes.0)
            .decompress()
            .ok_or(Error::MalformedPublicKey)?;

        Ok(VerificationKey {
            A_bytes: bytes,
            minus_A: -A,
        })
    }
}

impl TryFrom<&[u8]> for VerificationKey {
    type Error = Error;
    fn try_from(slice: &[u8]) -> Result<VerificationKey, Error> {
        VerificationKeyBytes::try_from(slice).and_then(|vkb| vkb.try_into())
    }
}

impl TryFrom<[u8; 32]> for VerificationKey {
    type Error = Error;
    fn try_from(bytes: [u8; 32]) -> Result<Self, Self::Error> {
        VerificationKeyBytes::from(bytes).try_into()
    }
}

impl VerificationKey {
    /// Verify a purported `signature` on the given `msg`.
    ///
    /// ## Zcash-specific consensus properties
    ///
    /// Ed25519 checks are described in [§5.4.5][ps] of the Zcash protocol specification and in
    /// [ZIP215].  The verification criteria for an (encoded) signature `(R_bytes, s_bytes)` with
    /// (encoded) verification key `A_bytes` are:
    ///
    /// * `A_bytes` and `R_bytes` MUST be encodings of points `A` and `R` respectively on the
    ///   twisted Edwards form of Curve25519, and non-canonical encodings MUST be accepted;
    ///
    /// * `s_bytes` MUST represent an integer `s` less than `l`, the order of the prime-order
    ///   subgroup of Curve25519;
    ///
    /// * the verification equation `[8][s]B = [8]R + [8][k]A` MUST be satisfied;
    ///
    /// * the alternate verification equation `[s]B = R + [k]A`, allowed by RFC 8032, MUST NOT be
    ///   used.
    ///
    /// [ps]: https://zips.z.cash/protocol/protocol.pdf#concreteed25519
    /// [ZIP215]: https://zips.z.cash/zip-0215
    pub fn verify(&self, signature: &Signature, msg: &[u8]) -> Result<(), Error> {
        let k = Scalar::from_hash(
            Sha512::default()
                .chain(&signature.R_bytes[..])
                .chain(&self.A_bytes.0[..])
                .chain(msg),
        );
        self.verify_prehashed(signature, k)
    }

    /// Verify a signature with a prehashed `k` value. Note that this is not the
    /// same as "prehashing" in RFC8032.
    #[allow(non_snake_case)]
    pub(crate) fn verify_prehashed(&self, signature: &Signature, k: Scalar) -> Result<(), Error> {
        // `s_bytes` MUST represent an integer less than the prime `l`.
        let s = Option::<Scalar>::from(Scalar::from_canonical_bytes(signature.s_bytes))
            .ok_or(Error::InvalidSignature)?;
        // `R_bytes` MUST be an encoding of a point on the twisted Edwards form of Curve25519.
        let R = CompressedEdwardsY(signature.R_bytes)
            .decompress()
            .ok_or(Error::InvalidSignature)?;
        // We checked the encoding of A_bytes when constructing `self`.

        //       [8][s]B = [8]R + [8][k]A
        // <=>   [8]R = [8][s]B - [8][k]A
        // <=>   0 = [8](R - ([s]B - [k]A))
        // <=>   0 = [8](R - R')  where R' = [s]B - [k]A
        let R_prime = EdwardsPoint::vartime_double_scalar_mul_basepoint(&k, &self.minus_A, &s);

        if (R - R_prime).mul_by_cofactor().is_identity() {
            Ok(())
        } else {
            Err(Error::InvalidSignature)
        }
    }
}