dcrypt-sign 2.0.0

//! ECDSA implementation for NIST P-256 curve
//!
//! This implementation follows FIPS 186-4: Digital Signature Standard (DSS)
//! and SP 800-56A Rev. 3: Recommendation for Pair-Wise Key-Establishment Schemes
//! Using Discrete Logarithm Cryptography

use crate::ecdsa::common::{is_canonical_nonzero_scalar, is_high_s, SignatureComponents};
use dcrypt_algorithms::ec::p256 as ec;
use dcrypt_algorithms::hash::sha2::Sha256;
use dcrypt_algorithms::hash::HashFunction;
use dcrypt_algorithms::mac::hmac::Hmac;
use dcrypt_api::{error::Error as ApiError, Result as ApiResult, Signature as SignatureTrait};
use dcrypt_internal::constant_time::ct_eq;
use dcrypt_params::traditional::ecdsa::NIST_P256;
use rand::{CryptoRng, RngCore};
use zeroize::Zeroize;

/// ECDSA signature scheme using NIST P-256 curve (secp256r1)
///
/// Implements ECDSA as specified in FIPS 186-4, Section 6
pub struct EcdsaP256;

/// P-256 public key in uncompressed format (0x04 || X || Y)
///
/// Format: 65 bytes total (1 byte prefix + 32 bytes X + 32 bytes Y)
#[derive(Clone, Zeroize)]
pub struct EcdsaP256PublicKey(pub [u8; ec::P256_POINT_UNCOMPRESSED_SIZE]);

/// P-256 secret key
///
/// Contains both the raw scalar value and its byte representation
/// for efficient operations. The scalar d must satisfy 1 ≤ d ≤ n-1
/// where n is the order of the base point G.
#[derive(Clone)]
pub struct EcdsaP256SecretKey {
    raw: ec::Scalar,
    bytes: [u8; ec::P256_SCALAR_SIZE],
}

// Manual Zeroize implementation for EcdsaP256SecretKey
impl Zeroize for EcdsaP256SecretKey {
    fn zeroize(&mut self) {
        self.raw.zeroize();
        // Zeroize the byte representation
        self.bytes.zeroize();
    }
}

// Secure cleanup on drop
impl Drop for EcdsaP256SecretKey {
    fn drop(&mut self) {
        self.zeroize();
    }
}

/// P-256 signature encoded in ASN.1 DER format
///
/// Format: SEQUENCE { r INTEGER, s INTEGER }
#[derive(Clone)]
pub struct EcdsaP256Signature(pub Vec<u8>);

// AsRef/AsMut implementations for byte access
impl AsRef<[u8]> for EcdsaP256PublicKey {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

impl AsMut<[u8]> for EcdsaP256PublicKey {
    fn as_mut(&mut self) -> &mut [u8] {
        &mut self.0
    }
}

impl AsRef<[u8]> for EcdsaP256SecretKey {
    fn as_ref(&self) -> &[u8] {
        &self.bytes
    }
}

// REMOVED: AsMut<[u8]> for EcdsaP256SecretKey
// This implementation was removed for security reasons.
// Direct mutation of secret key bytes could create invalid keys
// outside the valid range [1, n-1], leading to security vulnerabilities.

impl AsRef<[u8]> for EcdsaP256Signature {
    fn as_ref(&self) -> &[u8] {
        &self.0
    }
}

impl AsMut<[u8]> for EcdsaP256Signature {
    fn as_mut(&mut self) -> &mut [u8] {
        &mut self.0
    }
}

impl SignatureTrait for EcdsaP256 {
    type PublicKey = EcdsaP256PublicKey;
    type SecretKey = EcdsaP256SecretKey;
    type SignatureData = EcdsaP256Signature;
    type KeyPair = (Self::PublicKey, Self::SecretKey);

    fn name() -> &'static str {
        "ECDSA-P256"
    }

    /// Generate an ECDSA key pair
    ///
    /// Generates a random private key d ∈ [1, n-1] and computes
    /// the corresponding public key Q = d·G where G is the base point.
    ///
    /// Reference: FIPS 186-4, Appendix B.4.1
    fn keypair<R: CryptoRng + RngCore>(rng: &mut R) -> ApiResult<Self::KeyPair> {
        // Generate EC keypair with private key in valid range [1, n-1]
        let (sk_scalar, pk_point) = ec::generate_keypair(rng).map_err(ApiError::from)?;

        // Serialize the private key scalar
        let sk_bytes: [u8; ec::P256_SCALAR_SIZE] = sk_scalar.serialize();

        // Verify the private key is non-zero (should never happen with proper generation)
        if sk_bytes.iter().all(|&b| b == 0) {
            return Err(ApiError::InvalidParameter {
                context: "ECDSA-P256 keypair",
                #[cfg(feature = "std")]
                message: "Generated secret key is zero (internal error)".to_string(),
            });
        }

        // Create the secret key structure
        let secret_key = EcdsaP256SecretKey {
            raw: sk_scalar,
            bytes: sk_bytes,
        };

        // Serialize public key in uncompressed format
        let public_key = EcdsaP256PublicKey(pk_point.serialize_uncompressed());

        Ok((public_key, secret_key))
    }

    fn public_key(keypair: &Self::KeyPair) -> Self::PublicKey {
        keypair.0.clone()
    }

    fn secret_key(keypair: &Self::KeyPair) -> Self::SecretKey {
        keypair.1.clone()
    }

    /// Sign a message using ECDSA
    ///
    /// Implements the ECDSA signature generation algorithm as specified in
    /// FIPS 186-4, Section 6.3, with deterministic nonce generation per
    /// RFC 6979 hedged with additional entropy (FIPS 186-5, Section 6.4).
    ///
    /// Algorithm:
    /// 1. e = HASH(M), where HASH is SHA-256
    /// 2. z = the leftmost min(N, bitlen(e)) bits of e, where N = 256
    /// 3. Generate k deterministically per RFC 6979 with extra entropy
    /// 4. (x₁, y₁) = k·G
    /// 5. r = x₁ mod n; if r = 0, go back to step 3
    /// 6. s = k⁻¹(z + rd) mod n; if s = 0, go back to step 3
    /// 7. Return signature (r, s)
    fn sign(message: &[u8], secret_key: &Self::SecretKey) -> ApiResult<Self::SignatureData> {
        // Step 1: Hash the message using SHA-256 (FIPS 186-4 approved hash function)
        let mut hasher = Sha256::new();
        hasher.update(message).map_err(ApiError::from)?;
        let hash_output = hasher.finalize().map_err(ApiError::from)?;

        // Step 2: Convert hash to integer z
        // For P-256, we use all 256 bits of the hash output
        let mut z_bytes = [0u8; ec::P256_SCALAR_SIZE];
        z_bytes.copy_from_slice(hash_output.as_ref());
        let z = ec::Scalar::from_bytes_reduced(z_bytes);

        // Get the private key scalar d
        let d = secret_key.raw.clone();

        // Generate ephemeral key using RFC 6979 deterministic + hedged nonce generation
        let mut rng = rand::thread_rng();

        loop {
            // Step 3: Derive deterministic k with extra entropy
            let k = deterministic_k_hedged(&d, &z, &mut rng);

            // Step 4: Compute (x₁, y₁) = k·G
            let kg = ec::scalar_mult_base_g(&k).map_err(ApiError::from)?;
            let r_bytes = kg.x_coordinate_bytes();

            // Step 5: Compute r = x₁ mod n
            let r = ec::Scalar::from_bytes_reduced(r_bytes);
            if r.is_zero() {
                continue;
            }

            // Compute k⁻¹ mod n
            let k_inv = k.inv_mod_n().map_err(ApiError::from)?;

            // Step 6: Compute s = k⁻¹(z + rd) mod n
            let rd = r.mul_mod_n(&d).map_err(ApiError::from)?;

            let z_plus_rd = z.add_mod_n(&rd).map_err(ApiError::from)?;

            let mut s = k_inv.mul_mod_n(&z_plus_rd).map_err(ApiError::from)?;

            // If s = 0, try again (extremely unlikely)
            if s.is_zero() {
                continue;
            }
            if is_high_s(&s.serialize(), &NIST_P256.n) {
                s = s.negate();
            }

            // Step 7: Create signature (r, s)
            let sig = SignatureComponents {
                r: r.serialize().to_vec(),
                s: s.serialize().to_vec(),
            };

            // Encode signature in DER format
            let der_sig = sig.to_der();

            return Ok(EcdsaP256Signature(der_sig));
        }
    }

    /// Verify an ECDSA signature
    ///
    /// Implements the ECDSA signature verification algorithm as specified in
    /// FIPS 186-4, Section 6.4.
    ///
    /// Algorithm:
    /// 1. Verify that r and s are integers in [1, n-1]
    /// 2. e = HASH(M), where HASH is SHA-256
    /// 3. z = the leftmost min(N, bitlen(e)) bits of e, where N = 256
    /// 4. w = s⁻¹ mod n
    /// 5. u₁ = zw mod n and u₂ = rw mod n
    /// 6. (x₁, y₁) = u₁·G + u₂·Q
    /// 7. If (x₁, y₁) = O, reject the signature
    /// 8. v = x₁ mod n
    /// 9. Accept the signature if and only if v = r
    fn verify(
        message: &[u8],
        signature: &Self::SignatureData,
        public_key: &Self::PublicKey,
    ) -> ApiResult<()> {
        // Parse signature from DER format
        let sig = SignatureComponents::from_der(&signature.0)?;

        // Step 1: Verify r and s are in valid range [1, n-1]
        if sig.r.len() > ec::P256_SCALAR_SIZE || sig.s.len() > ec::P256_SCALAR_SIZE {
            return Err(ApiError::InvalidSignature {
                context: "ECDSA-P256 verify",
                #[cfg(feature = "std")]
                message: "Invalid signature component size".to_string(),
            });
        }

        // Convert r and s to scalars (with proper padding)
        let mut r_bytes = [0u8; ec::P256_SCALAR_SIZE];
        let mut s_bytes = [0u8; ec::P256_SCALAR_SIZE];
        r_bytes[ec::P256_SCALAR_SIZE - sig.r.len()..].copy_from_slice(&sig.r);
        s_bytes[ec::P256_SCALAR_SIZE - sig.s.len()..].copy_from_slice(&sig.s);

        if !is_canonical_nonzero_scalar(&r_bytes, &NIST_P256.n)
            || !is_canonical_nonzero_scalar(&s_bytes, &NIST_P256.n)
        {
            return Err(ApiError::InvalidSignature {
                context: "ECDSA-P256 verify",
                #[cfg(feature = "std")]
                message: "signature components must be canonical integers in [1, n-1]".to_string(),
            });
        }

        let r = ec::Scalar::new(r_bytes).map_err(|_| ApiError::InvalidSignature {
            context: "ECDSA-P256 verify",
            #[cfg(feature = "std")]
            message: "Invalid r component".to_string(),
        })?;

        let s = ec::Scalar::new(s_bytes).map_err(|_| ApiError::InvalidSignature {
            context: "ECDSA-P256 verify",
            #[cfg(feature = "std")]
            message: "Invalid s component".to_string(),
        })?;
        if is_high_s(&s.serialize(), &NIST_P256.n) {
            return Err(ApiError::InvalidSignature {
                context: "ECDSA-P256 verify",
                #[cfg(feature = "std")]
                message: "high-s signatures are non-canonical".to_string(),
            });
        }

        // Step 2: Hash the message using SHA-256
        let mut hasher = Sha256::new();
        hasher.update(message).map_err(ApiError::from)?;
        let hash_output = hasher.finalize().map_err(ApiError::from)?;

        // Step 3: Convert hash to integer z
        let mut z_bytes = [0u8; ec::P256_SCALAR_SIZE];
        z_bytes.copy_from_slice(hash_output.as_ref());
        let z = ec::Scalar::from_bytes_reduced(z_bytes);

        // Step 4: Compute w = s⁻¹ mod n
        let s_inv = s.inv_mod_n().map_err(ApiError::from)?;

        // Step 5: Compute u₁ = zw mod n and u₂ = rw mod n
        let u1 = z.mul_mod_n(&s_inv).map_err(ApiError::from)?;
        let u2 = r.mul_mod_n(&s_inv).map_err(ApiError::from)?;

        // Parse the public key point Q
        let q = ec::Point::deserialize_uncompressed(&public_key.0).map_err(ApiError::from)?;

        // Step 6: Compute point (x₁, y₁) = u₁·G + u₂·Q
        let u1g = ec::scalar_mult_base_g(&u1).map_err(ApiError::from)?;

        let u2q = ec::scalar_mult(&u2, &q).map_err(ApiError::from)?;

        let point = u1g.add(&u2q);

        // Step 7: Check if point is identity (point at infinity)
        if point.is_identity() {
            return Err(ApiError::InvalidSignature {
                context: "ECDSA-P256 verify",
                #[cfg(feature = "std")]
                message: "Invalid signature: verification point is identity".to_string(),
            });
        }

        // Step 8: Compute v = x₁ mod n
        let x1_bytes = point.x_coordinate_bytes();
        let x1 = ec::Scalar::from_bytes_reduced(x1_bytes);

        // Step 9: Verify v = r using constant-time comparison
        if !ct_eq(r.serialize(), x1.serialize()) {
            return Err(ApiError::InvalidSignature {
                context: "ECDSA-P256 verify",
                #[cfg(feature = "std")]
                message: "Signature verification failed".to_string(),
            });
        }

        Ok(())
    }
}

/* ------------------------------------------------------------------------- */
/*                      RFC 6979 + extra-entropy helper                      */
/* ------------------------------------------------------------------------- */

/// Derive a deterministic nonce k as per RFC 6979 §3.2, hedged with
/// 32 bytes of RNG-supplied entropy (recommended by §3.6 / FIPS 186-5 §6.4).
///
/// This implementation combines deterministic nonce generation with additional
/// randomness to provide defense against weak RNG states while maintaining
/// the benefits of deterministic signatures for fault attack resistance.
fn deterministic_k_hedged<R: RngCore + CryptoRng>(
    d: &ec::Scalar,
    z: &ec::Scalar,
    rng: &mut R,
) -> ec::Scalar {
    use zeroize::Zeroize;

    let mut rbuf = [0u8; 32];
    rng.fill_bytes(&mut rbuf); // extra entropy R

    let mut v = [0x01u8; 32];
    let mut k = [0x00u8; 32];

    // ----- step C -----
    // K = HMAC_K(V || 0x00 || int2octets(x) || bits2octets(h1) || R)
    {
        let mut mac = Hmac::<Sha256>::new(&k).unwrap();
        mac.update(&v).unwrap();
        mac.update(&[0x00]).unwrap();
        mac.update(&d.serialize()).unwrap();
        mac.update(&z.serialize()).unwrap();
        mac.update(&rbuf).unwrap();
        k.copy_from_slice(&mac.finalize().unwrap());
    }

    // ----- step D -----
    // V = HMAC_K(V)
    let v_new = Hmac::<Sha256>::mac(&k, &v).unwrap();
    v.copy_from_slice(&v_new);

    // ----- step E -----
    // K = HMAC_K(V || 0x01 || int2octets(x) || bits2octets(h1) || R)
    {
        let mut mac = Hmac::<Sha256>::new(&k).unwrap();
        mac.update(&v).unwrap();
        mac.update(&[0x01]).unwrap();
        mac.update(&d.serialize()).unwrap();
        mac.update(&z.serialize()).unwrap();
        mac.update(&rbuf).unwrap();
        k.copy_from_slice(&mac.finalize().unwrap());
    }

    // ----- step F -----
    // V = HMAC_K(V)
    let v_new = Hmac::<Sha256>::mac(&k, &v).unwrap();
    v.copy_from_slice(&v_new);

    // ----- step G/H -----
    // Generate candidate k values until we find a valid one
    loop {
        let v_new = Hmac::<Sha256>::mac(&k, &v).unwrap();
        v.copy_from_slice(&v_new);
        if let Ok(candidate) = ec::Scalar::new(v) {
            if !candidate.is_zero() {
                rbuf.zeroize(); // scrub extra entropy
                return candidate;
            }
        }

        // retry path (step H): update K and V if candidate was invalid
        let mut mac = Hmac::<Sha256>::new(&k).unwrap();
        mac.update(&v).unwrap();
        mac.update(&[0x00]).unwrap();
        k.copy_from_slice(&mac.finalize().unwrap());
        let v_new = Hmac::<Sha256>::mac(&k, &v).unwrap();
        v.copy_from_slice(&v_new);
    }
}

#[cfg(test)]
mod tests;