ic-ec 0.2.3

X25519, Ed25519, and elliptic-curve arithmetic for IronCrypto
Documentation 
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//! SP 800-56A elliptic-curve Diffie-Hellman over the NIST prime curves.
//!
//! The shared secret is the x-coordinate of `[d]Q`, as SP 800-56A ยง5.7.1.2
//! specifies. It is **not** a key: it is a field element that must go through a
//! KDF before use. The ontology records that as a `serious` constraint.

use super::arith::Field;
use super::gentable::HasGeneratorTable;
use super::point::{AffinePoint, Curve, Point};
use ic_core::{ensure, Result};

/// Load and validate a private scalar.
fn load_scalar<C: Curve>(bytes: &[u8]) -> Result<C::Scalar> {
    ensure!(
        bytes.len() == C::SCALAR_BYTES,
        InvalidLength,
        "ecdh private key"
    );
    let d = C::scalar_from_slice(bytes).ok_or(ic_core::err!(
        InvalidParameter,
        "ecdh private key is not less than n"
    ))?;
    ensure!(
        !bool::from(d.is_zero()),
        InvalidParameter,
        "ecdh private key must not be zero"
    );
    Ok(d)
}

/// Compute the public key, SEC1 uncompressed.
pub fn public_key<C: HasGeneratorTable>(private_key: &[u8], out: &mut [u8]) -> Result<()> {
    ensure!(
        out.len() == 1 + 2 * C::FIELD_BYTES,
        InvalidLength,
        "ecdh public key buffer"
    );
    let d = load_scalar::<C>(private_key)?;
    let q = Point::<C>::mul_generator(&d)
        .to_affine()
        .ok_or(ic_core::err!(Internal, "public key is the identity"))?;
    // The length was checked above, so this can only fail if that check
    // and the encoder disagree. Propagating it keeps such a drift loud
    // rather than silently returning success with nothing written.
    ensure!(
        q.write_uncompressed(out),
        Internal,
        "public key buffer length disagrees with the encoder"
    );
    Ok(())
}

/// Compute the public key, SEC1 compressed.
pub fn public_key_compressed<C: HasGeneratorTable>(
    private_key: &[u8],
    out: &mut [u8],
) -> Result<()> {
    ensure!(
        out.len() == 1 + C::FIELD_BYTES,
        InvalidLength,
        "ecdh compressed key buffer"
    );
    let d = load_scalar::<C>(private_key)?;
    let q = Point::<C>::mul_generator(&d)
        .to_affine()
        .ok_or(ic_core::err!(Internal, "public key is the identity"))?;
    ensure!(
        q.write_compressed(out),
        Internal,
        "compressed key buffer length disagrees with the encoder"
    );
    Ok(())
}

/// Compute the shared secret: the x-coordinate of `[d]Q`.
pub fn agree<C: Curve>(private_key: &[u8], peer_public_key: &[u8], out: &mut [u8]) -> Result<()> {
    ensure!(
        out.len() == C::FIELD_BYTES,
        InvalidLength,
        "ecdh shared secret buffer"
    );
    let d = load_scalar::<C>(private_key)?;

    // SP 800-56A full public-key validation: the decoder checks that the point
    // is on the curve and refuses the identity, and every NIST prime curve has
    // cofactor 1, so there is no small subgroup left to land in.
    let q = AffinePoint::<C>::from_sec1(peer_public_key).ok_or(ic_core::err!(
        InvalidParameter,
        "ecdh peer public key is not a valid curve point"
    ))?;

    let shared = Point::<C>::from_affine(&q).mul_scalar(&d);
    let affine = shared.to_affine().ok_or(ic_core::err!(
        InvalidParameter,
        "ecdh key agreement produced the identity"
    ))?;

    out.copy_from_slice(affine.x.to_bytes().as_ref());
    Ok(())
}