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|
//! Elliptic Curve Diffie-Hellman.
use nettle::{curve25519, ecc, ecdh, random::Yarrow};
use crate::{Error, Result};
use crate::crypto::SessionKey;
use crate::crypto::ecdh::{encrypt_shared, decrypt_shared};
use crate::crypto::mem::Protected;
use crate::crypto::mpi::{MPI, PublicKey, SecretKeyMaterial, Ciphertext};
use crate::packet::{key, Key};
use crate::types::Curve;
/// Wraps a session key using Elliptic Curve Diffie-Hellman.
#[allow(non_snake_case)]
pub fn encrypt<R>(recipient: &Key<key::PublicParts, R>,
session_key: &SessionKey)
-> Result<Ciphertext>
where R: key::KeyRole
{
let mut rng = Yarrow::default();
if let &PublicKey::ECDH {
ref curve, ref q,..
} = recipient.mpis() {
match curve {
Curve::Cv25519 => {
// Obtain the authenticated recipient public key R
let R = q.decode_point(curve)?.0;
// Generate an ephemeral key pair {v, V=vG}
let v: Protected =
curve25519::private_key(&mut rng).into();
// Compute the public key. We need to add an encoding
// octet in front of the key.
let mut VB = [0x40; 1 + curve25519::CURVE25519_SIZE];
curve25519::mul_g(&mut VB[1..], &v)
.expect("buffers are of the wrong size");
let VB = MPI::new(&VB);
// Compute the shared point S = vR;
let mut S: Protected =
vec![0; curve25519::CURVE25519_SIZE].into();
curve25519::mul(&mut S, &v, R)
.expect("buffers are of the wrong size");
encrypt_shared(recipient, session_key, VB, &S)
}
Curve::NistP256 | Curve::NistP384 | Curve::NistP521 => {
// Obtain the authenticated recipient public key R and
// generate an ephemeral private key v.
// Note: ecc::Point and ecc::Scalar are cleaned up by
// nettle.
let (Rx, Ry) = q.decode_point(curve)?;
let (R, v, field_sz) = match curve {
Curve::NistP256 => {
let R = ecc::Point::new::<ecc::Secp256r1>(Rx, Ry)?;
let v =
ecc::Scalar::new_random::<ecc::Secp256r1, _>(&mut rng);
let field_sz = 256;
(R, v, field_sz)
}
Curve::NistP384 => {
let R = ecc::Point::new::<ecc::Secp384r1>(Rx, Ry)?;
let v =
ecc::Scalar::new_random::<ecc::Secp384r1, _>(&mut rng);
let field_sz = 384;
(R, v, field_sz)
}
Curve::NistP521 => {
let R = ecc::Point::new::<ecc::Secp521r1>(Rx, Ry)?;
let v =
ecc::Scalar::new_random::<ecc::Secp521r1, _>(&mut rng);
let field_sz = 521;
(R, v, field_sz)
}
_ => unreachable!(),
};
// Compute the public key.
let VB = ecdh::point_mul_g(&v);
let (VBx, VBy) = VB.as_bytes();
let VB = MPI::new_weierstrass(&VBx, &VBy, field_sz);
// Compute the shared point S = vR;
let S = ecdh::point_mul(&v, &R)?;
// Get the X coordinate, safely dispose of Y.
let (Sx, Sy) = S.as_bytes();
Protected::from(Sy); // Just a precaution.
// Zero-pad to the size of the underlying field,
// rounded to the next byte.
let mut Sx = Vec::from(Sx);
while Sx.len() < (field_sz + 7) / 8 {
Sx.insert(0, 0);
}
encrypt_shared(recipient, session_key, VB, &Sx.into())
}
// Not implemented in Nettle
Curve::BrainpoolP256 | Curve::BrainpoolP512 =>
Err(Error::UnsupportedEllipticCurve(curve.clone()).into()),
// N/A
Curve::Unknown(_) | Curve::Ed25519 =>
Err(Error::UnsupportedEllipticCurve(curve.clone()).into()),
Curve::__Nonexhaustive => unreachable!(),
}
} else {
Err(Error::InvalidArgument("Expected an ECDHPublicKey".into()).into())
}
}
/// Unwraps a session key using Elliptic Curve Diffie-Hellman.
#[allow(non_snake_case)]
pub fn decrypt<R>(recipient: &Key<key::PublicParts, R>,
recipient_sec: &SecretKeyMaterial,
ciphertext: &Ciphertext)
-> Result<SessionKey>
where R: key::KeyRole
{
match (recipient.mpis(), recipient_sec, ciphertext) {
(PublicKey::ECDH { ref curve, ..},
SecretKeyMaterial::ECDH { ref scalar, },
Ciphertext::ECDH { ref e, .. }) =>
{
let S: Protected = match curve {
Curve::Cv25519 => {
// Get the public part V of the ephemeral key.
let V = e.decode_point(curve)?.0;
// Nettle expects the private key to be exactly
// CURVE25519_SIZE bytes long but OpenPGP allows leading
// zeros to be stripped.
// Padding has to be unconditional; otherwise we have a
// secret-dependent branch.
//
// Reverse the scalar. See
// https://lists.gnupg.org/pipermail/gnupg-devel/2018-February/033437.html.
let missing = curve25519::CURVE25519_SIZE
.saturating_sub(scalar.value().len());
let mut r = [0u8; curve25519::CURVE25519_SIZE];
r[missing..].copy_from_slice(scalar.value());
r.reverse();
// Compute the shared point S = rV = rvG, where (r, R)
// is the recipient's key pair.
let mut S: Protected =
vec![0; curve25519::CURVE25519_SIZE].into();
let res = curve25519::mul(&mut S, &r[..], V);
unsafe {
memsec::memzero(r.as_mut_ptr(),
curve25519::CURVE25519_SIZE);
}
res.expect("buffers are of the wrong size");
S
}
Curve::NistP256 | Curve::NistP384 | Curve::NistP521 => {
// Get the public part V of the ephemeral key and
// compute the shared point S = rV = rvG, where (r, R)
// is the recipient's key pair.
let (Vx, Vy) = e.decode_point(curve)?;
let (V, r, field_sz) = match curve {
Curve::NistP256 => {
let V =
ecc::Point::new::<ecc::Secp256r1>(&Vx
|
