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// Copyright 2015-2016 Brian Smith.
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice appear in all copies.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHORS DISCLAIM ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY
// SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
// WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
// OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
// CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
//! X25519 Key agreement.
use super::ops;
use crate::{agreement, constant_time, ec, error, rand};
use untrusted;
static CURVE25519: ec::Curve = ec::Curve {
public_key_len: PUBLIC_KEY_LEN,
elem_and_scalar_len: ELEM_AND_SCALAR_LEN,
id: ec::CurveID::Curve25519,
check_private_key_bytes: x25519_check_private_key_bytes,
generate_private_key: x25519_generate_private_key,
public_from_private: x25519_public_from_private,
};
/// X25519 (ECDH using Curve25519) as described in [RFC 7748].
///
/// Everything is as described in RFC 7748. Key agreement will fail if the
/// result of the X25519 operation is zero; see the notes on the
/// "all-zero value" in [RFC 7748 section 6.1].
///
/// [RFC 7748]: https://tools.ietf.org/html/rfc7748
/// [RFC 7748 section 6.1]: https://tools.ietf.org/html/rfc7748#section-6.1
pub static X25519: agreement::Algorithm = agreement::Algorithm {
i: ec::AgreementAlgorithmImpl {
curve: &CURVE25519,
ecdh: x25519_ecdh,
},
};
fn x25519_check_private_key_bytes(bytes: &[u8]) -> Result<(), error::Unspecified> {
debug_assert_eq!(bytes.len(), PRIVATE_KEY_LEN);
Ok(())
}
fn x25519_generate_private_key(
rng: &rand::SecureRandom,
) -> Result<ec::PrivateKey, error::Unspecified> {
let mut result = ec::PrivateKey {
bytes: [0; ec::SCALAR_MAX_BYTES],
};
rng.fill(&mut result.bytes[..PRIVATE_KEY_LEN])?;
Ok(result)
}
fn x25519_public_from_private(
public_out: &mut [u8], private_key: &ec::PrivateKey,
) -> Result<(), error::Unspecified> {
let public_out = slice_as_array_ref_mut!(public_out, PUBLIC_KEY_LEN)?;
// XXX: This shouldn't require dynamic checks, but rustc can't slice an
// array reference to a shorter array reference. TODO(perf): Fix this.
let private_key = slice_as_array_ref!(&private_key.bytes[..PRIVATE_KEY_LEN], PRIVATE_KEY_LEN)?;
unsafe {
GFp_x25519_public_from_private(public_out, private_key);
}
Ok(())
}
fn x25519_ecdh(
out: &mut [u8], my_private_key: &ec::PrivateKey, peer_public_key: untrusted::Input,
) -> Result<(), error::Unspecified> {
let out = slice_as_array_ref_mut!(out, SHARED_SECRET_LEN)?;
// XXX: This shouldn't require dynamic checks, but rustc can't slice an
// array reference to a shorter array reference. TODO(perf): Fix this.
let my_private_key =
slice_as_array_ref!(&my_private_key.bytes[..PRIVATE_KEY_LEN], PRIVATE_KEY_LEN)?;
let peer_public_key =
slice_as_array_ref!(peer_public_key.as_slice_less_safe(), PUBLIC_KEY_LEN)?;
unsafe {
GFp_x25519_scalar_mult(out, my_private_key, peer_public_key);
}
let zeros: SharedSecret = [0; SHARED_SECRET_LEN];
if constant_time::verify_slices_are_equal(out, &zeros).is_ok() {
// All-zero output results when the input is a point of small order.
return Err(error::Unspecified);
}
Ok(())
}
const ELEM_AND_SCALAR_LEN: usize = ops::ELEM_LEN;
// An X25519 private key as an unmasked scalar.
type PrivateKey = [u8; PRIVATE_KEY_LEN];
const PRIVATE_KEY_LEN: usize = ELEM_AND_SCALAR_LEN;
// An X25519 public key as an encoded Curve25519 point.
type PublicKey = [u8; PUBLIC_KEY_LEN];
const PUBLIC_KEY_LEN: usize = ELEM_AND_SCALAR_LEN;
// An X25519 shared secret as an encoded Curve25519 point.
type SharedSecret = [u8; SHARED_SECRET_LEN];
const SHARED_SECRET_LEN: usize = ELEM_AND_SCALAR_LEN;
extern "C" {
fn GFp_x25519_public_from_private(public_key_out: &mut PublicKey, private_key: &PrivateKey);
fn GFp_x25519_scalar_mult(
out: &mut ops::EncodedPoint, scalar: &ops::Scalar, point: &ops::EncodedPoint,
);
}