gnir 0.14.2

// Copyright 2015-2017 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.

//! Key Agreement: ECDH, including X25519.
//!
//! # Example
//!
//! Note that this example uses X25519, but ECDH using NIST P-256/P-384 is done
//! exactly the same way, just substituting
//! `agreement::ECDH_P256`/`agreement::ECDH_P384` for `agreement::X25519`.
//!
//! ```
//! # fn x25519_agreement_example() -> Result<(), ring::error::Unspecified> {
//! use ring::{agreement, rand};
//! use untrusted;
//!
//! let rng = rand::SystemRandom::new();
//!
//! let my_private_key = agreement::EphemeralPrivateKey::generate(&agreement::X25519, &rng)?;
//!
//! // Make `my_public_key` a byte slice containing my public key. In a real
//! // application, this would be sent to the peer in an encoded protocol
//! // message.
//! let my_public_key = my_private_key.compute_public_key()?;
//!
//! // In a real application, the peer public key would be parsed out of a
//! // protocol message. Here we just generate one.
//! let peer_public_key = {
//!     let peer_private_key = agreement::EphemeralPrivateKey::generate(&agreement::X25519, &rng)?;
//!     peer_private_key.compute_public_key()?
//! };
//! let peer_public_key = untrusted::Input::from(peer_public_key.as_ref());
//!
//! // In a real application, the protocol specifies how to determine what
//! // algorithm was used to generate the peer's private key. Here, we know it
//! // is X25519 since we just generated it.
//! let peer_public_key_alg = &agreement::X25519;
//!
//! agreement::agree_ephemeral(
//!     my_private_key,
//!     peer_public_key_alg,
//!     peer_public_key,
//!     ring::error::Unspecified,
//!     |_key_material| {
//!         // In a real application, we'd apply a KDF to the key material and the
//!         // public keys (as recommended in RFC 7748) and then derive session
//!         // keys from the result. We omit all that here.
//!         Ok(())
//!     },
//! )
//! # }
//! # fn main() { x25519_agreement_example().unwrap() }
//! ```

// The "NSA Guide" steps here are from from section 3.1, "Ephemeral Unified
// Model."

use crate::{ec, error, rand};
use untrusted;

pub use crate::ec::{
    curve25519::x25519::X25519,
    suite_b::ecdh::{ECDH_P256, ECDH_P384},
};

/// A key agreement algorithm.
pub struct Algorithm {
    pub(crate) curve: &'static ec::Curve,
    pub(crate) ecdh: fn(
        out: &mut [u8],
        private_key: &ec::Seed,
        peer_public_key: untrusted::Input,
    ) -> Result<(), error::Unspecified>,
}

derive_debug_via_self!(Algorithm, self.curve);

impl Eq for Algorithm {}
impl PartialEq for Algorithm {
    fn eq(&self, other: &Algorithm) -> bool { self.curve.id == other.curve.id }
}

/// An ephemeral private key for use (only) with `agree_ephemeral`. The
/// signature of `agree_ephemeral` ensures that an `EphemeralPrivateKey` can be
/// used for at most one key agreement.
pub struct EphemeralPrivateKey {
    private_key: ec::Seed,
    alg: &'static Algorithm,
}

impl<'a> EphemeralPrivateKey {
    /// Generate a new ephemeral private key for the given algorithm.
    pub fn generate(
        alg: &'static Algorithm, rng: &rand::SecureRandom,
    ) -> Result<Self, error::Unspecified> {
        // NSA Guide Step 1.
        //
        // This only handles the key generation part of step 1. The rest of
        // step one is done by `compute_public_key()`.
        let private_key = ec::Seed::generate(&alg.curve, rng)?;
        Ok(Self { private_key, alg })
    }

    /// Computes the public key from the private key.
    #[inline(always)]
    pub fn compute_public_key(&self) -> Result<PublicKey, error::Unspecified> {
        // NSA Guide Step 1.
        //
        // Obviously, this only handles the part of Step 1 between the private
        // key generation and the sending of the public key to the peer. `out`
        // is what should be sent to the peer.
        self.private_key.compute_public_key().map(PublicKey)
    }

    #[cfg(test)]
    pub fn bytes(&'a self) -> &'a [u8] { self.private_key.bytes_less_safe() }
}

/// A public key for key agreement.
#[derive(Clone, Debug)]
pub struct PublicKey(ec::PublicKey);

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

/// Performs a key agreement with an ephemeral private key and the given public
/// key.
///
/// `my_private_key` is the ephemeral private key to use. Since it is moved, it
/// will not be usable after calling `agree_ephemeral`, thus guaranteeing that
/// the key is used for only one key agreement.
///
/// `peer_public_key_alg` is the algorithm/curve for the peer's public key
/// point; `agree_ephemeral` will return `Err(error_value)` if it does not
/// match `my_private_key's` algorithm/curve.
///
/// `peer_public_key` is the peer's public key. `agree_ephemeral` verifies that
/// it is encoded in the standard form for the algorithm and that the key is
/// *valid*; see the algorithm's documentation for details on how keys are to
/// be encoded and what constitutes a valid key for that algorithm.
///
/// `error_value` is the value to return if an error occurs before `kdf` is
/// called, e.g. when decoding of the peer's public key fails or when the public
/// key is otherwise invalid.
///
/// After the key agreement is done, `agree_ephemeral` calls `kdf` with the raw
/// key material from the key agreement operation and then returns what `kdf`
/// returns.
pub fn agree_ephemeral<F, R, E>(
    my_private_key: EphemeralPrivateKey, peer_public_key_alg: &Algorithm,
    peer_public_key: untrusted::Input, error_value: E, kdf: F,
) -> Result<R, E>
where
    F: FnOnce(&[u8]) -> Result<R, E>,
{
    // NSA Guide Prerequisite 1.
    //
    // The domain parameters are hard-coded. This check verifies that the
    // peer's public key's domain parameters match the domain parameters of
    // this private key.
    if peer_public_key_alg != my_private_key.alg {
        return Err(error_value);
    }

    let alg = &my_private_key.alg;

    // NSA Guide Prerequisite 2, regarding which KDFs are allowed, is delegated
    // to the caller.

    // NSA Guide Prerequisite 3, "Prior to or during the key-agreement process,
    // each party shall obtain the identifier associated with the other party
    // during the key-agreement scheme," is delegated to the caller.

    // NSA Guide Step 1 is handled by `EphemeralPrivateKey::generate()` and
    // `EphemeralPrivateKey::compute_public_key()`.

    let mut shared_key = [0u8; ec::ELEM_MAX_BYTES];
    let shared_key = &mut shared_key[..alg.curve.elem_scalar_seed_len];

    // NSA Guide Steps 2, 3, and 4.
    //
    // We have a pretty liberal interpretation of the NIST's spec's "Destroy"
    // that doesn't meet the NSA requirement to "zeroize."
    (alg.ecdh)(shared_key, &my_private_key.private_key, peer_public_key)
        .map_err(|_| error_value)?;

    // NSA Guide Steps 5 and 6.
    //
    // Again, we have a pretty liberal interpretation of the NIST's spec's
    // "Destroy" that doesn't meet the NSA requirement to "zeroize."
    kdf(shared_key)
}