x25519.rs

// -*- mode: rust; -*-
//
// This file is part of x25519-dalek.
// Copyright (c) 2017-2019 isis lovecruft
// Copyright (c) 2019 DebugSteven
// See LICENSE for licensing information.
//
// Authors:
// - isis agora lovecruft <isis@patternsinthevoid.net>
// - DebugSteven <debugsteven@gmail.com>

//! x25519 Diffie-Hellman key exchange
//!
//! This implements x25519 key exchange as specified by Mike Hamburg
//! and Adam Langley in [RFC7748](https://tools.ietf.org/html/rfc7748).

use curve25519_dalek::constants::ED25519_BASEPOINT_TABLE;
use curve25519_dalek::montgomery::MontgomeryPoint;
use curve25519_dalek::scalar::Scalar;

use rand_core::CryptoRng;
use rand_core::RngCore;

use zeroize::Zeroize;

/// A Diffie-Hellman public key, corresponding to an [`EphemeralSecret`] or [`StaticSecret`] key.
#[cfg_attr(feature = "serde", serde(crate = "our_serde"))]
#[cfg_attr(
    feature = "serde",
    derive(our_serde::Serialize, our_serde::Deserialize)
)]
#[derive(PartialEq, Eq, Hash, Copy, Clone, Debug)]
pub struct PublicKey(pub(crate) MontgomeryPoint);

impl From<[u8; 32]> for PublicKey {
    /// Given a byte array, construct a x25519 `PublicKey`.
    fn from(bytes: [u8; 32]) -> PublicKey {
        PublicKey(MontgomeryPoint(bytes))
    }
}

impl PublicKey {
    /// Convert this public key to a byte array.
    #[inline]
    pub fn to_bytes(&self) -> [u8; 32] {
        self.0.to_bytes()
    }

    /// View this public key as a byte array.
    #[inline]
    pub fn as_bytes(&self) -> &[u8; 32] {
        self.0.as_bytes()
    }
}

/// A short-lived Diffie-Hellman secret key that can only be used to compute a single
/// [`SharedSecret`].
///
/// This type is identical to the [`StaticSecret`] type, except that the
/// [`EphemeralSecret::diffie_hellman`] method consumes and then wipes the secret key, and there
/// are no serialization methods defined.  This means that [`EphemeralSecret`]s can only be
/// generated from fresh randomness by [`EphemeralSecret::new`] and the compiler statically checks
/// that the resulting secret is used at most once.
#[derive(Zeroize)]
#[zeroize(drop)]
pub struct EphemeralSecret(pub(crate) Scalar);

impl EphemeralSecret {
    /// Perform a Diffie-Hellman key agreement between `self` and
    /// `their_public` key to produce a [`SharedSecret`].
    pub fn diffie_hellman(self, their_public: &PublicKey) -> SharedSecret {
        SharedSecret(self.0 * their_public.0)
    }

    /// Generate an x25519 [`EphemeralSecret`] key.
    pub fn new<T: RngCore + CryptoRng>(mut csprng: T) -> Self {
        let mut bytes = [0u8; 32];

        csprng.fill_bytes(&mut bytes);

        EphemeralSecret(clamp_scalar(bytes))
    }
}

impl<'a> From<&'a EphemeralSecret> for PublicKey {
    /// Given an x25519 [`EphemeralSecret`] key, compute its corresponding [`PublicKey`].
    fn from(secret: &'a EphemeralSecret) -> PublicKey {
        PublicKey((&ED25519_BASEPOINT_TABLE * &secret.0).to_montgomery())
    }
}

/// A Diffie-Hellman secret key that can be used to compute multiple [`SharedSecret`]s.
///
/// This type is identical to the [`EphemeralSecret`] type, except that the
/// [`StaticSecret::diffie_hellman`] method does not consume the secret key, and the type provides
/// serialization methods to save and load key material.  This means that the secret may be used
/// multiple times (but does not *have to be*).
///
/// Some protocols, such as Noise, already handle the static/ephemeral distinction, so the
/// additional guarantees provided by [`EphemeralSecret`] are not helpful or would cause duplicate
/// code paths.  In this case, it may be useful to
/// ```rust,ignore
/// use x25519_dalek::StaticSecret as SecretKey;
/// ```
/// since the only difference between the two is that [`StaticSecret`] does not enforce at
/// compile-time that the key is only used once.
#[cfg_attr(feature = "serde", serde(crate = "our_serde"))]
#[cfg_attr(
    feature = "serde",
    derive(our_serde::Serialize, our_serde::Deserialize)
)]
#[derive(Clone, Zeroize)]
#[zeroize(drop)]
pub struct StaticSecret(
    #[cfg_attr(feature = "serde", serde(with = "AllowUnreducedScalarBytes"))] pub(crate) Scalar,
);

impl StaticSecret {
    /// Perform a Diffie-Hellman key agreement between `self` and
    /// `their_public` key to produce a `SharedSecret`.
    pub fn diffie_hellman(&self, their_public: &PublicKey) -> SharedSecret {
        SharedSecret(&self.0 * their_public.0)
    }

    /// Generate an x25519 key.
    pub fn new<T: RngCore + CryptoRng>(mut csprng: T) -> Self {
        let mut bytes = [0u8; 32];

        csprng.fill_bytes(&mut bytes);

        StaticSecret(clamp_scalar(bytes))
    }

    /// Extract this key's bytes for serialization.
    pub fn to_bytes(&self) -> [u8; 32] {
        self.0.to_bytes()
    }
}

impl From<[u8; 32]> for StaticSecret {
    /// Load a secret key from a byte array.
    fn from(bytes: [u8; 32]) -> StaticSecret {
        StaticSecret(clamp_scalar(bytes))
    }
}

impl<'a> From<&'a StaticSecret> for PublicKey {
    /// Given an x25519 [`StaticSecret`] key, compute its corresponding [`PublicKey`].
    fn from(secret: &'a StaticSecret) -> PublicKey {
        PublicKey((&ED25519_BASEPOINT_TABLE * &secret.0).to_montgomery())
    }
}

/// The result of a Diffie-Hellman key exchange.
///
/// Each party computes this using their [`EphemeralSecret`] or [`StaticSecret`] and their
/// counterparty's [`PublicKey`].
#[derive(Zeroize)]
#[zeroize(drop)]
pub struct SharedSecret(pub(crate) MontgomeryPoint);

impl SharedSecret {
    /// Convert this shared secret to a byte array.
    #[inline]
    pub fn to_bytes(&self) -> [u8; 32] {
        self.0.to_bytes()
    }

    /// View this shared secret key as a byte array.
    #[inline]
    pub fn as_bytes(&self) -> &[u8; 32] {
        self.0.as_bytes()
    }
}

/// "Decode" a scalar from a 32-byte array.
///
/// By "decode" here, what is really meant is applying key clamping by twiddling
/// some bits.
///
/// # Returns
///
/// A `Scalar`.
fn clamp_scalar(mut scalar: [u8; 32]) -> Scalar {
    scalar[0] &= 248;
    scalar[31] &= 127;
    scalar[31] |= 64;

    Scalar::from_bits(scalar)
}

/// The bare, byte-oriented x25519 function, exactly as specified in RFC7748.
///
/// This can be used with [`X25519_BASEPOINT_BYTES`] for people who
/// cannot use the better, safer, and faster DH API.
pub fn x25519(k: [u8; 32], u: [u8; 32]) -> [u8; 32] {
    (clamp_scalar(k) * MontgomeryPoint(u)).to_bytes()
}

/// The X25519 basepoint, for use with the bare, byte-oriented x25519
/// function.  This is provided for people who cannot use the typed
/// DH API for some reason.
pub const X25519_BASEPOINT_BYTES: [u8; 32] = [
    9, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
];

/// Derived serialization methods will not work on a StaticSecret because x25519 requires
/// non-canonical scalars which are rejected by curve25519-dalek. Thus we provide a way to convert
/// the bytes directly to a scalar using Serde's remote derive functionality.
#[cfg_attr(feature = "serde", serde(crate = "our_serde"))]
#[cfg_attr(
    feature = "serde",
    derive(our_serde::Serialize, our_serde::Deserialize)
)]
#[cfg_attr(feature = "serde", serde(remote = "Scalar"))]
struct AllowUnreducedScalarBytes(
    #[cfg_attr(feature = "serde", serde(getter = "Scalar::to_bytes"))] [u8; 32],
);
impl From<AllowUnreducedScalarBytes> for Scalar {
    fn from(bytes: AllowUnreducedScalarBytes) -> Scalar {
        clamp_scalar(bytes.0)
    }
}

#[cfg(test)]
mod test {
    use super::*;

    use rand_core::OsRng;

    #[test]
    fn byte_basepoint_matches_edwards_scalar_mul() {
        let mut scalar_bytes = [0x37; 32];

        for i in 0..32 {
            scalar_bytes[i] += 2;

            let result = x25519(scalar_bytes, X25519_BASEPOINT_BYTES);

            let expected = (&ED25519_BASEPOINT_TABLE * &clamp_scalar(scalar_bytes))
                .to_montgomery()
                .to_bytes();

            assert_eq!(result, expected);
        }
    }

    #[test]
    #[cfg(feature = "serde")]
    fn serde_bincode_public_key_roundtrip() {
        use bincode;

        let public_key = PublicKey::from(X25519_BASEPOINT_BYTES);

        let encoded = bincode::serialize(&public_key).unwrap();
        let decoded: PublicKey = bincode::deserialize(&encoded).unwrap();

        assert_eq!(encoded.len(), 32);
        assert_eq!(decoded.as_bytes(), public_key.as_bytes());
    }

    #[test]
    #[cfg(feature = "serde")]
    fn serde_bincode_public_key_matches_from_bytes() {
        use bincode;

        let expected = PublicKey::from(X25519_BASEPOINT_BYTES);
        let decoded: PublicKey = bincode::deserialize(&X25519_BASEPOINT_BYTES).unwrap();

        assert_eq!(decoded.as_bytes(), expected.as_bytes());
    }

    #[test]
    #[cfg(feature = "serde")]
    fn serde_bincode_static_secret_roundtrip() {
        use bincode;

        let static_secret = StaticSecret(clamp_scalar([0x24; 32]));

        let encoded = bincode::serialize(&static_secret).unwrap();
        let decoded: StaticSecret = bincode::deserialize(&encoded).unwrap();

        assert_eq!(encoded.len(), 32);
        assert_eq!(decoded.to_bytes(), static_secret.to_bytes());
    }

    #[test]
    #[cfg(feature = "serde")]
    fn serde_bincode_static_secret_matches_from_bytes() {
        use bincode;

        let expected = StaticSecret(clamp_scalar([0x24; 32]));
        let clamped_bytes = clamp_scalar([0x24; 32]).to_bytes();
        let decoded: StaticSecret = bincode::deserialize(&clamped_bytes).unwrap();

        assert_eq!(decoded.to_bytes(), expected.to_bytes());
    }

    fn do_rfc7748_ladder_test1(input_scalar: [u8; 32], input_point: [u8; 32], expected: [u8; 32]) {
        let result = x25519(input_scalar, input_point);

        assert_eq!(result, expected);
    }

    #[test]
    fn rfc7748_ladder_test1_vectorset1() {
        let input_scalar: [u8; 32] = [
            0xa5, 0x46, 0xe3, 0x6b, 0xf0, 0x52, 0x7c, 0x9d, 0x3b, 0x16, 0x15, 0x4b, 0x82, 0x46,
            0x5e, 0xdd, 0x62, 0x14, 0x4c, 0x0a, 0xc1, 0xfc, 0x5a, 0x18, 0x50, 0x6a, 0x22, 0x44,
            0xba, 0x44, 0x9a, 0xc4,
        ];
        let input_point: [u8; 32] = [
            0xe6, 0xdb, 0x68, 0x67, 0x58, 0x30, 0x30, 0xdb, 0x35, 0x94, 0xc1, 0xa4, 0x24, 0xb1,
            0x5f, 0x7c, 0x72, 0x66, 0x24, 0xec, 0x26, 0xb3, 0x35, 0x3b, 0x10, 0xa9, 0x03, 0xa6,
            0xd0, 0xab, 0x1c, 0x4c,
        ];
        let expected: [u8; 32] = [
            0xc3, 0xda, 0x55, 0x37, 0x9d, 0xe9, 0xc6, 0x90, 0x8e, 0x94, 0xea, 0x4d, 0xf2, 0x8d,
            0x08, 0x4f, 0x32, 0xec, 0xcf, 0x03, 0x49, 0x1c, 0x71, 0xf7, 0x54, 0xb4, 0x07, 0x55,
            0x77, 0xa2, 0x85, 0x52,
        ];

        do_rfc7748_ladder_test1(input_scalar, input_point, expected);
    }

    #[test]
    fn rfc7748_ladder_test1_vectorset2() {
        let input_scalar: [u8; 32] = [
            0x4b, 0x66, 0xe9, 0xd4, 0xd1, 0xb4, 0x67, 0x3c, 0x5a, 0xd2, 0x26, 0x91, 0x95, 0x7d,
            0x6a, 0xf5, 0xc1, 0x1b, 0x64, 0x21, 0xe0, 0xea, 0x01, 0xd4, 0x2c, 0xa4, 0x16, 0x9e,
            0x79, 0x18, 0xba, 0x0d,
        ];
        let input_point: [u8; 32] = [
            0xe5, 0x21, 0x0f, 0x12, 0x78, 0x68, 0x11, 0xd3, 0xf4, 0xb7, 0x95, 0x9d, 0x05, 0x38,
            0xae, 0x2c, 0x31, 0xdb, 0xe7, 0x10, 0x6f, 0xc0, 0x3c, 0x3e, 0xfc, 0x4c, 0xd5, 0x49,
            0xc7, 0x15, 0xa4, 0x93,
        ];
        let expected: [u8; 32] = [
            0x95, 0xcb, 0xde, 0x94, 0x76, 0xe8, 0x90, 0x7d, 0x7a, 0xad, 0xe4, 0x5c, 0xb4, 0xb8,
            0x73, 0xf8, 0x8b, 0x59, 0x5a, 0x68, 0x79, 0x9f, 0xa1, 0x52, 0xe6, 0xf8, 0xf7, 0x64,
            0x7a, 0xac, 0x79, 0x57,
        ];

        do_rfc7748_ladder_test1(input_scalar, input_point, expected);
    }

    #[test]
    #[ignore] // Run only if you want to burn a lot of CPU doing 1,000,000 DH operations
    fn rfc7748_ladder_test2() {
        use curve25519_dalek::constants::X25519_BASEPOINT;

        let mut k: [u8; 32] = X25519_BASEPOINT.0;
        let mut u: [u8; 32] = X25519_BASEPOINT.0;
        let mut result: [u8; 32];

        macro_rules! do_iterations {
            ($n:expr) => {
                for _ in 0..$n {
                    result = x25519(k, u);
                    // OBVIOUS THING THAT I'M GOING TO NOTE ANYWAY BECAUSE I'VE
                    // SEEN PEOPLE DO THIS WITH GOLANG'S STDLIB AND YOU SURE AS
                    // HELL SHOULDN'T DO HORRIBLY STUPID THINGS LIKE THIS WITH
                    // MY LIBRARY:
                    //
                    // NEVER EVER TREAT SCALARS AS POINTS AND/OR VICE VERSA.
                    //
                    //                ↓↓ DON'T DO THIS ↓↓
                    u = k.clone();
                    k = result;
                }
            };
        }

        // After one iteration:
        //     422c8e7a6227d7bca1350b3e2bb7279f7897b87bb6854b783c60e80311ae3079
        // After 1,000 iterations:
        //     684cf59ba83309552800ef566f2f4d3c1c3887c49360e3875f2eb94d99532c51
        // After 1,000,000 iterations:
        //     7c3911e0ab2586fd864497297e575e6f3bc601c0883c30df5f4dd2d24f665424

        do_iterations!(1);
        assert_eq!(
            k,
            [
                0x42, 0x2c, 0x8e, 0x7a, 0x62, 0x27, 0xd7, 0xbc, 0xa1, 0x35, 0x0b, 0x3e, 0x2b, 0xb7,
                0x27, 0x9f, 0x78, 0x97, 0xb8, 0x7b, 0xb6, 0x85, 0x4b, 0x78, 0x3c, 0x60, 0xe8, 0x03,
                0x11, 0xae, 0x30, 0x79,
            ]
        );
        do_iterations!(999);
        assert_eq!(
            k,
            [
                0x68, 0x4c, 0xf5, 0x9b, 0xa8, 0x33, 0x09, 0x55, 0x28, 0x00, 0xef, 0x56, 0x6f, 0x2f,
                0x4d, 0x3c, 0x1c, 0x38, 0x87, 0xc4, 0x93, 0x60, 0xe3, 0x87, 0x5f, 0x2e, 0xb9, 0x4d,
                0x99, 0x53, 0x2c, 0x51,
            ]
        );
        do_iterations!(999_000);
        assert_eq!(
            k,
            [
                0x7c, 0x39, 0x11, 0xe0, 0xab, 0x25, 0x86, 0xfd, 0x86, 0x44, 0x97, 0x29, 0x7e, 0x57,
                0x5e, 0x6f, 0x3b, 0xc6, 0x01, 0xc0, 0x88, 0x3c, 0x30, 0xdf, 0x5f, 0x4d, 0xd2, 0xd2,
                0x4f, 0x66, 0x54, 0x24,
            ]
        );
    }
}