rust/bssl-crypto/src/ed25519.rs

// Copyright 2023 The BoringSSL Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//! Ed25519, a signature scheme.
//!
//! Ed25519 builds a signature scheme over a curve that is isogenous to
//! curve25519. This module provides the "pure" signature scheme described in
//! <https://datatracker.ietf.org/doc/html/rfc8032>.
//!
//! ```
//! use bssl_crypto::ed25519;
//!
//! let key = ed25519::PrivateKey::generate();
//! // Publish your public key.
//! let public_key_bytes = *key.to_public().as_bytes();
//!
//! // Sign and publish some message.
//! let signed_message = b"hello world";
//! let mut sig = key.sign(signed_message);
//!
//! // Anyone with the public key can verify it.
//! let public_key = ed25519::PublicKey::from_bytes(&public_key_bytes);
//! assert!(public_key.verify(signed_message, &sig).is_ok());
//! ```

use crate::{
    cbb_to_buffer, scoped, with_output_array, Buffer, FfiMutSlice, FfiSlice, InvalidSignatureError,
};

/// The length in bytes of an Ed25519 public key.
pub const PUBLIC_KEY_LEN: usize = bssl_sys::ED25519_PUBLIC_KEY_LEN as usize;

/// The length in bytes of an Ed25519 seed which is the 32-byte private key
/// representation defined in RFC 8032.
pub const SEED_LEN: usize =
    (bssl_sys::ED25519_PRIVATE_KEY_LEN - bssl_sys::ED25519_PUBLIC_KEY_LEN) as usize;

/// The length in bytes of an Ed25519 signature.
pub const SIGNATURE_LEN: usize = bssl_sys::ED25519_SIGNATURE_LEN as usize;

// The length in bytes of an Ed25519 keypair. In BoringSSL, the private key is suffixed with the
// public key, so the keypair length is the same as the private key length.
const KEYPAIR_LEN: usize = bssl_sys::ED25519_PRIVATE_KEY_LEN as usize;

// The longest Ed25519ph context permitted by RFC 8032.
const MAX_CONTEXT_LEN: usize = 255;

/// An Ed25519 private key.
#[derive(Clone)]
pub struct PrivateKey([u8; KEYPAIR_LEN]);

/// An Ed25519 public key used to verify a signature + message.
pub struct PublicKey([u8; PUBLIC_KEY_LEN]);

/// An Ed25519 signature created by signing a message with a private key.
pub type Signature = [u8; SIGNATURE_LEN];

impl PrivateKey {
    /// Generates a new Ed25519 keypair.
    pub fn generate() -> Self {
        let mut public_key = [0u8; PUBLIC_KEY_LEN];
        let mut private_key = [0u8; KEYPAIR_LEN];

        // Safety:
        // - Public key and private key are the correct length.
        unsafe {
            bssl_sys::ED25519_keypair(public_key.as_mut_ffi_ptr(), private_key.as_mut_ffi_ptr())
        }

        PrivateKey(private_key)
    }

    /// Returns the "seed" of this private key, as defined in RFC 8032.
    pub fn to_seed(&self) -> [u8; SEED_LEN] {
        // This code will never panic because a length 32 slice will always fit into a
        // size 32 byte array. The private key is the first 32 bytes of the keypair.
        #[allow(clippy::expect_used)]
        self.0[..SEED_LEN]
            .try_into()
            .expect("A slice of length SEED_LEN will always fit into an array of length SEED_LEN")
    }

    /// Derives a key-pair from `seed`, which is the 32-byte private key representation defined
    /// in RFC 8032.
    pub fn from_seed(seed: &[u8; SEED_LEN]) -> Self {
        let mut public_key = [0u8; PUBLIC_KEY_LEN];
        let mut private_key = [0u8; KEYPAIR_LEN];

        // Safety:
        // - Public key, private key, and seed are the correct lengths.
        unsafe {
            bssl_sys::ED25519_keypair_from_seed(
                public_key.as_mut_ffi_ptr(),
                private_key.as_mut_ffi_ptr(),
                seed.as_ffi_ptr(),
            )
        }
        PrivateKey(private_key)
    }

    /// Signs the given message and returns the signature.
    pub fn sign(&self, msg: &[u8]) -> Signature {
        let mut sig_bytes = [0u8; SIGNATURE_LEN];

        // Safety:
        // - On allocation failure we panic.
        // - Signature and private keys are always the correct length.
        let result = unsafe {
            bssl_sys::ED25519_sign(
                sig_bytes.as_mut_ffi_ptr(),
                msg.as_ffi_ptr(),
                msg.len(),
                self.0.as_ffi_ptr(),
            )
        };
        assert_eq!(result, 1, "allocation failure in bssl_sys::ED25519_sign");

        sig_bytes
    }

    /// Signs the given `sha512_digest` with the given `context` using Ed25519ph.
    ///
    /// Note that Ed25519ph (pre-hashed) and "pure" Ed25519 are different algorithms
    /// defined in RFC 8032. Calling `sign` with the digest of a message will *not*
    /// return the same output as `sign_prehashed`.
    ///
    /// This function returns None if `context` is longer than 255 bytes.
    pub fn sign_prehashed(&self, context: &[u8], sha512_digest: &[u8; 64]) -> Option<Signature> {
        if context.len() > MAX_CONTEXT_LEN {
            return None;
        }

        let mut sig_bytes = [0u8; SIGNATURE_LEN];

        // Safety:
        // - On allocation failure we panic.
        // - The context length is checked above.
        // - Signature and private keys are always the correct length.
        let result = unsafe {
            bssl_sys::ED25519_sign_prehashed(
                sig_bytes.as_mut_ffi_ptr(),
                context.as_ffi_ptr(),
                context.len(),
                sha512_digest.as_ffi_ptr(),
                self.0.as_ffi_ptr(),
            )
        };
        assert_eq!(
            result, 1,
            "allocation failure in bssl_sys::ED25519_sign_prehashed"
        );

        Some(sig_bytes)
    }

    /// Returns the [`PublicKey`] corresponding to this private key.
    pub fn to_public(&self) -> PublicKey {
        let keypair_bytes = &self.0;

        // This code will never panic because a length 32 slice will always fit into a
        // size 32 byte array. The public key is the last 32 bytes of the keypair.
        #[allow(clippy::expect_used)]
        PublicKey(
            keypair_bytes[PUBLIC_KEY_LEN..]
                .try_into()
                .expect("The slice is always the correct size for a public key"),
        )
    }

    // Safety: caller must make sure that the key type is ED25519
    pub(crate) unsafe fn from_evp_pkey(mut pkey: scoped::EvpPkey) -> Self {
        let mut seed = [0; SEED_LEN];
        let len = &mut { SEED_LEN };
        // Safety: pkey is now owned and len is set
        let ret = unsafe {
            bssl_sys::EVP_PKEY_get_raw_private_key(
                pkey.as_ffi_ptr(),
                seed.as_mut_ptr(),
                len as *mut _,
            )
        };
        // Sanity check, in case the seed is not as long as expected.
        assert_eq!(ret, 1);
        assert_eq!(*len, SEED_LEN);
        Self::from_seed(&seed)
    }
}

impl PublicKey {
    /// Builds the public key from an array of bytes.
    pub fn from_bytes(bytes: &[u8; PUBLIC_KEY_LEN]) -> Self {
        PublicKey(*bytes)
    }

    /// Returns the bytes of the public key.
    pub fn as_bytes(&self) -> &[u8; PUBLIC_KEY_LEN] {
        &self.0
    }

    /// Parse a public key in SubjectPublicKeyInfo format.
    pub fn from_der_subject_public_key_info(spki: &[u8]) -> Option<Self> {
        // Safety: `EVP_pkey_ed25519` is always safe to call.
        let alg = unsafe { bssl_sys::EVP_pkey_ed25519() };
        let mut pkey =
            scoped::EvpPkey::from_der_subject_public_key_info(spki, core::slice::from_ref(&alg))?;
        let raw_pkey: [u8; PUBLIC_KEY_LEN] = unsafe {
            with_output_array(|out, mut out_len| {
                // We only passed one key type, so `pkey` must be an Ed25519
                // key. The raw public key then must be available, and must be
                // `PUBLIC_KEY_LEN` bytes.
                assert_eq!(
                    1,
                    bssl_sys::EVP_PKEY_get_raw_public_key(pkey.as_ffi_ptr(), out, &mut out_len)
                );
                assert_eq!(out_len, PUBLIC_KEY_LEN);
            })
        };
        Some(PublicKey(raw_pkey))
    }

    /// Serialize this key in SubjectPublicKeyInfo format.
    pub fn to_der_subject_public_key_info(&self) -> Buffer {
        // Safety: this only copies from the `self.0` buffer.
        let mut pkey = scoped::EvpPkey::from_ptr(unsafe {
            bssl_sys::EVP_PKEY_from_raw_public_key(
                bssl_sys::EVP_pkey_ed25519(),
                self.0.as_ffi_ptr(),
                PUBLIC_KEY_LEN,
            )
        });
        // Safety: we are only testing pointer nullness, we do not mutate the data
        assert!(!pkey.as_ffi_ptr().is_null());

        cbb_to_buffer(PUBLIC_KEY_LEN + 32, |cbb| unsafe {
            // The arguments are valid so this will only fail if out of memory,
            // which this crate doesn't handle.
            assert_eq!(1, bssl_sys::EVP_marshal_public_key(cbb, pkey.as_ffi_ptr()));
        })
    }

    /// Verifies that `signature` is a valid signature, by this key, of `msg`.
    pub fn verify(&self, msg: &[u8], signature: &Signature) -> Result<(), InvalidSignatureError> {
        let ret = unsafe {
            // Safety: `self.0` is the correct length and other buffers are valid.
            bssl_sys::ED25519_verify(
                msg.as_ffi_ptr(),
                msg.len(),
                signature.as_ffi_ptr(),
                self.0.as_ffi_ptr(),
            )
        };
        if ret == 1 {
            Ok(())
        } else {
            Err(InvalidSignatureError)
        }
    }

    /// Verifies that `signature` is a valid signature under the given `context`
    /// of `sha512_digest` using Ed25519ph.
    ///
    /// Note that Ed25519ph (pre-hashed) and "pure" Ed25519 are different algorithms
    /// defined in RFC 8032. Calling `verify` with the digest of a message will *not*
    /// return the same output as `verify_prehashed`.
    ///
    /// A `context` longer than 255 bytes, the maximum permitted by RFC 8032, is
    /// reported as an invalid signature.
    pub fn verify_prehashed(
        &self,
        context: &[u8],
        sha512_digest: &[u8; 64],
        signature: &Signature,
    ) -> Result<(), InvalidSignatureError> {
        let ret = unsafe {
            // Safety: `self.0` is the correct length and other buffers are valid.
            bssl_sys::ED25519_verify_prehashed(
                context.as_ffi_ptr(),
                context.len(),
                sha512_digest.as_ffi_ptr(),
                signature.as_ffi_ptr(),
                self.0.as_ffi_ptr(),
            )
        };
        if ret == 1 {
            Ok(())
        } else {
            Err(InvalidSignatureError)
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::digest::Sha512;
    use crate::test_helpers;

    #[test]
    fn gen_roundtrip() {
        let private_key = PrivateKey::generate();
        assert_ne!([0u8; 64], private_key.0);
        let seed = private_key.to_seed();
        let new_private_key = PrivateKey::from_seed(&seed);
        assert_eq!(private_key.0, new_private_key.0);
    }

    #[test]
    fn der_subject_public_key_info() {
        let priv_key = PrivateKey::generate();
        let msg = [0u8; 0];
        let sig = priv_key.sign(&msg);

        let pub_key = priv_key.to_public();
        assert!(pub_key.verify(&msg, &sig).is_ok());

        let pub_key_der = pub_key.to_der_subject_public_key_info();
        let pub_key_from_der =
            PublicKey::from_der_subject_public_key_info(pub_key_der.as_ref()).unwrap();
        assert_eq!(pub_key.as_bytes(), pub_key_from_der.as_bytes());
        assert!(pub_key_from_der.verify(&msg, &sig).is_ok());

        assert!(PublicKey::from_der_subject_public_key_info(
            &pub_key_from_der.as_bytes()[0..PUBLIC_KEY_LEN / 2]
        )
        .is_none());

        assert!(PublicKey::from_der_subject_public_key_info(b"").is_none());
    }

    #[test]
    fn der_subject_public_key_info_wrong_type() {
        // This is an X25519 key, not an Ed25519 key.
        let spki = test_helpers::decode_hex_into_vec("302a300506032b656e032100e6db6867583030db3594c1a424b15f7c726624ec26b3353b10a903a6d0ab1c4c");
        // `from_der_subject_public_key_info` should reject it.
        assert!(PublicKey::from_der_subject_public_key_info(&spki).is_none());
    }

    #[test]
    fn empty_msg() {
        // Test Case 1 from RFC test vectors: https://www.rfc-editor.org/rfc/rfc8032#section-7.1
        let pk = test_helpers::decode_hex(
            "d75a980182b10ab7d54bfed3c964073a0ee172f3daa62325af021a68f707511a",
        );
        let seed = test_helpers::decode_hex(
            "9d61b19deffd5a60ba844af492ec2cc44449c5697b326919703bac031cae7f60",
        );
        let msg = [0u8; 0];
        let sig_expected  = test_helpers::decode_hex("e5564300c360ac729086e2cc806e828a84877f1eb8e5d974d873e065224901555fb8821590a33bacc61e39701cf9b46bd25bf5f0595bbe24655141438e7a100b");
        let kp = PrivateKey::from_seed(&seed);
        let sig = kp.sign(&msg);
        assert_eq!(sig_expected, sig);

        let pub_key = PublicKey::from_bytes(&pk);
        assert_eq!(pub_key.as_bytes(), kp.to_public().as_bytes());
        assert!(pub_key.verify(&msg, &sig).is_ok());
    }

    #[test]
    fn ed25519_sign_and_verify() {
        // Test Case 15 from RFC test vectors: https://www.rfc-editor.org/rfc/rfc8032#section-7.1
        let pk = test_helpers::decode_hex(
            "cf3af898467a5b7a52d33d53bc037e2642a8da996903fc252217e9c033e2f291",
        );
        let sk = test_helpers::decode_hex(
            "9acad959d216212d789a119252ebfe0c96512a23c73bd9f3b202292d6916a738",
        );
        let msg: [u8; 14] = test_helpers::decode_hex("55c7fa434f5ed8cdec2b7aeac173");
        let sig_expected  = test_helpers::decode_hex("6ee3fe81e23c60eb2312b2006b3b25e6838e02106623f844c44edb8dafd66ab0671087fd195df5b8f58a1d6e52af42908053d55c7321010092748795ef94cf06");
        let kp = PrivateKey::from_seed(&sk);

        let sig = kp.sign(&msg);
        assert_eq!(sig_expected, sig);

        let pub_key = PublicKey::from_bytes(&pk);
        assert_eq!(pub_key.as_bytes(), kp.to_public().as_bytes());
        assert!(pub_key.verify(&msg, &sig).is_ok());
    }

    #[test]
    fn ed25519ph_sign_and_verify() {
        // Test Case 15 from RFC test vectors: https://www.rfc-editor.org/rfc/rfc8032#section-7.3
        let pk = test_helpers::decode_hex(
            "ec172b93ad5e563bf4932c70e1245034c35467ef2efd4d64ebf819683467e2bf",
        );
        let sk = test_helpers::decode_hex(
            "833fe62409237b9d62ec77587520911e9a759cec1d19755b7da901b96dca3d42",
        );
        let msg: [u8; 3] = test_helpers::decode_hex("616263");
        let mut ctx = Sha512::new();
        ctx.update(&msg);
        let sha512_digest = ctx.digest();
        let sig_expected = test_helpers::decode_hex("98a70222f0b8121aa9d30f813d683f809e462b469c7ff87639499bb94e6dae4131f85042463c2a355a2003d062adf5aaa10b8c61e636062aaad11c2a26083406");
        let kp = PrivateKey::from_seed(&sk);

        let sig = kp.sign_prehashed(&[], &sha512_digest).unwrap();
        assert_eq!(sig_expected, sig);

        let pub_key = PublicKey::from_bytes(&pk);
        assert_eq!(pub_key.as_bytes(), kp.to_public().as_bytes());
        assert!(pub_key.verify_prehashed(&[], &sha512_digest, &sig).is_ok());
    }

    #[test]
    fn ed25519ph_max_context_length() {
        let kp = PrivateKey::generate();
        let pub_key = kp.to_public();
        let mut ctx = Sha512::new();
        ctx.update(b"test message");
        let sha512_digest = ctx.digest();

        let context = [0u8; 255]; // Maximum allowed context length.
        let sig = kp.sign_prehashed(&context, &sha512_digest).unwrap();
        assert!(pub_key
            .verify_prehashed(&context, &sha512_digest, &sig)
            .is_ok());

        let too_long_context = [0u8; 256];
        assert!(kp
            .sign_prehashed(&too_long_context, &sha512_digest)
            .is_none());
        assert!(pub_key
            .verify_prehashed(&too_long_context, &sha512_digest, &sig)
            .is_err());
    }
}