Jędrzej Stuczyński
66 lines
2.4 KiB
Rust
66 lines
2.4 KiB
Rust
// Copyright 2021 - Nym Technologies SA <contact@nymtech.net>
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// SPDX-License-Identifier: Apache-2.0
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use crate::asymmetric::x25519;
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use crate::hkdf;
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use cipher::{Key, KeyIvInit, StreamCipher};
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use digest::crypto_common::BlockSizeUser;
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use digest::Digest;
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#[cfg(feature = "rand")]
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use rand::{CryptoRng, RngCore};
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/// Generate an ephemeral encryption keypair and perform diffie-hellman to establish
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/// shared key with the remote.
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#[cfg(feature = "rand")]
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pub fn new_ephemeral_shared_key<C, D, R>(
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rng: &mut R,
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remote_key: &x25519::PublicKey,
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) -> (x25519::KeyPair, Key<C>)
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where
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C: StreamCipher + KeyIvInit,
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D: Digest + BlockSizeUser + Clone,
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R: RngCore + CryptoRng,
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{
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let ephemeral_keypair = x25519::KeyPair::new(rng);
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// after performing diffie-hellman we don't care about the private component anymore
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let dh_result = ephemeral_keypair.private_key().diffie_hellman(remote_key);
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// SAFETY: while this is a relatively weak assumption, it's unlikely that any stream cipher has `C::key_size()`
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// larger than 255 * chunk_size of the digest (so for example keys larger than 8160 bytes if sh256 is used)
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#[allow(clippy::expect_used)]
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let okm = hkdf::extract_then_expand::<D>(None, &dh_result, None, C::key_size())
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.expect("somehow too long okm was provided");
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// SAFETY: the generated okm has exactly `C::key_size()` elements,
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// so this call is safe
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#[allow(clippy::unwrap_used)]
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let derived_shared_key = Key::<C>::from_exact_iter(okm).unwrap();
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(ephemeral_keypair, derived_shared_key)
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}
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/// Recompute shared key using remote public key and local private key.
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pub fn recompute_shared_key<C, D>(
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remote_key: &x25519::PublicKey,
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local_key: &x25519::PrivateKey,
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) -> Key<C>
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where
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C: StreamCipher + KeyIvInit,
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D: Digest + BlockSizeUser + Clone,
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{
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let dh_result = local_key.diffie_hellman(remote_key);
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// SAFETY: while this is a relatively weak assumption, it's unlikely that any stream cipher has `C::key_size()`
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// larger than 255 * chunk_size of the digest (so for example keys larger than 8160 bytes if sh256 is used)
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#[allow(clippy::expect_used)]
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let okm = hkdf::extract_then_expand::<D>(None, &dh_result, None, C::key_size())
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.expect("somehow too long okm was provided");
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// SAFETY: the generated okm has exactly `C::key_size()` elements,
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// so this call is safe
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#[allow(clippy::unwrap_used)]
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Key::<C>::from_exact_iter(okm).unwrap()
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}
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