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Use Sha256s for tweaks in sign to enforce randomness

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We assume that tweaks are the output of a SHA-256 hash function
(and thus that failing to create a private key from the has
negligible probability) in `add_public_key_tweak` and elsewhere.

Thus, we really shouldn't be taking byte arrays in the public API
but rather `Sha256` objects, and communicating in the docs for
`add_public_key_tweak` that we can panic if its not the output of
a hash function, both of which we do here.
This commit is contained in:
Matt Corallo 2024-04-19 00:36:52 +00:00
parent 23d9514259
commit 9f1c9062db
2 changed files with 8 additions and 6 deletions
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12
lightning/src/ln/channel_keys.rs
View file

@ -45,11 +45,11 @@ macro_rules! basepoint_impl {
///
/// This calculates the hash part in the tweak derivation process, which is used to
/// ensure that each key is unique and cannot be guessed by an external party.
pub fn derive_add_tweak(&self, per_commitment_point: &PublicKey) -> [u8; 32] {
pub fn derive_add_tweak(&self, per_commitment_point: &PublicKey) -> Sha256 {
let mut sha = Sha256::engine();
sha.input(&per_commitment_point.serialize());
sha.input(&self.to_public_key().serialize());
Sha256::from_engine(sha).to_byte_array()
Sha256::from_engine(sha)
}
}
@ -166,18 +166,20 @@ fn derive_public_key<T: secp256k1::Signing>(
let mut sha = Sha256::engine();
sha.input(&per_commitment_point.serialize());
sha.input(&base_point.serialize());
let res = Sha256::from_engine(sha).to_byte_array();
let res = Sha256::from_engine(sha);
add_public_key_tweak(secp_ctx, base_point, &res)
}
/// Adds a tweak to a public key to derive a new public key.
///
/// May panic if `tweak` is not the output of a SHA-256 hash.
pub fn add_public_key_tweak<T: secp256k1::Signing>(
secp_ctx: &Secp256k1<T>, base_point: &PublicKey, tweak: &[u8; 32],
secp_ctx: &Secp256k1<T>, base_point: &PublicKey, tweak: &Sha256,
) -> PublicKey {
let hashkey = PublicKey::from_secret_key(
&secp_ctx,
&SecretKey::from_slice(tweak)
&SecretKey::from_slice(tweak.as_byte_array())
.expect("Hashes should always be valid keys unless SHA-256 is broken"),
);
base_point.combine(&hashkey)

2
lightning/src/sign/mod.rs
View file

@ -401,7 +401,7 @@ impl SpendableOutputDescriptor {
subtype: 0,
key: "add_tweak".as_bytes().to_vec(),
},
add_tweak.to_vec(),
add_tweak.as_byte_array().to_vec(),
)]
.into_iter()
.collect()