Handle fuzztarget 0 hashes in key derivation functions

Since upstream secp256k1 now rejects 0-tweak values, we need to
ensure we don't pass them in to tweak functions.

Author: TheBlueMatt

lightning/src/ln/chan_utils.rs

@@ -219,7 +219,16 @@ pub fn derive_public_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1<T>, per_com
 	let mut sha = Sha256::engine();
 	sha.input(&per_commitment_point.serialize());
 	sha.input(&base_point.serialize());
-	let res = Sha256::from_engine(sha).into_inner();
+	let mut res = Sha256::from_engine(sha).into_inner();
+
+	// In fuzztarget we can get 0-hashes, but they are invalid tweaks, so just increment them.
+	#[cfg(feature = "fuzztarget")]
+	{
+		if res == [0; 32] {
+			res[31] = 1;
+		}
+	}
+	res[0] = res[0]; // Ignore fuzztarget-only mut
 
 	let hashkey = PublicKey::from_secret_key(&secp_ctx, &SecretKey::from_slice(&res)?);
 	base_point.combine(&hashkey)
@@ -273,21 +282,35 @@ pub fn derive_private_revocation_key<T: secp256k1::Signing>(secp_ctx: &Secp256k1
 /// Note that this is infallible iff we trust that at least one of the two input keys are randomly
 /// generated (ie our own).
 pub fn derive_public_revocation_key<T: secp256k1::Verification>(secp_ctx: &Secp256k1<T>, per_commitment_point: &PublicKey, countersignatory_revocation_base_point: &PublicKey) -> Result<PublicKey, SecpError> {
-	let rev_append_commit_hash_key = {
+	let mut rev_append_commit_hash_key = {
 		let mut sha = Sha256::engine();
 		sha.input(&countersignatory_revocation_base_point.serialize());
 		sha.input(&per_commitment_point.serialize());
 
 		Sha256::from_engine(sha).into_inner()
 	};
-	let commit_append_rev_hash_key = {
+	let mut commit_append_rev_hash_key = {
 		let mut sha = Sha256::engine();
 		sha.input(&per_commitment_point.serialize());
 		sha.input(&countersignatory_revocation_base_point.serialize());
 
 		Sha256::from_engine(sha).into_inner()
 	};
 
+	// In fuzztarget we can get 0-hashes, but they are invalid tweaks, so just increment them.
+	#[cfg(feature = "fuzztarget")]
+	{
+		if rev_append_commit_hash_key == [0; 32] {
+			rev_append_commit_hash_key[31] = 1;
+		}
+
+		if commit_append_rev_hash_key == [0; 32] {
+			commit_append_rev_hash_key[31] = 1;
+		}
+	}
+	rev_append_commit_hash_key[0] = rev_append_commit_hash_key[0]; // Ignore fuzztarget-only mut
+	commit_append_rev_hash_key[0] = commit_append_rev_hash_key[0]; // Ignore fuzztarget-only mut
+
 	let mut countersignatory_contrib = countersignatory_revocation_base_point.clone();
 	countersignatory_contrib.mul_assign(&secp_ctx, &rev_append_commit_hash_key)?;
 	let mut broadcaster_contrib = per_commitment_point.clone();