Small QoL improvements for wallet developers (#2651)

* Small changes for wallet devs

* Move create_nonce into Keychain trait

* Replace match by map_err

* Add flag to Slate to skip fee check

* Fix secp dependency

* Remove check_fee flag in Slate

core/src/libtx/build.rs

@@ -34,7 +34,8 @@ pub struct Context<'a, K>
 where
 	K: Keychain,
 {
-	keychain: &'a K,
+	/// The keychain used for key derivation
+	pub keychain: &'a K,
 }
 
 /// Function type returned by the transaction combinators. Transforms a

core/src/libtx/proof.rs

@@ -14,29 +14,12 @@
 
 //! Rangeproof library functions
 
-use crate::blake2;
 use crate::keychain::{Identifier, Keychain};
 use crate::libtx::error::{Error, ErrorKind};
 use crate::util::secp::key::SecretKey;
 use crate::util::secp::pedersen::{Commitment, ProofInfo, ProofMessage, RangeProof};
 use crate::util::secp::{self, Secp256k1};
 
-fn create_nonce<K>(k: &K, commit: &Commitment) -> Result<SecretKey, Error>
-where
-	K: Keychain,
-{
-	// hash(commit|wallet root secret key (m)) as nonce
-	let root_key = k.derive_key(0, &K::root_key_id())?;
-	let res = blake2::blake2b::blake2b(32, &commit.0, &root_key.0[..]);
-	let res = res.as_bytes();
-	match SecretKey::from_slice(k.secp(), &res) {
-		Ok(sk) => Ok(sk),
-		Err(e) => Err(ErrorKind::RangeProof(
-			format!("Unable to create nonce: {:?}", e).to_string(),
-		))?,
-	}
-}
-
 /// Create a bulletproof
 pub fn create<K>(
 	k: &K,
@@ -50,7 +33,9 @@ where
 {
 	let commit = k.commit(amount, key_id)?;
 	let skey = k.derive_key(amount, key_id)?;
-	let nonce = create_nonce(k, &commit)?;
+	let nonce = k
+		.create_nonce(&commit)
+		.map_err(|e| ErrorKind::RangeProof(e.to_string()))?;
 	let message = ProofMessage::from_bytes(&key_id.serialize_path());
 	Ok(k.secp()
 		.bullet_proof(amount, skey, nonce, extra_data, Some(message)))
@@ -80,7 +65,9 @@ pub fn rewind<K>(
 where
 	K: Keychain,
 {
-	let nonce = create_nonce(k, &commit)?;
+	let nonce = k
+		.create_nonce(&commit)
+		.map_err(|e| ErrorKind::RangeProof(e.to_string()))?;
 	let proof_message = k
 		.secp()
 		.rewind_bullet_proof(commit, nonce, extra_data, proof);