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

keychain/src/keychain.rs

@@ -142,6 +142,15 @@ impl Keychain for ExtKeychain {
 		Ok(BlindingFactor::from_secret_key(sum))
 	}
 
+	fn create_nonce(&self, commit: &Commitment) -> Result<SecretKey, Error> {
+		// hash(commit|wallet root secret key (m)) as nonce
+		let root_key = self.derive_key(0, &Self::root_key_id())?;
+		let res = blake2::blake2b::blake2b(32, &commit.0, &root_key.0[..]);
+		let res = res.as_bytes();
+		SecretKey::from_slice(&self.secp, &res)
+			.map_err(|e| Error::RangeProof(format!("Unable to create nonce: {:?}", e).to_string()))
+	}
+
 	fn sign(&self, msg: &Message, amount: u64, id: &Identifier) -> Result<Signature, Error> {
 		let skey = self.derive_key(amount, id)?;
 		let sig = self.secp.sign(msg, &skey)?;

keychain/src/types.rs

@@ -468,6 +468,7 @@ pub trait Keychain: Sync + Send + Clone {
 	fn derive_key(&self, amount: u64, id: &Identifier) -> Result<SecretKey, Error>;
 	fn commit(&self, amount: u64, id: &Identifier) -> Result<Commitment, Error>;
 	fn blind_sum(&self, blind_sum: &BlindSum) -> Result<BlindingFactor, Error>;
+	fn create_nonce(&self, commit: &Commitment) -> Result<SecretKey, Error>;
 	fn sign(&self, msg: &Message, amount: u64, id: &Identifier) -> Result<Signature, Error>;
 	fn sign_with_blinding(&self, _: &Message, _: &BlindingFactor) -> Result<Signature, Error>;
 	fn set_use_switch_commits(&mut self, value: bool);