contracthash: expose key tweak in new function

Needed for applications where the tweak and the secret key material are on different
devices (and the one with the secret material does not want to know how to compute
the tweak itself).

Cargo.toml

@@ -1,7 +1,7 @@
 
 [package]
 name = "bitcoin"
-version = "0.5.10"
+version = "0.5.11"
 authors = ["Andrew Poelstra <apoelstra@wpsoftware.net>"]
 license = "CC0-1.0"
 homepage = "https://github.com/apoelstra/rust-bitcoin/"

src/util/contracthash.rs

@@ -181,16 +181,21 @@ pub fn tweak_keys(secp: &Secp256k1, keys: &[PublicKey], contract: &[u8]) -> Resu
     Ok(ret)
 }
 
-/// Tweak a secret key using some arbitrary data
+/// Compute a tweak from some given data for the given public key
-pub fn tweak_secret_key(secp: &Secp256k1, key: &SecretKey, contract: &[u8]) -> Result<SecretKey, Error> {
+pub fn compute_tweak(secp: &Secp256k1, pk: &PublicKey, contract: &[u8]) -> Result<SecretKey, Error> {
-    // Compute public key
-    let pk = try!(PublicKey::from_secret_key(secp, &key).map_err(Error::Secp));
-    // Compute HMAC tweak
     let mut hmac_raw = [0; 32];
     let mut hmac = hmac::Hmac::new(sha2::Sha256::new(), &pk.serialize_vec(&secp, true));
     hmac.input(contract);
     hmac.raw_result(&mut hmac_raw);
-    let hmac_sk = try!(SecretKey::from_slice(&secp, &hmac_raw).map_err(Error::BadTweak));
+    SecretKey::from_slice(&secp, &hmac_raw).map_err(Error::BadTweak)
+}
+
+/// Tweak a secret key using some arbitrary data (calls `compute_tweak` internally)
+pub fn tweak_secret_key(secp: &Secp256k1, key: &SecretKey, contract: &[u8]) -> Result<SecretKey, Error> {
+    // Compute public key
+    let pk = try!(PublicKey::from_secret_key(secp, &key).map_err(Error::Secp));
+    // Compute tweak
+    let hmac_sk = try!(compute_tweak(secp, &pk, contract));
     // Execute the tweak
     let mut key = *key;
     try!(key.add_assign(&secp, &hmac_sk).map_err(Error::Secp));