Add transparent key derivation

Reviewed-on: #75

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Type for transaction creation errors
 - Types for Sapling circuit parameters
 - Function to create transaction
+- Function to derive distinct transparent spending keys
 
 ### Changed
 

src/ZcashHaskell/Transparent.hs

@@ -101,7 +101,22 @@ genTransparentReceiver i scope xprvk = do
     ScriptAddress j -> return $ TransparentReceiver P2SH $ fromBinary j
     _anyOtherKind -> throwIO $ userError "Unsupported transparent address type"
 
--- } decode a Transparent Address in HRF and return a TransparentAddress object
+-- | Generate a transparent receiver
+genTransparentSecretKey ::
+     Int -- ^ The index of the address to be created
+  -> Scope -- ^ `External` for wallet addresses or `Internal` for change addresses
+  -> XPrvKey -- ^ The transparent private key
+  -> IO TransparentSpendingKey
+genTransparentSecretKey i scope xprvk = do
+  ioCtx <- createContext
+  let s =
+        case scope of
+          External -> 0
+          Internal -> 1
+  let path = Deriv :/ s :/ fromIntegral i :: DerivPath
+  return $ derivePath ioCtx path xprvk
+
+-- | decode a Transparent Address in HRF and return a TransparentAddress object
 decodeTransparentAddress :: BS.ByteString -> Maybe TransparentAddress
 decodeTransparentAddress taddress = do
   if BS.length taddress < 34