Derive Sprout ReceivingKey's from SpendingKey's via SHA256Compress

Test is incomplete, also the type aliases block us from impl'ing Debug or Display.

Cargo.lock

@@ -2077,6 +2077,7 @@ dependencies = [
  "lazy_static",
  "proptest",
  "proptest-derive",
+ "rand_core 0.5.1",
  "redjubjub",
  "ripemd160",
  "secp256k1",

zebra-chain/Cargo.toml

@@ -14,6 +14,7 @@ chrono = "0.4"
 futures = "0.3"
 hex = "0.4"
 lazy_static = "1.4.0"
+rand_core = "0.5.1"
 ripemd160 = "0.8.0"
 secp256k1 = { version = "0.17.2", features = ["serde"] }
 serde = { version = "1", features = ["serde_derive"] }

zebra-chain/src/keys/sprout.rs

@@ -11,6 +11,8 @@ use std::{
     io::{self},
 };
 
+use byteorder::{ByteOrder, LittleEndian};
+
 #[cfg(test)]
 use proptest::{array, collection::vec, prelude::*};
 #[cfg(test)]
@@ -24,6 +26,7 @@ use crate::serialization::{SerializationError, ZcashDeserialize, ZcashSerialize}
 ///
 /// All other Sprout key types derive from the SpendingKey value.
 /// Actually 252 bits.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct SpendingKey(pub [u8; 32]);
 
 /// Derived from a _SpendingKey_.
@@ -31,12 +34,23 @@ pub type ReceivingKey = x25519_dalek::StaticSecret;
 
 impl From<SpendingKey> for ReceivingKey {
     fn from(spending_key: SpendingKey) -> ReceivingKey {
-        ReceivingKey::from(spending_key.0)
+        let mut state = [0u32; 8];
+        let mut block = [0u8; 64];
+
+        block[0..32].copy_from_slice(&spending_key.0[..]);
+        block[0] |= 0b11000000;
+
+        compress256(&mut state, &block);
+
+        let mut derived_bytes = [0u8; 32];
+        LittleEndian::write_u32_into(&state, &mut derived_bytes);
+
+        ReceivingKey::from(derived_bytes)
     }
 }
 
 /// Derived from a _SpendingKey_.
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 pub struct PayingKey(pub [u8; 32]);
 
 /// Derived from a _ReceivingKey_.
@@ -48,6 +62,28 @@ pub struct IncomingViewingKey {
     receiving_key: ReceivingKey,
 }
 
+#[cfg(test)]
+mod tests {
+
+    use rand_core::{OsRng, RngCore};
+
+    use super::*;
+
+    #[test]
+    fn derive_receiving_key() {
+        let mut bytes = [0u8; 32];
+        OsRng.fill_bytes(&mut bytes);
+
+        let spending_key = SpendingKey(bytes);
+
+        println!("{:?}", spending_key);
+
+        let receiving_key = ReceivingKey::from(spending_key);
+
+        // println!("{}", receiving_key);
+    }
+}
+
 #[cfg(test)]
 proptest! {