Add proof-of-possession hash fn

beacon_chain/utils/hashing/src/lib.rs

@@ -6,3 +6,12 @@ pub fn canonical_hash(input: &[u8]) -> Vec<u8> {
     let result = blake2b(64, &[], input);
     result.as_bytes()[0..32].to_vec()
 }
+
+pub fn proof_of_possession_hash(input: &[u8]) -> Vec<u8> {
+    let result = blake2b(64, &[], input);
+    let mut hash = result.as_bytes()[32..64].to_vec();
+    // TODO: this padding is not part of the spec, it is required otherwise Milagro will panic.
+    // We should either drop the padding or ensure the padding is in the spec.
+    hash.append(&mut vec![0; 18]);
+    hash
+}