From 42cb9c1ff9edc8b2ce5993a8256c2a146b68ae5c Mon Sep 17 00:00:00 2001
From: Henry de Valence <hdevalence@hdevalence.ca>
Date: Thu, 12 Sep 2019 03:46:39 -0700
Subject: [PATCH] Add a `Sha256dChecksum` type for truncated double SHA256.

---
 zebra-chain/Cargo.toml   |  1 +
 zebra-chain/src/types.rs | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/zebra-chain/Cargo.toml b/zebra-chain/Cargo.toml
index 20580e65..0b03b9e0 100644
--- a/zebra-chain/Cargo.toml
+++ b/zebra-chain/Cargo.toml
@@ -7,3 +7,4 @@ edition = "2018"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+sha2 = "0.8"
diff --git a/zebra-chain/src/types.rs b/zebra-chain/src/types.rs
index 090454af..9a33fe7b 100644
--- a/zebra-chain/src/types.rs
+++ b/zebra-chain/src/types.rs
@@ -1,5 +1,20 @@
 //! Newtype wrappers for primitive data types with semantic meaning.
 
+/// A 4-byte checksum using truncated double SHA256.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+pub struct Sha256dChecksum(pub [u8; 4]);
+
+impl<'a> From<&'a [u8]> for Sha256dChecksum {
+    fn from(bytes: &'a [u8]) -> Self {
+        use sha2::{Sha256, Digest};
+        let hash1 = Sha256::digest(bytes);
+        let hash2 = Sha256::digest(&hash1);
+        let mut checksum = [0u8; 4];
+        checksum[0..4].copy_from_slice(&hash2[0..4]);
+        Self(checksum)
+    }
+}
+
 /// A u32 which represents a block height value.
 pub struct BlockHeight(pub u32);
 
@@ -34,3 +49,17 @@ pub enum InventoryType {
 
 /// Inventory Vector
 pub struct InventoryVector(pub InventoryType, pub [u8; 32]);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn sha256d_checksum() {
+        // https://en.bitcoin.it/wiki/Protocol_documentation#Hashes
+        let input = b"hello";
+        let checksum = Sha256dChecksum::from(&input[..]);
+        let expected = Sha256dChecksum([0x95, 0x95, 0xc9, 0xdf]);
+        assert_eq!(checksum, expected);
+    }
+}
\ No newline at end of file