Add a `Sha256dChecksum` type for truncated double SHA256.

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"

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);
+    }
+}