Merge #878: Add chain hash type using consts

8e29f2b Add ChainHash type (Tobin Harding)
cd8f511 blockdata: constants: Use wildcard import in unit tests (Tobin Harding)
71bf196 Use fully qualified path in macro (Tobin Harding)

Pull request description:

  The Lightning network defines a type called 'chain hash' that is used to uniquely represent the various Bitcoin networks as a 32 byte hash value. Chain hash is now being used by the DLC folks, as such it is useful to have it implemented in rust-bitcoin.

  One method of calculating a chain hash is by hashing the genesis block for the respective network.

  Add a `ChainHash` type that can be used to get the unique identifier of each of the 4 Bitcoin networks we support. Add a method that calculates the chain hash for a network using the double sha256 of the genesis block. Do so using hard coded consts and add unit tests (regression/sanity) that show these hard coded byte arrays match the hash of the data we return for the genesis block for the respective network.

  The chain hash for the main Bitcoin network can be verified from LN docs (BOLT 0), add a link to this document.

  Closes: #481

ACKs for top commit:
  Kixunil:
    ACK 8e29f2b
  sanket1729:
    ACK 8e29f2b.

Tree-SHA512: 8156bb55838b73694ddf77a606cbe403f53a31d363aa0dee11b97dc31aa9b62609d7d84b8f0f92c08e90372a3e8c7b416fb07989d6da9633763373b41339b1f5

src/blockdata/constants.rs

@@ -176,15 +176,40 @@ pub fn genesis_block(network: Network) -> Block {
     }
 }
 
+// Mainnet value can be verified at https://github.com/lightning/bolts/blob/master/00-introduction.md
+const GENESIS_BLOCK_HASH_BITCOIN: [u8; 32] = [111, 226, 140, 10, 182, 241, 179, 114, 193, 166, 162, 70, 174, 99, 247, 79, 147, 30, 131, 101, 225, 90, 8, 156, 104, 214, 25, 0, 0, 0, 0, 0];
+const GENESIS_BLOCK_HASH_TESTNET: [u8; 32] = [67, 73, 127, 215, 248, 38, 149, 113, 8, 244, 163, 15, 217, 206, 195, 174, 186, 121, 151, 32, 132, 233, 14, 173, 1, 234, 51, 9, 0, 0, 0, 0];
+const GENESIS_BLOCK_HASH_SIGNET: [u8; 32] = [246, 30, 238, 59, 99, 163, 128, 164, 119, 160, 99, 175, 50, 178, 187, 201, 124, 159, 249, 240, 31, 44, 66, 37, 233, 115, 152, 129, 8, 0, 0, 0];
+const GENESIS_BLOCK_HASH_REGTEST: [u8; 32] = [6, 34, 110, 70, 17, 26, 11, 89, 202, 175, 18, 96, 67, 235, 91, 191, 40, 195, 79, 58, 94, 51, 42, 31, 199, 178, 183, 60, 241, 136, 145, 15];
+
+/// The uniquely identifying hash of the target blockchain.
+#[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ChainHash([u8; 32]);
+impl_array_newtype!(ChainHash, u8, 32);
+impl_bytes_newtype!(ChainHash, 32);
+
+impl ChainHash {
+    /// Returns the hash of the `network` genesis block for use as a chain hash.
+    ///
+    /// See [BOLT 0](https://github.com/lightning/bolts/blob/ffeece3dab1c52efdb9b53ae476539320fa44938/00-introduction.md#chain_hash)
+    /// for specification.
+    pub fn using_genesis_block(network: Network) -> Self {
+        match network {
+            Network::Bitcoin => ChainHash(GENESIS_BLOCK_HASH_BITCOIN),
+            Network::Testnet => ChainHash(GENESIS_BLOCK_HASH_TESTNET),
+            Network::Signet => ChainHash(GENESIS_BLOCK_HASH_SIGNET),
+            Network::Regtest => ChainHash(GENESIS_BLOCK_HASH_REGTEST),
+        }
+    }
+}
+
 #[cfg(test)]
 mod test {
     use core::default::Default;
+    use super::*;
     use crate::hashes::hex::FromHex;
-
     use crate::network::constants::Network;
     use crate::consensus::encode::serialize;
-    use crate::blockdata::constants::{genesis_block, bitcoin_genesis_tx};
-    use crate::blockdata::constants::{MAX_SEQUENCE, COIN_VALUE};
 
     #[test]
     fn bitcoin_genesis_first_transaction() {
@@ -250,5 +275,49 @@ mod test {
         assert_eq!(format!("{:x}", gen.header.block_hash()),
                    "00000008819873e925422c1ff0f99f7cc9bbb232af63a077a480a3633bee1ef6".to_string());
     }
+
+    // The *_chain_hash tests are sanity/regression tests, they verify that the const byte array
+    // representing the genesis block is the same as that created by hashing the genesis block.
+    fn chain_hash_and_genesis_block(network: Network) {
+        use hashes::{sha256, Hash};
+
+        // The genesis block hash is a double-sha256 and it is displayed backwards.
+        let genesis_hash = genesis_block(network).block_hash();
+        // We abuse the sha256 hash here so we get a LowerHex impl that does not print the hex backwards.
+        let hash = sha256::Hash::from_slice(&genesis_hash.into_inner()).unwrap();
+        let want = format!("{:02x}", hash);
+
+        let chain_hash = ChainHash::using_genesis_block(network);
+        let got = format!("{:02x}", chain_hash);
+
+        // Compare strings because the spec specifically states how the chain hash must encode to hex.
+        assert_eq!(got, want);
+    }
+
+    macro_rules! chain_hash_genesis_block {
+        ($($test_name:ident, $network:expr);* $(;)*) => {
+            $(
+                #[test]
+                fn $test_name() {
+                    chain_hash_and_genesis_block($network);
+                }
+            )*
+        }
+    }
+
+    chain_hash_genesis_block! {
+        mainnet_chain_hash_genesis_block, Network::Bitcoin;
+        testnet_chain_hash_genesis_block, Network::Testnet;
+        signet_chain_hash_genesis_block, Network::Signet;
+        regtest_chain_hash_genesis_block, Network::Regtest;
+    }
+
+    // Test vector taken from: https://github.com/lightning/bolts/blob/master/00-introduction.md
+    #[test]
+    fn mainnet_chain_hash_test_vector() {
+        let got = format!("{:x}", ChainHash::using_genesis_block(Network::Bitcoin));
+        let want = "6fe28c0ab6f1b372c1a6a246ae63f74f931e8365e15a089c68d6190000000000";
+        assert_eq!(got, want);
+    }
 }
 

src/internal_macros.rs

@@ -94,7 +94,7 @@ macro_rules! impl_array_newtype {
             pub fn into_bytes(self) -> [$ty; $len] { self.0 }
         }
 
-        impl<'a> ::core::convert::From<&'a [$ty]> for $thing {
+        impl<'a> core::convert::From<&'a [$ty]> for $thing {
             fn from(data: &'a [$ty]) -> $thing {
                 assert_eq!(data.len(), $len);
                 let mut ret = [0; $len];
@@ -119,7 +119,7 @@ macro_rules! impl_array_newtype {
 
 macro_rules! display_from_debug {
     ($thing:ident) => {
-        impl ::core::fmt::Display for $thing {
+        impl core::fmt::Display for $thing {
             fn fmt(&self, f: &mut ::core::fmt::Formatter) -> Result<(), ::core::fmt::Error> {
                 ::core::fmt::Debug::fmt(self, f)
             }
@@ -375,13 +375,13 @@ macro_rules! impl_bytes_newtype {
 
         impl ::core::fmt::Display for $t {
             fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
-                fmt::LowerHex::fmt(self, f)
+                ::core::fmt::LowerHex::fmt(self, f)
             }
         }
 
         impl ::core::fmt::Debug for $t {
             fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
-                fmt::LowerHex::fmt(self, f)
+                ::core::fmt::LowerHex::fmt(self, f)
             }
         }