Create tagged taproot hashes

Cargo.toml

@@ -25,7 +25,7 @@ secp-recovery = ["secp256k1/recovery"]
 [dependencies]
 base64-compat = { version = "1.0.0", optional = true }
 bech32 = "0.7.2"
-bitcoin_hashes = "0.9.0"
+bitcoin_hashes = "0.9.1"
 secp256k1 = { version = "0.19.0", features = [ "recovery" ] }
 
 bitcoinconsensus = { version = "0.19.0-1", optional = true }

src/util/mod.rs

@@ -27,6 +27,7 @@ pub mod hash;
 pub mod merkleblock;
 pub mod misc;
 pub mod psbt;
+pub mod taproot;
 pub mod uint;
 pub mod bip158;
 

src/util/taproot.rs

@@ -0,0 +1,166 @@
+// Rust Bitcoin Library
+// Written in 2019 by
+//     The rust-bitcoin developers.
+// To the extent possible under law, the author(s) have dedicated all
+// copyright and related and neighboring rights to this software to
+// the public domain worldwide. This software is distributed without
+// any warranty.
+//
+// You should have received a copy of the CC0 Public Domain Dedication
+// along with this software.
+// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+//
+
+//! Taproot
+//!
+
+use hashes::{sha256, sha256t, Hash};
+
+/// The SHA-256 midstate value for the TapLeaf hash.
+const MIDSTATE_TAPLEAF: [u8; 32] = [
+    156, 224, 228, 230, 124, 17, 108, 57, 56, 179, 202, 242, 195, 15, 80, 137, 211, 243, 147, 108,
+    71, 99, 110, 96, 125, 179, 62, 234, 221, 198, 240, 201,
+];
+// 9ce0e4e67c116c3938b3caf2c30f5089d3f3936c47636e607db33eeaddc6f0c9
+
+/// The SHA-256 midstate value for the TapBranch hash.
+const MIDSTATE_TAPBRANCH: [u8; 32] = [
+    35, 168, 101, 169, 184, 164, 13, 167, 151, 124, 30, 4, 196, 158, 36, 111, 181, 190, 19, 118,
+    157, 36, 201, 183, 181, 131, 181, 212, 168, 210, 38, 210,
+];
+// 23a865a9b8a40da7977c1e04c49e246fb5be13769d24c9b7b583b5d4a8d226d2
+
+/// The SHA-256 midstate value for the TapTweak hash.
+const MIDSTATE_TAPTWEAK: [u8; 32] = [
+    209, 41, 162, 243, 112, 28, 101, 93, 101, 131, 182, 195, 185, 65, 151, 39, 149, 244, 226, 50,
+    148, 253, 84, 244, 162, 174, 141, 133, 71, 202, 89, 11,
+];
+// d129a2f3701c655d6583b6c3b941972795f4e23294fd54f4a2ae8d8547ca590b
+
+/// The SHA-256 midstate value for the TapSigHash hash.
+const MIDSTATE_TAPSIGHASH: [u8; 32] = [
+    245, 4, 164, 37, 215, 248, 120, 59, 19, 99, 134, 138, 227, 229, 86, 88, 110, 238, 148, 93, 188,
+    120, 136, 221, 2, 166, 226, 195, 24, 115, 254, 159,
+];
+// f504a425d7f8783b1363868ae3e556586eee945dbc7888dd02a6e2c31873fe9f
+
+/// Internal macro to speficy the different taproot tagged hashes.
+macro_rules! sha256t_hash_newtype {
+    ($newtype:ident, $tag:ident, $midstate:ident, $midstate_len:expr, $docs:meta, $reverse: expr) => {
+        /// The tag used for [$newtype].
+        pub struct $tag;
+
+        impl sha256t::Tag for $tag {
+            fn engine() -> sha256::HashEngine {
+                let midstate = sha256::Midstate::from_inner($midstate);
+                sha256::HashEngine::from_midstate(midstate, $midstate_len)
+            }
+        }
+
+        hash_newtype!($newtype, sha256t::Hash<$tag>, 32, $docs, $reverse);
+    };
+}
+
+// Currently all taproot hashes are defined as being displayed backwards,
+// but that can be specified individually per hash.
+sha256t_hash_newtype!(TapLeafHash, TapLeafTag, MIDSTATE_TAPLEAF, 64,
+    doc="Taproot-tagged hash for tapscript Merkle tree leafs", true
+);
+sha256t_hash_newtype!(TapBranchHash, TapBranchTag, MIDSTATE_TAPBRANCH, 64,
+    doc="Taproot-tagged hash for tapscript Merkle tree branches", true
+);
+sha256t_hash_newtype!(TapTweakHash, TapTweakTag, MIDSTATE_TAPTWEAK, 64,
+    doc="Taproot-tagged hash for public key tweaks", true
+);
+sha256t_hash_newtype!(TapSighashHash, TapSighashTag, MIDSTATE_TAPSIGHASH, 64,
+    doc="Taproot-tagged hash for the taproot signature hash", true
+);
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use hashes::hex::ToHex;
+    use hashes::sha256t::Tag;
+    use hashes::{sha256, Hash, HashEngine};
+
+    fn tag_engine(tag_name: &str) -> sha256::HashEngine {
+        let mut engine = sha256::Hash::engine();
+        let tag_hash = sha256::Hash::hash(tag_name.as_bytes());
+        engine.input(&tag_hash[..]);
+        engine.input(&tag_hash[..]);
+        engine
+    }
+
+    #[test]
+    fn test_midstates() {
+        // check midstate against hard-coded values
+        assert_eq!(MIDSTATE_TAPLEAF, tag_engine("TapLeaf").midstate().into_inner());
+        assert_eq!(MIDSTATE_TAPBRANCH, tag_engine("TapBranch").midstate().into_inner());
+        assert_eq!(MIDSTATE_TAPTWEAK, tag_engine("TapTweak").midstate().into_inner());
+        assert_eq!(MIDSTATE_TAPSIGHASH, tag_engine("TapSighash").midstate().into_inner());
+
+        // test that engine creation roundtrips
+        assert_eq!(tag_engine("TapLeaf").midstate(), TapLeafTag::engine().midstate());
+        assert_eq!(tag_engine("TapBranch").midstate(), TapBranchTag::engine().midstate());
+        assert_eq!(tag_engine("TapTweak").midstate(), TapTweakTag::engine().midstate());
+        assert_eq!(tag_engine("TapSighash").midstate(), TapSighashTag::engine().midstate());
+
+        // check that hash creation is the same as building into the same engine
+        fn empty_hash(tag_name: &str) -> [u8; 32] {
+            let mut e = tag_engine(tag_name);
+            e.input(&[]);
+            sha256::Hash::from_engine(e).into_inner()
+        }
+        assert_eq!(empty_hash("TapLeaf"), TapLeafHash::hash(&[]).into_inner());
+        assert_eq!(empty_hash("TapBranch"), TapBranchHash::hash(&[]).into_inner());
+        assert_eq!(empty_hash("TapTweak"), TapTweakHash::hash(&[]).into_inner());
+        assert_eq!(empty_hash("TapSighash"), TapSighashHash::hash(&[]).into_inner());
+    }
+
+    #[test]
+    fn test_vectors_core() {
+        //! Test vectors taken from Core
+
+        // uninitialized writers
+        //   CHashWriter writer = HasherTapLeaf;
+        //   writer.GetSHA256().GetHex()
+        assert_eq!(
+            TapLeafHash::from_engine(TapLeafTag::engine()).to_hex(),
+            "cbfa0621df37662ca57697e5847b6abaf92934a1a5624916f8d177a388c21252"
+        );
+        assert_eq!(
+            TapBranchHash::from_engine(TapBranchTag::engine()).to_hex(),
+            "dffd9fbe4c21c893fa934f8774eda0e1efdc06f52ffbf5c1533c6f4dec73c353"
+        );
+        assert_eq!(
+            TapTweakHash::from_engine(TapTweakTag::engine()).to_hex(),
+            "e4156b45ff9b277dd92a042af9eed8c91f1d037f68f0d6b20001ab749422a48a"
+        );
+        assert_eq!(
+            TapSighashHash::from_engine(TapSighashTag::engine()).to_hex(),
+            "03c8b9d47cdb5f7bf924e282ce99ba8d2fe581262a04002907d8bc4a9111bcda"
+        );
+
+        // 0-byte
+        //   CHashWriter writer = HasherTapLeaf;
+        //   writer << std::vector<unsigned char>{};
+        //   writer.GetSHA256().GetHex()
+        // Note that Core writes the 0 length prefix when an empty vector is written.
+        assert_eq!(
+            TapLeafHash::hash(&[0]).to_hex(),
+            "29589d5122ec666ab5b4695070b6debc63881a4f85d88d93ddc90078038213ed"
+        );
+        assert_eq!(
+            TapBranchHash::hash(&[0]).to_hex(),
+            "1deb45569eb6b2da88b5c2ab46d6a64ab08d58a2fdd5f75a24e6c760194b5392"
+        );
+        assert_eq!(
+            TapTweakHash::hash(&[0]).to_hex(),
+            "1eea90d42a359c89bbf702ddf6bde140349e95b9e8036ff1c37f04e6b53787cd"
+        );
+        assert_eq!(
+            TapSighashHash::hash(&[0]).to_hex(),
+            "cd10c023c300fb9a507dff136370fba1d8a0566667cfafc4099a8803e00dfdc2"
+        );
+    }
+}