Provide a Rustier wrapper for zcash_script (ZcashFoundation#171)

* Move C++ bindings out of lib.rs

Have lib.rs re-export them, but make room for the upcoming Rust implementation.

* Provide a Rustier wrapper for zcash_script

This adds a `Script` trait that exposes slightly Rustier types in order
to have a common interface for the existing C++ implementation as well
as the upcoming Rust implementation (and a third instance that runs both
and checks that the Rust result matches the C++ one).

The module structure (interpreter.rs, zcash_script.rs) and locations of
definitions are intended to mirror the structure of the C++ code, especially as
we get the Rust implementation in place, for easier comparison. That
organization is very likely to change once everything has been checked.

* Address review feedback

Thanks to @nuttycom and @arya2 for getting the closure to work.

* Additional cleanup

* Use `try_from`/`_into` instead of `as`

* Address review feedback

* Widen the `Unknown` error type

This should fix the Windows build.

Cargo.toml

@@ -60,6 +60,7 @@ path = "src/lib.rs"
 external-secp = []
 
 [dependencies]
+bitflags = "2.5"
 
 [build-dependencies]
 # The `bindgen` dependency should automatically upgrade to match the version used by zebra-state's `rocksdb` dependency in:

src/cxx.rs

@@ -0,0 +1,115 @@
+//! Rust bindings for Zcash transparent scripts.
+
+#![allow(missing_docs)]
+#![allow(clippy::needless_lifetimes)]
+#![allow(non_upper_case_globals)]
+#![allow(non_camel_case_types)]
+#![allow(non_snake_case)]
+#![allow(unsafe_code)]
+#![allow(unused_imports)]
+#![allow(clippy::unwrap_or_default)]
+
+// Use the generated C++ bindings
+include!(concat!(env!("OUT_DIR"), "/bindings.rs"));
+#[cfg(test)]
+mod tests {
+    use std::ffi::{c_int, c_uint, c_void};
+
+    pub use super::zcash_script_error_t;
+    use hex::FromHex;
+
+    lazy_static::lazy_static! {
+        pub static ref SCRIPT_PUBKEY: Vec<u8> = <Vec<u8>>::from_hex("a914c117756dcbe144a12a7c33a77cfa81aa5aeeb38187").unwrap();
+        pub static ref SCRIPT_SIG: Vec<u8> = <Vec<u8>>::from_hex("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").expect("Block bytes are in valid hex representation");
+    }
+
+    extern "C" fn sighash(
+        sighash_out: *mut u8,
+        sighash_out_len: c_uint,
+        ctx: *const c_void,
+        _script_code: *const u8,
+        _script_code_len: c_uint,
+        _hash_type: c_int,
+    ) {
+        unsafe {
+            assert!(ctx.is_null());
+            let sighash =
+                hex::decode("e8c7bdac77f6bb1f3aba2eaa1fada551a9c8b3b5ecd1ef86e6e58a5f1aab952c")
+                    .unwrap();
+            assert!(sighash_out_len == sighash.len() as c_uint);
+            std::ptr::copy_nonoverlapping(sighash.as_ptr(), sighash_out, sighash.len());
+        }
+    }
+
+    extern "C" fn invalid_sighash(
+        sighash_out: *mut u8,
+        sighash_out_len: c_uint,
+        ctx: *const c_void,
+        _script_code: *const u8,
+        _script_code_len: c_uint,
+        _hash_type: c_int,
+    ) {
+        unsafe {
+            assert!(ctx.is_null());
+            let sighash =
+                hex::decode("08c7bdac77f6bb1f3aba2eaa1fada551a9c8b3b5ecd1ef86e6e58a5f1aab952c")
+                    .unwrap();
+            assert!(sighash_out_len == sighash.len() as c_uint);
+            std::ptr::copy_nonoverlapping(sighash.as_ptr(), sighash_out, sighash.len());
+        }
+    }
+
+    #[test]
+    fn it_works() {
+        let nLockTime: i64 = 2410374;
+        let isFinal: u8 = 1;
+        let script_pub_key = &*SCRIPT_PUBKEY;
+        let script_sig = &*SCRIPT_SIG;
+        let flags: c_uint = 513;
+        let mut err = 0;
+
+        let ret = unsafe {
+            super::zcash_script_verify_callback(
+                std::ptr::null(),
+                Some(sighash),
+                nLockTime,
+                isFinal,
+                script_pub_key.as_ptr(),
+                script_pub_key.len() as c_uint,
+                script_sig.as_ptr(),
+                script_sig.len() as c_uint,
+                flags,
+                &mut err,
+            )
+        };
+
+        assert!(ret == 1);
+    }
+
+    #[test]
+    fn it_fails_on_invalid_sighash() {
+        let nLockTime: i64 = 2410374;
+        let isFinal: u8 = 1;
+        let script_pub_key = &*SCRIPT_PUBKEY;
+        let script_sig = &*SCRIPT_SIG;
+        let flags: c_uint = 513;
+        let mut err = 0;
+
+        let ret = unsafe {
+            super::zcash_script_verify_callback(
+                std::ptr::null(),
+                Some(invalid_sighash),
+                nLockTime,
+                isFinal,
+                script_pub_key.as_ptr(),
+                script_pub_key.len() as c_uint,
+                script_sig.as_ptr(),
+                script_sig.len() as c_uint,
+                flags,
+                &mut err,
+            )
+        };
+
+        assert!(ret != 1);
+    }
+}

src/interpreter.rs

@@ -0,0 +1,71 @@
+bitflags::bitflags! {
+    /// The different SigHash types, as defined in <https://zips.z.cash/zip-0143>
+    ///
+    /// TODO: This is currently defined as `i32` to match the `c_int` constants in this package, but
+    ///       should use librustzcash’s `u8` constants once we’ve removed the C++.
+    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
+    pub struct HashType: i32 {
+        /// Sign all the outputs
+        const All = 1;
+        /// Sign none of the outputs - anyone can spend
+        const None = 2;
+        /// Sign one of the outputs - anyone can spend the rest
+        const Single = 3;
+        /// Anyone can add inputs to this transaction
+        const AnyoneCanPay = 0x80;
+    }
+}
+
+bitflags::bitflags! {
+    #[derive(Copy, Clone, Debug, PartialEq, Eq)]
+    /// Script verification flags
+    pub struct VerificationFlags: u32 {
+        /// Evaluate P2SH subscripts (softfork safe,
+        /// [BIP16](https://github.com/bitcoin/bips/blob/master/bip-0016.mediawiki).
+        const P2SH = 1 << 0;
+
+        /// Passing a non-strict-DER signature or one with undefined hashtype to a checksig operation causes script failure.
+        /// Evaluating a pubkey that is not (0x04 + 64 bytes) or (0x02 or 0x03 + 32 bytes) by checksig causes script failure.
+        /// (softfork safe, but not used or intended as a consensus rule).
+        const StrictEnc = 1 << 1;
+
+        /// Passing a non-strict-DER signature or one with S > order/2 to a checksig operation causes script failure
+        /// (softfork safe, [BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 5).
+        const LowS = 1 << 3;
+
+        /// verify dummy stack item consumed by CHECKMULTISIG is of zero-length (softfork safe, [BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 7).
+        const NullDummy = 1 << 4;
+
+        /// Using a non-push operator in the scriptSig causes script failure (softfork safe, [BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 2).
+        const SigPushOnly = 1 << 5;
+
+        /// Require minimal encodings for all push operations (OP_0... OP_16, OP_1NEGATE where possible, direct
+        /// pushes up to 75 bytes, OP_PUSHDATA up to 255 bytes, OP_PUSHDATA2 for anything larger). Evaluating
+        /// any other push causes the script to fail ([BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 3).
+        /// In addition, whenever a stack element is interpreted as a number, it must be of minimal length ([BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 4).
+        /// (softfork safe)
+        const MinimalData = 1 << 6;
+
+        /// Discourage use of NOPs reserved for upgrades (NOP1-10)
+        ///
+        /// Provided so that nodes can avoid accepting or mining transactions
+        /// containing executed NOP's whose meaning may change after a soft-fork,
+        /// thus rendering the script invalid; with this flag set executing
+        /// discouraged NOPs fails the script. This verification flag will never be
+        /// a mandatory flag applied to scripts in a block. NOPs that are not
+        /// executed, e.g.  within an unexecuted IF ENDIF block, are *not* rejected.
+        const DiscourageUpgradableNOPs = 1 << 7;
+
+        /// Require that only a single stack element remains after evaluation. This changes the success criterion from
+        /// "At least one stack element must remain, and when interpreted as a boolean, it must be true" to
+        /// "Exactly one stack element must remain, and when interpreted as a boolean, it must be true".
+        /// (softfork safe, [BIP62](https://github.com/bitcoin/bips/blob/master/bip-0062.mediawiki) rule 6)
+        /// Note: CLEANSTACK should never be used without P2SH.
+        const CleanStack = 1 << 8;
+
+        /// Verify CHECKLOCKTIMEVERIFY
+        ///
+        /// See [BIP65](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki) for details.
+        const CHECKLOCKTIMEVERIFY = 1 << 9;
+    }
+}