Merge rust-bitcoin/rust-bitcoin#2078: Script helper to classify bare multisig

acbf23aaa5 Add `is_multisig` helper to Script type (Clark Moody)

Pull request description:

  A new `is_multisig` helper method to classify bare multisig output scripts.

  The form of a valid multisig script is:
  - Pushnum `M`
  - <N> pubkeys
  - Pushnum `N`
  - `OP_CHECKMULTISIG`

  `N` must equal the number of pushed pubkeys, and `M` must be less than or equal to `N`.

  I've tested this against the RPC output of Core at the block level, checking that the total number of multisig outputs matches.

  ```
  Block 350338, 89 multisig
  Block 350340, 29 multisig
  Block 350341, 4 multisig
  Block 350343, 579 multisig
  Block 350344, 48 multisig
  Block 350346, 11 multisig
  Block 350347, 404 multisig
  Block 350350, 127 multisig
  Block 350351, 1 multisig
  Block 350353, 40 multisig
  Block 350356, 13 multisig
  Block 350357, 2 multisig
  Block 350358, 1 multisig
  ```

ACKs for top commit:
  tcharding:
    ACK acbf23aaa5
  apoelstra:
    ACK acbf23aaa5

Tree-SHA512: b8feeaa8725ac63a658897dac3b303fc8b3d56674d796b14569548124928329993bea45482928d9ce85231f1b5837922af8c0a77b2601a92f88b5e2a9394e97f

bitcoin/src/blockdata/script/borrowed.rs

@@ -218,6 +218,59 @@ impl Script {
         }
     }
 
+    /// Checks whether a script pubkey is a bare multisig output.
+    ///
+    /// In a bare multisig pubkey script the keys are not hashed, the script
+    /// is of the form:
+    ///
+    ///    `2 <pubkey1> <pubkey2> <pubkey3> 3 OP_CHECKMULTISIG`
+    #[inline]
+    pub fn is_multisig(&self) -> bool {
+        let required_sigs;
+
+        let mut instructions = self.instructions();
+        if let Some(Ok(Instruction::Op(op))) = instructions.next() {
+            if let Some(pushnum) = op.decode_pushnum() {
+                required_sigs = pushnum;
+            } else {
+                return false;
+            }
+        } else {
+            return false;
+        }
+
+        let mut num_pubkeys: u8 = 0;
+        while let Some(Ok(instruction)) = instructions.next() {
+            match instruction {
+                Instruction::PushBytes(_) => {
+                    num_pubkeys += 1;
+                }
+                Instruction::Op(op) => {
+                    if let Some(pushnum) = op.decode_pushnum() {
+                        if pushnum != num_pubkeys {
+                            return false;
+                        }
+                    }
+                    break;
+                }
+            }
+        }
+
+        if required_sigs > num_pubkeys {
+            return false;
+        }
+
+        if let Some(Ok(Instruction::Op(op))) = instructions.next() {
+            if op != OP_CHECKMULTISIG {
+                return false;
+            }
+        } else {
+            return false;
+        }
+
+        instructions.next().is_none()
+    }
+
     /// Checks whether a script pubkey is a Segregated Witness (segwit) program.
     #[inline]
     pub fn is_witness_program(&self) -> bool {

bitcoin/src/blockdata/script/tests.rs

@@ -376,6 +376,53 @@ fn op_return_test() {
     assert!(!ScriptBuf::from_hex("").unwrap().is_op_return());
 }
 
+#[test]
+fn multisig() {
+    // First multisig? 1-of-2
+    // In block 164467, txid 60a20bd93aa49ab4b28d514ec10b06e1829ce6818ec06cd3aabd013ebcdc4bb1
+    assert!(
+        ScriptBuf::from_hex("514104cc71eb30d653c0c3163990c47b976f3fb3f37cccdcbedb169a1dfef58bbfbfaff7d8a473e7e2e6d317b87bafe8bde97e3cf8f065dec022b51d11fcdd0d348ac4410461cbdcc5409fb4b4d42b51d33381354d80e550078cb532a34bfa2fcfdeb7d76519aecc62770f5b0e4ef8551946d8a540911abe3e7854a26f39f58b25c15342af52ae")
+            .unwrap()
+            .is_multisig()
+    );
+    // 2-of-2
+    assert!(
+        ScriptBuf::from_hex("5221021c4ac2ecebc398e390e07f045aac5cc421f82f0739c1ce724d3d53964dc6537d21023a2e9155e0b62f76737605504819a2b4e5ce20653f6c397d7a178ae42ba702f452ae")
+            .unwrap()
+            .is_multisig()
+    );
+
+    // Extra opcode after OP_CHECKMULTISIG
+    assert!(
+        !ScriptBuf::from_hex("5221021c4ac2ecebc398e390e07f045aac5cc421f82f0739c1ce724d3d53964dc6537d21023a2e9155e0b62f76737605504819a2b4e5ce20653f6c397d7a178ae42ba702f452ae52")
+            .unwrap()
+            .is_multisig()
+    );
+    // Required sigs > num pubkeys
+    assert!(
+        !ScriptBuf::from_hex("5321021c4ac2ecebc398e390e07f045aac5cc421f82f0739c1ce724d3d53964dc6537d21023a2e9155e0b62f76737605504819a2b4e5ce20653f6c397d7a178ae42ba702f452ae")
+            .unwrap()
+            .is_multisig()
+    );
+    // Num pubkeys != pushnum
+    assert!(
+        !ScriptBuf::from_hex("5221021c4ac2ecebc398e390e07f045aac5cc421f82f0739c1ce724d3d53964dc6537d21023a2e9155e0b62f76737605504819a2b4e5ce20653f6c397d7a178ae42ba702f453ae")
+            .unwrap()
+            .is_multisig()
+    );
+
+    // Taproot hash from another test
+    assert!(!ScriptBuf::from_hex(
+        "20d85a959b0290bf19bb89ed43c916be835475d013da4b362117393e25a48229b8ac"
+    )
+    .unwrap()
+    .is_multisig());
+    // OP_RETURN from another test
+    assert!(!ScriptBuf::from_hex("6aa9149eb21980dc9d413d8eac27314938b9da920ee53e87")
+        .unwrap()
+        .is_multisig());
+}
+
 #[test]
 #[cfg(feature = "serde")]
 fn script_json_serialize() {