signet: miner skips PSBT step for OP_TRUE

contrib/signet/README.md

@@ -81,3 +81,4 @@ These steps can instead be done explicitly:
 
 This is intended to allow you to replace part of the pipeline for further experimentation (eg, to sign the block with a hardware wallet).
 
+For custom signets with a trivial challenge such as  `OP_TRUE` the walletprocesspsbt step can be skipped.

contrib/signet/miner

@@ -77,15 +77,20 @@ def decode_challenge_psbt(b64psbt):
 def get_block_from_psbt(psbt):
     return from_binary(CBlock, psbt.g.map[PSBT_SIGNET_BLOCK])
 
-def get_solution_from_psbt(psbt):
+def get_solution_from_psbt(psbt, emptyok=False):
     scriptSig = psbt.i[0].map.get(PSBT_IN_FINAL_SCRIPTSIG, b"")
     scriptWitness = psbt.i[0].map.get(PSBT_IN_FINAL_SCRIPTWITNESS, b"\x00")
+    if emptyok and len(scriptSig) == 0 and scriptWitness == b"\x00":
+        return None
     return ser_string(scriptSig) + scriptWitness
 
 def finish_block(block, signet_solution, grind_cmd):
-    block.vtx[0].vout[-1].scriptPubKey += CScriptOp.encode_op_pushdata(SIGNET_HEADER + signet_solution)
-    block.vtx[0].rehash()
-    block.hashMerkleRoot = block.calc_merkle_root()
+    if signet_solution is None:
+        pass # Don't need to add a signet commitment if there's no signet signature needed
+    else:
+        block.vtx[0].vout[-1].scriptPubKey += CScriptOp.encode_op_pushdata(SIGNET_HEADER + signet_solution)
+        block.vtx[0].rehash()
+        block.hashMerkleRoot = block.calc_merkle_root()
     if grind_cmd is None:
         block.solve()
     else:
@@ -97,10 +102,7 @@ def finish_block(block, signet_solution, grind_cmd):
         block.rehash()
     return block
 
-def generate_psbt(tmpl, reward_spk, *, blocktime=None, poolid=None):
-    signet_spk = tmpl["signet_challenge"]
-    signet_spk_bin = bytes.fromhex(signet_spk)
-
+def new_block(tmpl, reward_spk, *, blocktime=None, poolid=None):
     scriptSig = script_BIP34_coinbase_height(tmpl["height"])
     if poolid is not None:
         scriptSig = CScript(b"" + scriptSig + CScriptOp.encode_op_pushdata(poolid))
@@ -128,8 +130,14 @@ def generate_psbt(tmpl, reward_spk, *, blocktime=None, poolid=None):
     block.vtx[0].wit.vtxinwit = [cbwit]
     block.vtx[0].vout.append(CTxOut(0, bytes(get_witness_script(witroot, witnonce))))
 
-    signme, spendme = signet_txs(block, signet_spk_bin)
+    block.vtx[0].rehash()
+    block.hashMerkleRoot = block.calc_merkle_root()
 
+    return block
+
+def generate_psbt(block, signet_spk):
+    signet_spk_bin = bytes.fromhex(signet_spk)
+    signme, spendme = signet_txs(block, signet_spk_bin)
     psbt = PSBT()
     psbt.g = PSBTMap( {PSBT_GLOBAL_UNSIGNED_TX: signme.serialize(),
                        PSBT_SIGNET_BLOCK: block.serialize()
@@ -178,14 +186,16 @@ def get_reward_addr_spk(args, height):
 def do_genpsbt(args):
     poolid = get_poolid(args)
     tmpl = json.load(sys.stdin)
+    signet_spk = tmpl["signet_challenge"]
     _, reward_spk = get_reward_addr_spk(args, tmpl["height"])
-    psbt = generate_psbt(tmpl, reward_spk, poolid=poolid)
+    block = new_block(tmpl, reward_spk, poolid=poolid)
+    psbt = generate_psbt(block, signet_spk)
     print(psbt)
 
 def do_solvepsbt(args):
     psbt = decode_challenge_psbt(sys.stdin.read())
     block = get_block_from_psbt(psbt)
-    signet_solution = get_solution_from_psbt(psbt)
+    signet_solution = get_solution_from_psbt(psbt, emptyok=True)
     block = finish_block(block, signet_solution, args.grind_cmd)
     print(block.serialize().hex())
 
@@ -228,6 +238,21 @@ def seconds_to_hms(s):
         out = "-" + out
     return out
 
+def trivial_challenge(spkhex):
+    """
+    BIP325 allows omitting the signet commitment when scriptSig and
+    scriptWitness are both empty. This is the case for trivial
+    challenges such as OP_TRUE
+    """
+    spk = bytes.fromhex(spkhex)
+    if len(spk) == 1 and spk[0] == 0x51:
+        # OP_TRUE
+        return True
+    elif 2 <= len(spk) <= 76 and spk[0] + 1 == len(spk):
+        # Single fixed push of 1-75 bytes
+        return True
+    return False
+
 class Generate:
     INTERVAL = 600.0*2016/2015 # 10 minutes, adjusted for the off-by-one bug
 
@@ -328,16 +353,22 @@ class Generate:
         return tmpl
 
     def mine(self, bcli, grind_cmd, tmpl, reward_spk):
-        psbt = generate_psbt(tmpl, reward_spk, blocktime=self.mine_time, poolid=self.poolid)
-        input_stream = os.linesep.join([psbt, "true", "ALL"]).encode('utf8')
-        psbt_signed = json.loads(bcli("-stdin", "walletprocesspsbt", input=input_stream))
-        if not psbt_signed.get("complete",False):
-            logging.debug("Generated PSBT: %s" % (psbt,))
-            sys.stderr.write("PSBT signing failed\n")
-            return None
-        psbt = decode_challenge_psbt(psbt_signed["psbt"])
-        block = get_block_from_psbt(psbt)
-        signet_solution = get_solution_from_psbt(psbt)
+        block = new_block(tmpl, reward_spk, blocktime=self.mine_time, poolid=self.poolid)
+
+        signet_spk = tmpl["signet_challenge"]
+        if trivial_challenge(signet_spk):
+            signet_solution = None
+        else:
+            psbt = generate_psbt(block, signet_spk)
+            input_stream = os.linesep.join([psbt, "true", "ALL"]).encode('utf8')
+            psbt_signed = json.loads(bcli("-stdin", "walletprocesspsbt", input=input_stream))
+            if not psbt_signed.get("complete",False):
+               logging.debug("Generated PSBT: %s" % (psbt,))
+               sys.stderr.write("PSBT signing failed\n")
+               return None
+            psbt = decode_challenge_psbt(psbt_signed["psbt"])
+            signet_solution = get_solution_from_psbt(psbt)
+
         return finish_block(block, signet_solution, grind_cmd)
 
 def do_generate(args):

test/functional/tool_signet_miner.py

@@ -10,14 +10,26 @@
 import time
 
 from test_framework.key import ECKey
-from test_framework.script_util import key_to_p2wpkh_script
+from test_framework.script_util import CScript, key_to_p2wpkh_script
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
 from test_framework.wallet_util import bytes_to_wif
 
 
 CHALLENGE_PRIVATE_KEY = (42).to_bytes(32, 'big')
+SIGNET_COMMITMENT = 'ecc7daa2'
 
+def get_segwit_commitment(node):
+    coinbase = node.getblock(node.getbestblockhash(), 2)['tx'][0]
+    commitment = coinbase['vout'][1]['scriptPubKey']['hex']
+    assert_equal(commitment[0:12], '6a24aa21a9ed')
+    return commitment
+
+def get_signet_commitment(segwit_commitment):
+    for el in CScript.fromhex(segwit_commitment):
+        if isinstance(el, bytes) and el[0:4].hex() == SIGNET_COMMITMENT:
+            return el[4:].hex()
+    return None
 
 class SignetMinerTest(BitcoinTestFramework):
     def add_options(self, parser):
@@ -26,26 +38,32 @@ def add_options(self, parser):
     def set_test_params(self):
         self.chain = "signet"
         self.setup_clean_chain = True
-        self.num_nodes = 1
+        self.num_nodes = 3
 
         # generate and specify signet challenge (simple p2wpkh script)
         privkey = ECKey()
         privkey.set(CHALLENGE_PRIVATE_KEY, True)
         pubkey = privkey.get_pubkey().get_bytes()
         challenge = key_to_p2wpkh_script(pubkey)
-        self.extra_args = [[f'-signetchallenge={challenge.hex()}']]
+
+        self.extra_args = [
+            [f'-signetchallenge={challenge.hex()}'],
+            ["-signetchallenge=51"], # OP_TRUE
+            ["-signetchallenge=202cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824"], # sha256("hello")
+        ]
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_cli()
         self.skip_if_no_wallet()
         self.skip_if_no_bitcoin_util()
 
-    def run_test(self):
-        node = self.nodes[0]
-        # import private key needed for signing block
-        node.importprivkey(bytes_to_wif(CHALLENGE_PRIVATE_KEY))
+    def setup_network(self):
+        self.setup_nodes()
+        # Nodes with different signet networks are not connected
 
-        # generate block with signet miner tool
+    # generate block with signet miner tool
+    def mine_block(self, node):
+        n_blocks = node.getblockcount()
         base_dir = self.config["environment"]["SRCDIR"]
         signet_miner_path = os.path.join(base_dir, "contrib", "signet", "miner")
         subprocess.run([
@@ -59,7 +77,29 @@ def run_test(self):
                 f'--set-block-time={int(time.time())}',
                 '--poolnum=99',
             ], check=True, stderr=subprocess.STDOUT)
-        assert_equal(node.getblockcount(), 1)
+        assert_equal(node.getblockcount(), n_blocks + 1)
+
+    def run_test(self):
+        self.log.info("Signet node with single signature challenge")
+        node = self.nodes[0]
+        # import private key needed for signing block
+        node.importprivkey(bytes_to_wif(CHALLENGE_PRIVATE_KEY))
+        self.mine_block(node)
+        # MUST include signet commitment
+        assert get_signet_commitment(get_segwit_commitment(node))
+
+        node = self.nodes[1]
+        self.log.info("Signet node with trivial challenge (OP_TRUE)")
+        self.mine_block(node)
+        # MAY omit signet commitment (BIP 325). Do so for better compatibility
+        # with signet unaware mining software and hardware.
+        assert get_signet_commitment(get_segwit_commitment(node)) is None
+
+        node = self.nodes[2]
+        self.log.info("Signet node with trivial challenge (push sha256 hash)")
+        self.mine_block(node)
+        assert get_signet_commitment(get_segwit_commitment(node)) is None
+
 
 
 if __name__ == "__main__":