Make empty byte arrays pass CheckSignatureEncoding()

Makes it possible to compactly provide a delibrately invalid signature
for use with CHECK(MULTI)SIG. For instance with BIP19 if m != n invalid
signatures need to be provided in the scriptSig; prior to this change
those invalid signatures would need to be large DER-encoded signatures.

Note that we may want to further expand on this change in the future by
saying that only OP_0 is a "valid" invalid signature; BIP19 even with
this change is inherently malleable as the invalid signatures can be any
validly encoded DER signature.

src/script/interpreter.cpp

@@ -189,6 +189,11 @@ bool static IsDefinedHashtypeSignature(const valtype &vchSig) {
 }
 
 bool static CheckSignatureEncoding(const valtype &vchSig, unsigned int flags, ScriptError* serror) {
+    // Empty signature. Not strictly DER encoded, but allowed to provide a
+    // compact way to provide an invalid signature for use with CHECK(MULTI)SIG
+    if (vchSig.size() == 0) {
+        return true;
+    }
     if ((flags & (SCRIPT_VERIFY_DERSIG | SCRIPT_VERIFY_LOW_S | SCRIPT_VERIFY_STRICTENC)) != 0 && !IsDERSignature(vchSig)) {
         return set_error(serror, SCRIPT_ERR_SIG_DER);
     } else if ((flags & SCRIPT_VERIFY_LOW_S) != 0 && !IsLowDERSignature(vchSig, serror)) {

src/test/data/script_invalid.json

@@ -504,7 +504,7 @@
     "2-of-2 CHECKMULTISIG NOT with the first pubkey invalid, and both signatures validly encoded."
 ],
 [
-    "0 0x47 0x3044022044dc17b0887c161bb67ba9635bf758735bdde503e4b0a0987f587f14a4e1143d022009a215772d49a85dae40d8ca03955af26ad3978a0ff965faa12915e9586249a501 0",
+    "0 0x47 0x3044022044dc17b0887c161bb67ba9635bf758735bdde503e4b0a0987f587f14a4e1143d022009a215772d49a85dae40d8ca03955af26ad3978a0ff965faa12915e9586249a501 1",
     "2 0x21 0x02865c40293a680cb9c020e7b1e106d8c1916d3cef99aa431a56d253e69256dac0 0x21 0x02865c40293a680cb9c020e7b1e106d8c1916d3cef99aa431a56d253e69256dac0 2 CHECKMULTISIG NOT",
     "STRICTENC",
     "2-of-2 CHECKMULTISIG NOT with both pubkeys valid, but first signature invalid."

src/test/data/script_valid.json

@@ -662,6 +662,11 @@
 ["0 0 0x02 0x0000", "CHECKMULTISIGVERIFY 1", ""],
 ["0 0x02 0x0000 0", "CHECKMULTISIGVERIFY 1", ""],
 
+["While not really correctly DER encoded, the empty signature is allowed by"],
+["STRICTENC to provide a compact way to provide a delibrately invalid signature."],
+["0", "0x21 0x02865c40293a680cb9c020e7b1e106d8c1916d3cef99aa431a56d253e69256dac0 CHECKSIG NOT", "STRICTENC"],
+["0 0", "1 0x21 0x02865c40293a680cb9c020e7b1e106d8c1916d3cef99aa431a56d253e69256dac0 1 CHECKMULTISIG NOT", "STRICTENC"],
+
 ["CHECKMULTISIG evaluation order tests. CHECKMULTISIG evaluates signatures and"],
 ["pubkeys in a specific order, and will exit early if the number of signatures"],
 ["left to check is greater than the number of keys left. As STRICTENC fails the"],

src/test/multisig_tests.cpp

@@ -116,10 +116,6 @@ BOOST_AUTO_TEST_CASE(multisig_verify)
         }
     }
     s.clear();
-    s << OP_0 << OP_0;
-    BOOST_CHECK(!VerifyScript(s, a_or_b, flags, SignatureChecker(txTo[1], 0), &err));
-    BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_SIG_DER, ScriptErrorString(err));
-    s.clear();
     s << OP_0 << OP_1;
     BOOST_CHECK(!VerifyScript(s, a_or_b, flags, SignatureChecker(txTo[1], 0), &err));
     BOOST_CHECK_MESSAGE(err == SCRIPT_ERR_SIG_DER, ScriptErrorString(err));