Merge #497: [0.17] Add Liquid v1 based script matching functionality

592a622 Add detection of liquidv1 fedpeg script, startup sanity check (Gregory Sanders)

Pull request description:

  And use it to sanity check startup constants.

Tree-SHA512: 1e4c3910c5c269384b5dab7a3f38646356a8de7684515324320d610915466e5015e9993d7a2a619473ea3500f3f9145526414548d6da6ee22b4417b6836042bf

src/pegins.cpp

@@ -70,20 +70,23 @@ bool GetAmountFromParentChainPegin(CAmount& amount, const CTransaction& txBTC, u
 }
 
 // Takes federation redeem script and adds HMAC_SHA256(pubkey, scriptPubKey) as a tweak to each pubkey
-CScript calculate_contract(const CScript& federationRedeemScript, const CScript& scriptPubKey) {
+CScript calculate_contract(const CScript& federation_script, const CScript& scriptPubKey) {
     CScript scriptDestination;
-    txnouttype type;
     std::vector<std::vector<unsigned char> > solutions;
-    // Sanity check fedRedeemScript
-    if (!Solver(federationRedeemScript, type, solutions) || (type != TX_MULTISIG && type != TX_TRUE)) {
+    unsigned int required;
+    std::vector<std::vector<unsigned char>> keys;
+    // Sanity check federation_script only to match 3 templates
+    if (federation_script != CScript() << OP_TRUE &&
+            !MatchMultisig(federation_script, required, keys) &&
+            !MatchLiquidWatchman(federation_script)) {
        assert(false);
     }
 
     {
-        CScript::const_iterator sdpc = federationRedeemScript.begin();
+        CScript::const_iterator sdpc = federation_script.begin();
         std::vector<unsigned char> vch;
         opcodetype opcodeTmp;
-        while (federationRedeemScript.GetOp(sdpc, opcodeTmp, vch))
+        while (federation_script.GetOp(sdpc, opcodeTmp, vch))
         {
             size_t pub_len = 33;
             if (vch.size() == pub_len)
@@ -352,3 +355,75 @@ CTxOut GetPeginOutputFromWitness(const CScriptWitness& pegin_witness) {
     //return CTxOut(CAsset(pegin_witness.stack[1]), value, CScript(pegin_witness.stack[3].begin(), pegin_witness.stack[3].end()));
     return CTxOut(value, CScript(pegin_witness.stack[3].begin(), pegin_witness.stack[3].end()));
 }
+
+bool MatchLiquidWatchman(const CScript& script)
+{
+    CScript::const_iterator it = script.begin();
+    std::vector<unsigned char> data;
+    opcodetype opcode;
+
+    // Stack depth check for branch choice
+    if (!script.GetOp(it, opcode, data) || opcode != OP_DEPTH) {
+        return false;
+    }
+    // Take in value, then check equality
+    if (!script.GetOp(it, opcode, data) ||
+            !script.GetOp(it, opcode, data) ||
+            opcode != OP_EQUAL) {
+        return false;
+    }
+    // IF EQUAL
+    if (!script.GetOp(it, opcode, data) || opcode != OP_IF) {
+        return false;
+    }
+    // Take in value k, make sure minimally encoded number from 1 to 16
+    if (!script.GetOp(it, opcode, data) ||
+            opcode > OP_16 ||
+            (opcode < OP_1NEGATE && !CheckMinimalPush(data, opcode))) {
+        return false;
+    }
+    opcodetype opcode2 = opcode;
+    std::vector<unsigned char> num = data;
+    // Iterate through multisig stuff until ELSE is hit
+    while (opcode != OP_ELSE) {
+        if (!script.GetOp(it, opcode, data)) {
+            return false;
+        }
+    }
+    // Take minimally-encoded CSV push number k'
+    if (!script.GetOp(it, opcode, data) ||
+            opcode > OP_16 || (opcode < OP_1NEGATE && !CheckMinimalPush(data, opcode))) {
+        return false;
+    }
+    // CSV
+    if (!script.GetOp(it, opcode, data) || opcode != OP_CHECKSEQUENCEVERIFY) {
+        return false;
+    }
+    // Drop the CSV number
+    if (!script.GetOp(it, opcode, data) || opcode != OP_DROP) {
+        return false;
+    }
+    // Take the minimally-encoded n of k-of-n multisig arg
+    if (!script.GetOp(it, opcode, data) ||
+            opcode > OP_16 || (opcode < OP_1NEGATE && !CheckMinimalPush(data, opcode)) ) {
+        return false;
+    }
+
+    // The two multisig k-numbers must not match, otherwise ELSE branch can not be reached
+    if (opcode == opcode2 && num == data) {
+        return false;
+    }
+
+    // Find the ENDIF
+    while (opcode != OP_ENDIF) {
+        if (!script.GetOp(it, opcode, data)) {
+            return false;
+        }
+    }
+    // CHECKMULTISIG
+    if (!script.GetOp(it, opcode, data) || opcode != OP_CHECKMULTISIG) {
+        return false;
+    }
+    // No more pushes
+    return (it + 1 == script.end());
+}

src/pegins.h

@@ -22,4 +22,13 @@ bool IsValidPeginWitness(const CScriptWitness& pegin_witness, const COutPoint& p
 // Constructs unblinded output to be used in amount and scriptpubkey checks during pegin
 CTxOut GetPeginOutputFromWitness(const CScriptWitness& pegin_witness);
 
+/* Belt-and-suspenders-only matching against telescoped multisig used on Liquid v1:
+ * Pseudo-structure:
+ * Check number of elements on stack
+ * If enough for federation multisig, push all multisig args onto stack except OP_CMS
+ * If not, check CSV timeout, then if successful, push emergency key multisig args on
+ * stack except OP_CMS. End if, then push OP_CMS.
+ */
+bool MatchLiquidWatchman(const CScript& script);
+
 #endif // BITCOIN_PEGINS_H

src/script/interpreter.cpp

@@ -233,7 +233,7 @@ bool static CheckPubKeyEncoding(const valtype &vchPubKey, unsigned int flags, co
     return true;
 }
 
-bool static CheckMinimalPush(const valtype& data, opcodetype opcode) {
+bool CheckMinimalPush(const valtype& data, opcodetype opcode) {
     // Excludes OP_1NEGATE, OP_1-16 since they are by definition minimal
     assert(0 <= opcode && opcode <= OP_PUSHDATA4);
     if (data.size() == 0) {

src/script/interpreter.h

@@ -196,4 +196,6 @@ size_t CountWitnessSigOps(const CScript& scriptSig, const CScript& scriptPubKey,
 
 int FindAndDelete(CScript& script, const CScript& b);
 
+bool CheckMinimalPush(const std::vector<unsigned char>& data, opcodetype opcode);
+
 #endif // BITCOIN_SCRIPT_INTERPRETER_H

src/script/standard.cpp

@@ -72,7 +72,7 @@ static constexpr bool IsSmallInteger(opcodetype opcode)
     return opcode >= OP_1 && opcode <= OP_16;
 }
 
-static bool MatchMultisig(const CScript& script, unsigned int& required, std::vector<valtype>& pubkeys)
+bool MatchMultisig(const CScript& script, unsigned int& required, std::vector<valtype>& pubkeys)
 {
     opcodetype opcode;
     valtype data;

src/script/standard.h

@@ -203,4 +203,6 @@ CScript GetScriptForMultisig(int nRequired, const std::vector<CPubKey>& keys);
  */
 CScript GetScriptForWitness(const CScript& redeemscript);
 
+bool MatchMultisig(const CScript& script, unsigned int& required, std::vector<std::vector<unsigned char>>& pubkeys);
+
 #endif // BITCOIN_SCRIPT_STANDARD_H