txscript: add verification of the post-segwit minimal if policy

This commit modifies the op-code execution for OP_IF and OP_NOTIF to
enforce the additional “minimal if” constraints which require the
top-stack item when the op codes are encountered to be either an empty
vector, or exactly [0x01].

txscript/engine.go

@@ -84,6 +84,10 @@ const (
 	// ScriptVerifyDiscourageUpgradeableWitnessProgram makes witness
 	// program with versions 2-16 non-standard.
 	ScriptVerifyDiscourageUpgradeableWitnessProgram
+
+	// ScriptVerifyMinimalIf makes a script with an OP_IF/OP_NOTIF whose
+	// operand is anything other than empty vector or [0x01] non-standard.
+	ScriptVerifyMinimalIf
 )
 
 const (

txscript/opcode.go

@@ -918,6 +918,47 @@ func opcodeNop(op *parsedOpcode, vm *Engine) error {
 	return nil
 }
 
+// popIfBool enforces the "minimal if" policy during script execution if the
+// particular flag is set.  If so, in order to eliminate an additional source
+// of nuisance malleability, post-segwit for version 0 witness programs, we now
+// require the following: for OP_IF and OP_NOT_IF, the top stack item MUST
+// either be an empty byte slice, or [0x01]. Otherwise, the item at the top of
+// the stack will be popped and interpreted as a boolean.
+func popIfBool(vm *Engine) (bool, error) {
+	// When not in witness execution mode, not executing a v0 witness
+	// program, or the minimal if flag isn't set pop the top stack item as
+	// a normal bool.
+	if !vm.witness || !vm.hasFlag(ScriptVerifyMinimalIf) {
+		return vm.dstack.PopBool()
+	}
+
+	// At this point, a v0 witness program is being executed and the minimal
+	// if flag is set, so enforce additional constraints on the top stack
+	// item.
+	so, err := vm.dstack.PopByteArray()
+	if err != nil {
+		return false, err
+	}
+
+	// The top element MUST have a length of at least one.
+	if len(so) > 1 {
+		str := fmt.Sprintf("minimal if is active, top element MUST "+
+			"have a length of at least, instead length is %v",
+			len(so))
+		return false, scriptError(ErrMinimalIf, str)
+	}
+
+	// Additionally, if the length is one, then the value MUST be 0x01.
+	if len(so) == 1 && so[0] != 0x01 {
+		str := fmt.Sprintf("minimal if is active, top stack item MUST "+
+			"be an empty byte array or 0x01, is instead: %v",
+			so[0])
+		return false, scriptError(ErrMinimalIf, str)
+	}
+
+	return asBool(so), nil
+}
+
 // opcodeIf treats the top item on the data stack as a boolean and removes it.
 //
 // An appropriate entry is added to the conditional stack depending on whether
@@ -936,10 +977,11 @@ func opcodeNop(op *parsedOpcode, vm *Engine) error {
 func opcodeIf(op *parsedOpcode, vm *Engine) error {
 	condVal := OpCondFalse
 	if vm.isBranchExecuting() {
-		ok, err := vm.dstack.PopBool()
+		ok, err := popIfBool(vm)
 		if err != nil {
 			return err
 		}
+
 		if ok {
 			condVal = OpCondTrue
 		}
@@ -969,10 +1011,11 @@ func opcodeIf(op *parsedOpcode, vm *Engine) error {
 func opcodeNotIf(op *parsedOpcode, vm *Engine) error {
 	condVal := OpCondFalse
 	if vm.isBranchExecuting() {
-		ok, err := vm.dstack.PopBool()
+		ok, err := popIfBool(vm)
 		if err != nil {
 			return err
 		}
+
 		if !ok {
 			condVal = OpCondTrue
 		}