From 8df0af32d605fb56350ab8d989eeab08a4c992d7 Mon Sep 17 00:00:00 2001
From: "Owain G. Ainsworth" <oga@conformal.com>
Date: Fri, 14 Mar 2014 14:48:19 +0000
Subject: [PATCH] Encountering OP_VERIF and OP_VERNOTIF in execution is always
 an error.

I honestly thought we already handled this, but some tests dhill is
working on shows that we didn't.
---
 opcode.go | 16 ++++++++++++++++
 script.go |  9 ++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/opcode.go b/opcode.go
index 710cd212..3b26b329 100644
--- a/opcode.go
+++ b/opcode.go
@@ -862,6 +862,22 @@ type parsedOpcode struct {
 	opfunc func(op parsedOpcode, s Script) error
 }
 
+// The following opcodes are always illegal when passed over by the program
+// counter even if in a non-executed branch. (it isn't a coincidence that they
+// are conditionals).
+func (pop *parsedOpcode) alwaysIllegal() bool {
+	switch pop.opcode.value {
+	case OP_VERIF:
+		return true
+	case OP_VERNOTIF:
+		return true
+	default:
+		return false
+	}
+}
+
+// The following opcode are conditional and thus change the conditional
+// execution stack state when passed.
 func (pop *parsedOpcode) conditional() bool {
 	switch pop.opcode.value {
 	case OP_IF:
diff --git a/script.go b/script.go
index 2177df94..3fe74c3a 100644
--- a/script.go
+++ b/script.go
@@ -52,6 +52,11 @@ var (
 	// is encountered.
 	StackErrReservedOpcode = errors.New("Reserved Opcode")
 
+	// StackErrAlwaysIllegal is returned when an opcode marked as always
+	// illegal is encountered. Currently this is just OP_VERIF and
+	// OP_VERNOTIF.
+	StackErrAlwaysIllegal = errors.New("Always Illlegal instruction encountered")
+
 	// StackErrEarlyReturn is returned when OP_RETURN is executed in the
 	// script.
 	StackErrEarlyReturn = errors.New("Script returned early")
@@ -602,7 +607,9 @@ func (m *Script) Step() (done bool, err error) {
 	if m.condStack[0] != OpCondTrue {
 		// some opcodes still 'activate' if on the non-executing side
 		// of conditional execution
-		if opcode.conditional() {
+		if opcode.alwaysIllegal() {
+			return true, StackErrAlwaysIllegal
+		} else if opcode.conditional() {
 			executeInstr = true
 		} else {
 			executeInstr = false