Index: src/script/interpreter.cpp
===================================================================
--- src/script/interpreter.cpp
+++ src/script/interpreter.cpp
@@ -335,10 +335,9 @@
                 return set_error(serror, SCRIPT_ERR_OP_COUNT);
             }
 
-            if (opcode == OP_CAT || opcode == OP_SUBSTR || opcode == OP_LEFT ||
-                opcode == OP_RIGHT || opcode == OP_INVERT || opcode == OP_2MUL ||
-                opcode == OP_2DIV || opcode == OP_MUL || opcode == OP_LSHIFT ||
-                opcode == OP_RSHIFT) {
+            if (opcode == OP_SUBSTR || opcode == OP_LEFT || opcode == OP_RIGHT ||
+                opcode == OP_INVERT || opcode == OP_2MUL || opcode == OP_2DIV ||
+                opcode == OP_MUL || opcode == OP_LSHIFT || opcode == OP_RSHIFT) {
                 // Disabled opcodes.
                 return set_error(serror, SCRIPT_ERR_DISABLED_OPCODE);
             }
@@ -1234,6 +1233,24 @@
                         }
                     } break;
 
+                    //
+                    // Byte string operations
+                    //
+                    case OP_CAT: {
+                        // (x1 x2 -- out)
+                        if (stack.size() < 2) {
+                            return set_error(
+                                serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
+                        }
+                        valtype& vch1 = stacktop(-2);
+                        valtype& vch2 = stacktop(-1);
+                        if (vch1.size() + vch2.size() > MAX_SCRIPT_ELEMENT_SIZE) {
+                            return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
+                        }
+                        vch1.insert(vch1.end(), vch2.begin(), vch2.end());
+                        stack.pop_back();
+                    } break;
+
                     default:
                         return set_error(serror, SCRIPT_ERR_BAD_OPCODE);
                 }