OP_CAT is being proposed as a new tapscript opcode which simply concatenates the last two elements on the stack. A suprising consequence of this is that OP_CAT can allow for transaction introspection, also known as covenants. In this short work I demonstrate that one could also validate signatures over arbituary messages using just OP_CAT.

This BIP introduces OP_CAT as a tapscript opcode which allows the concatenation of two values on the stack. OP_CAT would be activated via a soft fork by redefining the opcode OP_SUCCESS126 (126 in decimal and 0x7e in hexadecimal). This is the same opcode value used by the original OP_CAT.