Bitcoin Covenants Part 4: OP_CHECKSIGFROMSTACK and OP_CAT
OP_CSFS authenticates signatures over arbitrary data. OP_CAT concatenates stack values into a single message. Together, they let Bitcoin scripts verify transaction structure at spend time, without pre-signed key management or new consensus rules beyond the two opcodes themselves.
This is Part 4 in the technical article series about Bitcoin covenants by Cointelegraph Research. To read the previous article, click here.
The following opcodes do not independently implement full covenant functionality. Instead, they are building blocks for constructing covenants when combined with other opcodes or script elements.
OP_CSFS is a proposed opcode that would allow Bitcoin script to verify signatures over arbitrary messages supplied on the stack. It is different from OP_CHECKSIG, which verifies signatures over the spending transaction according to the active SIGHASH mode. By enabling signature verification for data other than the serialized transaction details, OP_CSFS enables a broader class of constructions, including oracle-based scripts where an external party signs off-chain messages that represent real-world events. For example, a trusted oracle could publish Schnorr signatures over messages that encode external outcomes, and an OP_CSFS-based script could condition payments on the presence of a valid oracle signature.