Bitcoin Covenants: Applications of OP_CSFS and OP_CAT

Bitcoin covenants allow scripts to restrict how funds may be spent in the future and are considered a key foundation for vaults and Layer 2 scaling solutions. OP_CAT, an opcode that Satoshi Nakamoto disabled in 2010 over security concerns, has recently been proposed for reactivation under BIP 347. OP_CSFS, or OP_CHECKSIGFROMSTACK, is proposed under BIP 348 and would allow scripts to verify signatures for arbitrary messages on the stack. Together, the two are regarded as one of the simplest building-block combinations for covenant functionality.

The fourth article in the series examines how the two opcodes work together. OP_CSFS alone can provide only signature delegation. When paired with OP_CAT, however, it can reassemble transaction fields on the stack for verification, allowing a script to inspect a transaction’s structure at the time funds are spent. This avoids the key-management burden of presigned transactions and the need to create separate consensus rules for individual applications. Both proposals remain under review by the Bitcoin Core development community, with no timetable yet set for activation through a soft fork.