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OP_CSFS
OP_CHECKSIGFROMSTACK, CSFS, CHECKSIGFROMSTACK
A proposed opcode that checks a signature against an arbitrary message supplied by script, instead of only the spending transaction.
OP_CSFS, short for OP_CHECKSIGFROMSTACK, is a proposed opcode that verifies a signature against a message and public key placed on the script stack. Bitcoin's existing signature opcodes derive the signed message from the transaction being spent. OP_CSFS would make it possible to verify signatures over other precommitted data instead.
That extra flexibility matters for more expressive contract designs. Protocol designers often discuss OP_CSFS alongside OP_CTV, because the two together can help encode staged spending rules, delegated recovery flows, and other covenant-like constructions such as vaults.
OP_CSFS already exists on Elements-based sidechains, where it is used as an experimental opcode. On Bitcoin, it remains a proposal rather than a deployed consensus rule.