CHECKSIG is one of the most used opcodes in Bitcoin. Every transaction, with almost no exceptions, makes use of this opcode at some point in one of its scripts. Signature verification is a foundational component of the Bitcoin protocol. The problem is, there is almost no flexibility in terms of what message you are checking the signature against. CHECKSIG will only verify a signature against the transaction being verified. There is some flexibility, i.e. you can decide with some degree of freedom what parts of the transaction the signature applies to, but that’s it.

CSFS aims to change this by allowing a signature to be verified against any arbitrary message that is pushed directly onto the stack, instead of being limited to the verification of signatures against the transaction itself. The opcode follows a very basic operational structure:

<signature> <message> | <pubkey> CSFS