OP_CHECKSIGFROMSTACK (CSFS)

OP_CHECKSIGFROMSTACK, abbreviated CSFS, is a proposed Bitcoin opcode described in BIP 348, with authorship credited to Brandon Black and Jeremy Rubin. On Bitcoin's base layer it is not yet active and would require a soft fork. Notably, equivalent functionality has been live on the Liquid Network and Elements since 2016 and on Bitcoin Cash since 2018, so the primitive itself is well understood in practice. We describe it neutrally for readers tracking Bitcoin's scripting roadmap.

What it does

Standard signature checks like OP_CHECKSIG only verify a signature against the spending transaction. CSFS instead takes three items off the stack, a signature, a message, and a public key, and verifies the signature against that arbitrary message. This frees signature verification from the transaction itself.

Why it is useful

CSFS enables script delegation, where a committed key can authorize spending by some other key or condition without moving coins on-chain. Paired with CTV it supports floating signatures that can attach to multiple transactions, useful for reducing Lightning channel penalty complexity. Combined with OP_CAT it contributes to richer contract composition and introspection. CSFS is not itself a covenant, but it synergizes with covenant opcodes to build pre-signed and stateful protocols. Discussion continues on standardization and exact semantics.

CSFS is often discussed alongside covenant proposals such as OP_CHECKCONTRACTVERIFY and OP_TXHASH.