Bitcoin Covenants: CHECKSIGFROMSTACK (BIP 348)

CSFS, short for CHECKSIGFROMSTACK, is a proposal put forward by Brandon Black and Jeremy Rubin with BIP 348. While not technically a covenant, CSFS complements covenant proposals by allowing for enhanced flexibility in signature verification within Bitcoin scripts.

To understand how CSFS works, it’s important to grasp the basics of Bitcoin script language. Bitcoin script is stack-based, meaning data is stacked on top of each other and operated on by removing items from the stack based on opcode instructions. The script consists of a witness to unlock the script and the script included in the output being spent. These elements are added to the stack and operated on sequentially to verify the transaction.

The CSFS opcode introduces a new way of verifying signatures by allowing a signature to be verified against any arbitrary message pushed directly onto the stack. This differs from traditional signature verification, which is limited to verifying signatures against the transaction itself. The CSFS opcode structure involves dropping the signature, message, and public key onto the stack, then using CSFS to verify the signature against the message.

The flexibility provided by CSFS opens up a range of possibilities for Bitcoin script functionality. When combined with CTV, CSFS can enable floating signatures that attach to different transactions, a feature sought after by Lightning Network developers. Additionally, CSFS can facilitate delegation of control over UTXOs by allowing for the verification of variables such as new public keys against specific scripts.

Furthermore, when used in conjunction with CAT, CSFS can enhance the introspection functionality of Bitcoin scripts. While CSFS offers simple yet valuable functionality on its own, its compatibility with covenant opcodes allows for the creation of more complex script behaviors.

In conclusion, CSFS is a mature proposal with implementations dating back to 2016 on networks like Liquid and Elements. Its clear use cases and ability to enhance script functionality make it a valuable addition to the Bitcoin ecosystem. As Bitcoin script development continues to evolve, CSFS remains a key component in enabling new possibilities for script verification and control delegation.