checksig

checksig, in the context of Bitcoin Script, refers to the OP_CHECKSIG opcode. It verifies that a provided digital signature correctly signs the transaction for a given public key and the transaction’s input data. During evaluation, the top two items on the script stack are the signature and the public key. OP_CHECKSIG uses the current transaction context to compute the message hash (the sighash) that the signature is expected to cover, typically determined by the transaction’s inputs and the chosen sighash type (e.g., SIGHASH_ALL). The signature must be a DER-encoded ECDSA signature over the secp256k1 curve. The public key can be in compressed or uncompressed form. If the signature is valid for the public key and the sighash, OP_CHECKSIG pushes true (1) onto the stack; otherwise it pushes false (0). This result determines whether the script continues processing or fails.

There is also OP_CHECKSIGVERIFY, which performs the same check but requires a true result, and will cause script execution to fail if the check is false. OP_CHECKSIG is commonly used in standard spend scripts, such as P2PKH, where the signature proves authorization by the corresponding private key. The opcode is part of Bitcoin’s Script language, a stack-based, non-Turing-complete language used to specify spending conditions. It relies on the ECDSA verification implemented in the node’s cryptographic libraries and the consensus rules for sighash computation.