Opcode Explained

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OP_CHECKSIG ​

INFO

Opcode number: 172
Byte representation: 0xac
Short description: Verifies a digital signature against a public key and the transaction’s sighash.

OP_CHECKSIG is used to verify that a given digital signature is valid for a specific public key and a specific transaction context. It is a cornerstone of Bitcoin’s transaction validation and ensures that only the rightful owner of funds can authorize their spending.

See the bitcoin wiki page on OP_CHECKSIG for a more complete treatment of the opcode.

Operation ​

  1. Takes the top two items from the stack:
    • Signature (top)
    • Public key (second-to-top)
  2. Computes the sighash (a hash of the transaction, influenced by the SIGHASH flag).
  3. Uses the public key to verify that the signature is valid for the sighash.
  4. Pushes 1 (true) onto the stack if valid, or 0 (false) if invalid.

Notes ​

  • The signature must be a DER-encoded ECDSA signature followed by a one-byte sighash flag.
  • The public key can be compressed (33 bytes) or uncompressed (65 bytes).
  • If the signature is invalid or the format is incorrect, OP_CHECKSIG pushes 0 onto the stack (in Bitcoin Core, certain edge cases trigger a script failure instead).
  • OP_CHECKSIG is not a pure function—it depends on the transaction context, script location, and the sighash type.
  • The presence of OP_CODESEPARATOR may affect the portion of the script used in the sighash.

Example ​

A standard P2PK script (Pay-to-PubKey). This script checks if the provided signature corresponds to the given public key and the transaction’s sighash.

shell
# ASM script
<pubkey> OP_CHECKSIG

# Stack (after OP_CHECKSIG)
1      # if valid signature