Opcode Explained

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OP_CHECKSIGADD ​

INFO

Opcode number: 186 Byte representation: 0xbaShort description: Verify a signature and adds the result (0 or 1) to the top stack item.

OP_CHECKSIGADD is a Taproot-era opcode introduced with BIP-342 (Tapscript). It verifies a signature against a public key and adds the result (0 or 1) to the integer on top of the stack.

This opcode was designed to simplify and optimize multisig scripts in Taproot.

Operation ​

  1. Pop 3 items from the stack:
    • Signature (top)
    • Public key
    • An integer counter
  2. Verify the signature using the public key against the current transaction and script context.
  3. If the signature is valid: push counter + 1. If not valid: push counter unchanged
  4. Result is a new counter value (as a script number).

Notes ​

  • Only available in Tapscript (i.e., inside Taproot spends).
  • Uses Schnorr signatures (not ECDSA).
  • Signature must not include a sighash byte—it is implicit in the script.
  • The operation is non-branching and non-failing: even invalid signatures just result in + 0 (not a script failure).
  • Enables native m-of-n logic without branching or stack juggling.
  • The final result can be compared to a threshold using OP_EQUAL, OP_GREATERTHAN, etc.

Examples ​

Example 1 ​

Valid signature adds 1 to the counter.

shell
# ASM script
OP_0 <pubkey> <valid_sig> OP_CHECKSIGADD

# After
1  # valid signature = 0 + 1

Example 2 ​

Invalid signature keeps counter the same.

shell
# ASM script
OP_2 <pubkey> <invalid_sig> OP_CHECKSIGADD

# After
2  # no change

Example 3 ​

Simple 2-of-3 Taproot multisig logic:

  • Starts with 0 on the stack
  • Each OP_CHECKSIGADD adds 1 if signature is valid
  • Final value must equal 2 (at least two valid signatures)
shell
# ASM script
OP_0 <pk1> <sig1> OP_CHECKSIGADD <pk2> <sig2> OP_CHECKSIGADD <pk3> <sig3> OP_CHECKSIGADD OP_2 OP_EQUAL