Understanding Bitcoin’s OP_CHECKSIGADD Script
As a security feature, Bitcoin provides an additional layer of protection against attacks by including a threshold signature check in the unlocking script. Specifically, the CHECKSIGADD script for a 2-of-3 multisig configuration is designed to ensure that only two of the three signatures are required to unlock the wallet.
OP_CHECKSIG and OP_EQUAL
To understand what happens when we use these two opcodes together, let’s discuss their functions:
OP_CHECKSIGverifies the signature of a message.
OP_EQUALchecks whether two messages have the same content.
With this in mind, the CHECKSIGADD script performs an additional check: it adds an additional opcode OP_EQUAL to verify that each pair of signatures is equal. This ensures that only two of the three signatures are required to unlock.
What happens if you provide more than 2 valid signatures?
Now let’s see what happens if we provide more than two required signatures in the unlocking script.
When executing OP_CHECKSIGADD on a pair of signatures, it first verifies the signature. If the signature is invalid, it rejects the message and returns an error code. However, if the signature is valid, it adds another opcode to check if this additional signature matches the second signature.
If we provide more than two signatures in the unlocking script, OP_CHECKSIGADD will:
- Verify each pair of signatures (
OP_EQUAL) to make sure they are equal.
- If any of these pairs have a different signature, it rejects the message and returns an error code.
As a result, providing too many valid signatures will not cause CHECKSIGADD to fail; instead, it will simply return an error for each additional pair that does not match. This ensures that only two signatures are required for unlocking, while preventing attacks that exploit this weakness.
Best Practices and Security Considerations
While providing multiple valid signatures in an unlocking script may seem like a convenient security measure, it is important to consider the potential consequences of doing so. To mitigate this risk:
- Use
OP_EQUALconsistently in all scripts that require signature verification.
- Only add additional
OP_EQUALopcodes when necessary; excessive additions can slow down the execution process and increase the attack surface.
By understanding how the CHECKSIGADD script works, you can better appreciate the importance of carefully crafting your Bitcoin code to ensure strong security.