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 unlock script. Specifically, the CHECKSIGADD script for a 2-of-3 multisig setup is designed to ensure that only two out of three signatures are needed to unlock a wallet.
OP_CHECKSIG and OP_EQUAL
To understand what happens when we use these two opcodes together, let’s break down their functions:
OP_CHECKSIGverifies the signature of a message.
OP_EQUALverifies whether two messages have the same content.
Given this, the CHECKSIGADD script performs an additional check: it adds an additional OP_EQUAL opcode to verify that each pair of signatures is the same. This ensures that only two out of three signatures are needed to unlock.
What happens if you provide more than 2 valid signatures?
Now, let’s examine what would happen if we provided more than the required two signatures in the unlock script.
When running OP_CHECKSIGADD on a pair of signatures, it first checks 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 other signature.
If we provide more than two signatures in the unlock script, OP_CHECKSIGADD will:
- Check each pair of signatures (
OP_EQUAL) to ensure they are the same.
- If any of these pairs have a different signature, it will reject the message and return an error code.
As a result, providing too many valid signatures would not cause the CHECKSIGADD script to fail; instead, it would simply return an error for each additional pair that did not match. This ensures that only two signatures are required to unlock, while also preventing attacks from exploiting this weakness.
Best Practices and Security Considerations
While providing multiple valid signatures in the unlock script may seem like a convenient security measure, it is essential 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 as needed; 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 robust security.
