Understanding the Bitcoin OP_CHECKSIGADD script
As a security measure, Bitcoin provides an additional layer of protection against attacks by including a signature threshold check in the unlock script. Specifically, for a 2-of-3 multisig setup, the CHECKSIGADDD script is designed to ensure that only two of the three signatures are required to unlock wallets.
OP_CHECKSIG and OP_EQUAL
To understand what happens when we use these two opcodes together, let’s break down their functions:
OP_CHECKSIGchecks the message signature.
OP_EQUALchecks if two messages have the same content.
With that in mind, the CHECKSIGADD script performs an additional check: it adds an additional opcode OP_EQUAL to check if 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 two valid signatures?
Now let’s check what would happen if we provided more than the two required signatures in the unlock script.
When OP_CHECKSIGADD is run on a signature pair, the signature is checked first. If the signature is invalid, the message is rejected and an error code is returned. But if the signature is valid, another operation code is added to check if this additional signature matches the other signature.
When more than two signatures are provided in the unlock script, «OP_CHECKSIGADD»:
- Check each signature pair (OP_EQUAL) to make sure they are equal.
- If any of these pairs has a different signature, the message is rejected and an error code is returned.
So providing too many valid signatures will not cause the CHECKSIGADD script to fail. Instead, an error will simply be returned for each additional pair that does not match. This ensures that only two signatures are required to unlock, while preventing attacks from exploiting this flaw.
Best Practices and Security Considerations
Although providing multiple valid signatures in the unlock script may seem like a practical security measure at first glance, you should consider the potential consequences of doing so. Here’s how to mitigate this risk:
* Consistently use the OP_EQUAL option in all scripts that require signature verification.
- Add additional
OP_EQUALopcodes only when necessary; too many additions can slow down the execution process and increase the attack surface.
Understanding how the CHECKSIGADDD script works will help you better appreciate the importance of carefully designing your Bitcoin code to ensure solid security.