Fraud proof compiler

This repository implements a toolkit that helps people write fraud proof logic in Bitcoin script, particularly focusing on providing two "pseudo" opcodes:

• OP_RETURN_TRUE: mark the transaction as successful when being executed
• OP_IF_RETURN_TRUE: equivalent to OP_IF OP_RETURN_TRUE OP_ENDIF

Previously, in TapScript, we already have a number of OP_SUCCESSXX opcodes, as defined in BIP-342.

If any opcode numbered 80, 98, 126-129, 131-134, 137-138, 141-142, 149-153, 187-254 is encountered, validation succeeds (none of the rules below apply). This is true even if later bytes in the tapscript would fail to decode otherwise. These opcodes are renamed to OP_SUCCESS80, ..., OP_SUCCESS254, and collectively known as OP_SUCCESSx.

The problem is that these OP_SUCCESSx opcodes are way too powerful. The mere existence of them in the script will declare the script execution to be successful, even if the rest of the script is malformed (such as an OP_IF without the corresponding OP_ENDIF). Also, the OP_SUCCESSx opcodes are not intended to be used. Transactions that include them would be deemed nonstandard, and they are designed as placeholders while preventing people from using these opcodes before they are assigned with a different meaning.

Seeking for a weaker OP_SUCCESSx

It has been previously proposed to have a weaker version of OP_SUCCESSx, by Rusty Russell in a 2023 article, "Covenants: Examining ScriptPubkeys in Bitcoin Script", the idea of which is an OP_SUCCESS that only declares the script execution to be successful if it is executed. This would, of course, requires a consensus change, as we need to add a new opcode.

This repository is to provide a way that does not require adding new opcodes but is able to emulate such a weaker version of OP_SUCCESSx, as we discussed above.

Rewriting

In summary, the compiler will perform the following in order to rewrite the code.

• Start by having all the codes in OP_RETURN_TRUE. If there is any code in the same branch after OP_RETURN_TRUE, the code would not be executed, and can be removed.
• Convert all OP_RETURN_TRUE into the representation with OP_IF_RETURN_TRUE.
• Iterate the following steps:
  • if we can find the deepest “if” or “else” branch with OP_IF_RETURN_TRUE and the first OP_IF_RETURN_TRUE in it is not at the end (first priority):
    • apply the rule to push OP_IF_RETURN_TRUE to the end of this very branch, during which a deeper if-else statement might be created
  • else, if we can find the deepest if-else statement which has OP_IF_RETURN_TRUE but all of them are at the end of the if branch or the else branch (second priority):
    • adjust both branches to have an OP_IF_RETURN_TRUE at the end, remove the OP_IF_RETURN_TRUE at the end, and add OP_IF_RETURN_TRUE to the end of the if-else statement
    • If two OP_IF_RETURN_TRUE come next to each other as a result here, the two OP_IF_RETURN_TRUE would be replaced by OP_IF 1 OP_ENDIF OP_IF_RETURN_TRUE
  • else, if there is a top-level OP_IF_RETURN_TRUE but it is not the last instruction of the script (third priority):
    • apply the rule to push it to the end of the program, during which an if-else statement would be created
  • else, if there is a top-level OP_IF_RETURN_TRUE and this is the last opcode of the script (fourth priority):
    • convert it into OP_IF <Success Logic> OP_ENDIF

The success logic is as follows:

OP_DEPTH 512 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (256 OP_2DROP)
OP_ENDIF
OP_DEPTH 256 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (128 OP_2DROP)
OP_ENDIF
OP_DEPTH 128 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (64 OP_2DROP)
OP_ENDIF
OP_DEPTH 64 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (32 OP_2DROP)
OP_ENDIF
OP_DEPTH 32 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (16 OP_2DROP)
OP_ENDIF
OP_DEPTH 16 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (8 OP_2DROP)
OP_ENDIF
OP_DEPTH 8 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (4 OP_2DROP)
OP_ENDIF
OP_DEPTH 4 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP ... OP_2DROP (2 OP_2DROP)
OP_ENDIF
OP_DEPTH 2 OP_GREATERTHANOREQUAL OP_IF
    OP_2DROP
OP_ENDIF
OP_DEPTH OP_IF
    OP_DROP
OP_ENDIF
OP_TRUE