# [Paradigm-CTF 2023] Blacksheep Write Up

The key is to leverage the un-used call result of the method `CHECKVALUE`. The call result of the `CHECKVALUE` would help us bypass the noauth check.

/* Interface */
#define function withdraw(bytes32,uint8,bytes32,bytes32) payable returns ()


#define macro CHECKVALUE() = takes (0) returns (0) {
    callvalue 0x10 gt over jumpi // [callvalue < 0x10]
    0x00 dup1 revert
    over:
        0x00 
        0x00
        0x00
        0x00
        callvalue 0x02 mul
        caller
        0xFFFFFFFF
        call // [call_result]
}
#define macro CHECKSIG() = takes (0) returns (1) {
    // last call left a call result in stack
    0x04 calldataload // [call_result, calldata_32byte]
    0x00 mstore       // mem: [0x00 - calldata_32byte] stack: [call_result]
    0x24 calldataload 
    0x20 mstore       // mem: [0x20 - calldata_32byte, 0x00 -- calldata_32byte] stack: [call_result]
    0x44 calldataload
    0x40 mstore
    0x64 calldataload 
    0x60 mstore // mem: [0x20 - calldata_32byte, 0x00 -- calldata_32byte] stack: [call_result]
    0x20
    0x80
    0x80
    0x00
    0x1 // call the pre-compile contract -- ecrecover(hash,v,r,s) 
//           mem
//         [0; 31]	 hash
//         [32; 63]  v
//         [64; 95]	 r	
//         [96; 127] s	
    0xFFFFFFFF
    staticcall //  [call_result, static_callresult]
    iszero invalidSigner jumpi
    0x80 mload
    0xd8dA6Bf26964AF9D7eed9e03e53415D37AA96044 eq correctSigner jumpi
    end jump 

    correctSigner:
        0x00
        end jump
    invalidSigner:
        0x01
        end jump
    end: // [call_result]
}
#define macro WITHDRAW() = takes (0) returns (0){
    CHECKVALUE()
    CHECKSIG()
    // !call_result 
    iszero iszero noauth jumpi
    0x00 dup1 dup1 dup1
    selfbalance caller
    gas call
    end jump
    noauth:
        0x00 dup1 revert
    end:
}


#define macro MAIN() = takes (0) returns (0) {
    // Identify which function is being called.
    0x00 calldataload 0xE0 shr // [methodID]
    dup1 __FUNC_SIG(withdraw) eq withdrawj jumpi
    callvalue 0x00 lt recieve jumpi

    0x00 0x00 revert

    withdrawj:
        WITHDRAW()
    recieve:
}

With that context we can write any exploit contract that can return success when the black sheep contract call with a value.