contract ISignatureValidator {
    // bytes4(keccak256("isValidSignature(bytes,bytes)")
    bytes4 constant internal EIP1271_MAGIC_VALUE = 0x20c13b0b;

    /**
    * @dev Should return whether the signature provided is valid for the provided data
    * @param _data Arbitrary length data signed on the behalf of address(this)
    * @param _signature Signature byte array associated with _data
    *
    * MUST return the bytes4 magic value 0x20c13b0b when function passes.
    * MUST NOT modify state (using STATICCALL for solc < 0.5, view modifier for solc > 0.5)
    * MUST allow external calls
    */ 
    function isValidSignature(
        bytes calldata _data, 
        bytes calldata _signature)
        external 
        returns (bytes4);
}
function isValidSignature(
    bytes calldata _data, 
    bytes calldata _signature)
    external 
    returns (bytes4);

/**
 * @dev Should return whether the signature provided is valid for the provided data
 * @param _data Arbitrary length data signed on the behalf of address(this)
 * @param _signature Signature byte array associated with _data
 *
 * MUST return the bytes4 magic value 0x20c13b0b when function passes.
 * MUST NOT modify state (using STATICCALL for solc < 0.5, view modifier for solc > 0.5)
 * MUST allow external calls
 */