Possible Clues:

• Binary ___
• Spy's secret
• Pig Latin, e.g.
• System of ethics - cipher
• Spy's writing, perhaps
• Morse, for example, seen as an embodiment of the law?
• Set of rules applied to items for sale in the bar?
• See, the poetry is hard to decipher
• (Computer) programming instructions
• One if by land, two if by sea, etc.