Related Clues:

• Spy's writing, perhaps
• Ciphers
• Secret message
• One if by land, two if by sea, etc.
• Kind of word
• Value system
• See 71-Across
• Genetic or dress follower
• Systems of rules
• They may be cracked
• Preparation that needs a cracker?
• Set of principles
• Writes software
• Word with bar or area
• Secret signal