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Like secret messages, 6 letters
Like some diplomatic communication, 6 letters
Needing crackers, say, 6 letters
Needing cracking, perhaps, 6 letters
Needing to be cracked?, 6 letters
Not readily readable, 6 letters
One way to communicate, 6 letters
Rendered unreadable, 6 letters
Requiring decryption, 6 letters