cipher
n. countablen. a secret way of writing using a code or a special set of symbols. It can also mean a person or thing that is hidden or hard to understand.
n. a method of transforming a text in order to conceal its meaning; a code or cryptographic system. In a figurative sense, refers to a person or thing of no influence or one whose true character is hidden.
The spy used a complex cipher to send the message.
Historians spent decades trying to crack the mechanical cipher used by the military during the Great War.
The protagonist remains a cipher throughout the novel, his motivations obscured by a series of contradictory actions that leave the reader questioning his true allegiance until the final chapter.
14th century. From Middle English cifre, from Old French cyfre, cyffre (French chiffre), ultimately from Arabic صِفْر (ṣifr, “zero, empty”), from صَفَرَ (ṣafara, “to be empty”). Doublet of chiffre and zero. Sense 8 (a fault in an organ valve) may be a different word.
Often used with the verbs 'crack', 'break', or 'decode'.