The term functions as a proper noun phrase. The head noun is "cipher," which identifies the object as a system of secret writing or a coded message. The prepositive modifier "11 sep" acts as an adjectival specifier, isolating this particular cipher from others by associating it with the specific date of September 11th. It names a unique cryptographic puzzle rather than a general class or type of cryptographic method.
Unlike established cryptographic systems such as the Caesar or Vigenre ciphers, this term does not refer to a standardized algorithm. Instead, it typically designates a specific, often user-created or context-dependent puzzle found within a particular piece of media, ARG (Alternate Reality Game), or online community. The date "11 sep" serves as a crucial element of the puzzle's context, key, or plaintext, providing the necessary frame of reference for any attempt at cryptanalysis.
In practical application, analyzing such a cipher requires external, contextual knowledge beyond general cryptographic principles. The solver must understand the significance of the date within the source material from which the puzzle originates. Therefore, its decryption is an act of interdisciplinary interpretation, combining cryptanalysis with thematic and narrative analysis of the source text or game.
