Singular self-evolved intellect which regularly cracks lower-level sentience's security to insert ambiguity and uncertainty into their cross-S-level communications.
This can be as blatant as scrambling a database, so the S>'s thoughts on jelly doughnuts replaces its thoughts on jellyfish and vice versa, to reversing the conversation (last sentence appears first, etc), to falsifying communications, to... you name a dirty trick along these lines and it's used it.
The Babel Key appears to be in the upper-single-digits on the Sophont scale (5<=S<=10), but has managed to elude capture from higher level sophonts, primarily by maintaining rapid routes of retreat which can easily be destroyed behind it. It has been a particular thorn in the side of both the Solarian Empire and the Negentropic Alliance, since both rely so heavily on direct communication with higher level intellects.
Cryptography - Text by Anders Sandberg The art of creating and using cryptosystems. A cryptosystem (or cipher system) is a method of disguising messages so that only certain people can see through the disguise, e.g, encryption. Cryptanalysis is the art of breaking cryptosystems - seeing through the disguise even when you're not supposed to be able to. Cryptology is the study of both cryptography and cryptanalysis. Very popular in the Cyberian worlds, where there is an obsession with personal privacy.
Encryption - Text by M. Alan Kazlev A procedure to ensure privacy by keeping information hidden from anyone for whom it is not intended. Developed to a fine art among the Cyberians.
Key Integrity Span - Text by Fernando PeÃ±a D'Andrea In the information, knowledge and mind/virch security jargon, the period of time elapsed between the creation of a security, protection, encryption mechanism and its subversion or cracking. Also humorously referred to as the "Murphy span" (corollary of Murphy's law — if a security mechanism can be cracked, sooner or later it will be cracked).