Secret Communication in Sign Language

Question

In a world where spoken language is impossible, people still need to communicate secretly at times. After all, what would a culture be without movies about spies sharing secrets in public places?

I know that it's pretty easy to express simple, preset messages using a prearranged code, but what about juicy bits of gossip at a party? Lovers talking in a coffee shop? Shop clerks alerting each other to a seedy customer? In these cases, the people may know each other well or hardly at all, and could be communicating about almost any topic.

In our world, we can whisper. In a world where sound doesn't carry, what could be a substitute? What is a non-verbal equivalent of whispering?

I'm looking for a way to communicate that:

1. Is private
2. Requires no unusual equipment
3. Is unobtrusive and can be used in public places
4. Is non-auditory
5. Is non-telepathic
6. Leaves no physical record or evidence that a conversation took place
7. Can be ubiquitous in a non-verbal society

Answer 1

You just want one? Do the store clerks tracking a seedy customer use the same way of communication as the lovers talking in a coffee shop?

There are two major parts to making such a secret language. One is to prevent observation of the language being used, and the other is to make the language opaque to anyone who doesn't know the secret. This is true for spoken and non spoken languages.

Preventing observation is easy. In fact, its easier than it is with spoken words, because signs don't leave line of sight. All you have to do is make it hard for anyone besides the intended recipient to observe. Your lovers in the coffee shop should be more than capable of this. In fact, if they're having an affair, there's a good chance they already have their own shorthand, regardless of whether they can speak or not!

Preventing understanding is the other approach. Codewords, or in your case code-signs, would be easy to introduce for topics that we don't want others to pick up on. In our world, there's actually a remarkable amount of non-verbal communication in these forms, especially in situations where you are being hounded by someone and want others to get help.

A hybrid between them is to be subtle. Perhaps the symbols are all shown out in the open, but unless you know how to tie them together, they are meaningless. This approach is great for covert ops where the act of finding a "quiet place to talk," or in this case a hard to observe place to sign, is a hint that something is going down. Consider gang behaviors. When a police officer comes down the street, there is a sudden flurry of activity to ensure nobody is caught doing anything illegal. You might not even notice it because you aren't attuned to what they are doing.

Of course, hidden sign language like this isn't just limited to worlds that don't speak. Frank Herbert's Dune includes such a sign language for the Bene Gesserit to talk when others are watching, like here where a Reverend Mother is under the watchful guard of 8 imperial guards.

Answer 2

Maybe they are like Earthly cephalopods, which can signal on the side of the body facing the recipient. In one case, a rival male can get between a couple, flirt with the female on one side and impersonate another female on the other so the dominant male doesn't notice the intrusion of a rival.

These beings will be adapted to non-audio signaling of some kind, and might very well be able to be highly directional, with the evolved ability to communicate covertly predating full sentience. In fact, the modern intelligent species is decendant from the clever "sneaker" males and the females who found that attractive.

Answer 3

Specific, Predefined Messages

Sign language (and in general, body language) is widely used all across in our world. Hand gestures and face expression reveal a lot about the person's mood and intentions.

Also, different professions have their own sets of signs. For example, drivers use signs to communicate from within their cars. Usually a universal (or at least national) code set is formed for all the people of a specific profession/activity which makes no sense to a person who is unfamiliar with that profession/activity. This should explain you how shopkeepers in your world would communicate with each other about a seedy customer.

You also presented the example of lovers secretly communicating in a cafe. Love is an intimate relationship and with time partners tend to understand even each other's gaze and very subtle signs in their behavior. This is true even for this world where we have such extensive vocabulary of spoken language available to us. You can imagine yourself about your world where sign language is the primary form of communication.

Broader Communication

OK, so short, predefined messages can be easily conveyed through signs and body language, but what about messages which have not been predefined? Here we are getting in the realm of complete sets of communication systems.

You have two English speaking girls in a party. They are gossiping about a third girl. Which language would they use? English, obviously. Can two girls form a complete set of English code-vocabulary only legible to them? Yes, but they both have to have at least practiced it for 5 years or so or they both ought to have IQ levels exceeding 150 if they want to devise the whole set within a week or so.

So no. You cannot have secret sign communication between two people which is comprehensive and does not require extensive practice for a long time.