A good secret language

Just for fun, I thought I would say something about what could make a good secret language.

A secret language would be "good" to the extent that it met its requirements.  The obvious first requirement of a secret language is that it protect the communication that it encodes, so it should be different from known public languages.  Beyond mere strangeness, however, there are a number of supporting characteristics that a secret language might have.

1. Learnability.  In order to be used, a secret language must be learned.  The easier it is to learn, the more quickly it may be used.  Learnability is improved by:

1.1 Regularity.  Irregular forms make languages harder to learn, so a language with regular rules is easier to learn.

1.2 Phonological and Syntactic Familiarity.  If speakers of a secret language share a common public language, then the secret language will be easier to learn if it shares the phonology and syntax of the public language.  This occurs in Anglo-Romani and Thieves' Cant.

1.3 Redundancy.  A natural language makes relatively efficient use of the available phonological space for words.  For example, in Chinese, there is at least one morpheme for almost every possible Chinese monosyllable.  However, since the secret language is learned as a second language, it will benefit from making morphemes more distinct from each other than you would normally find in a natural language.  This occurs in Chinook Jargon.

2. Extensibility.  The core lexicon for a secret language may be relatively small, with a powerful derivational morphology to extend the lexicon over its intended domain of communication.  This makes the language easier to learn, because there are fewer morphemes to memorize, but also makes it possible to invent new terminology on the spot with a high likelihood that you will be understood.  I don't know of a secret language "in the wild" that does this.

3. Obfuscation. Some phonological features of natural languages could hypothetically be used for obfuscation.  Nearly all of these would come at the expense of lost redundancy.  For example:

3.1 Avoiding Labials.  If you wanted a secret language that could not be lip-read, you would avoid labials (including rounded vowels).  I have read a couple descriptions of the Chinook language that say Chinook speakers would hardly move their lips at all.

3.2 Avoiding Voiceless Fricatives, Affricates and Aspirates.  If you wanted a secret language whose sounds would not carry far, you might avoid sounds that are accompanied by high frequency formants, which carry farther.