Eric Jankiewicz has a Nautilus interview with Gary Klivans, a former corrections officer who became an expert in the codes used by gangs:
Codes are substitutes for the letters in our English alphabet. They could be anything—in addition to numbers, they use ornate symbols, a Chinese pictogram, or Mayan or Aztec symbols. Or they create their own symbols. And the languages vary widely. There is no one Blood or Crip gang code or Black Gangster Disciples code that all the members use. The codes are all local. So if you’re a law enforcement officer in Los Angeles or Chicago, you can travel 10 miles away and you’ll find a different code.
His account of how he learned to decipher them is quite interesting. (Thanks, Bob!)
About ten years ago I saw something on a listserve detailing how Mexican gangs in California prisons were starting to use modern nahuat to talk around guards, most of whom knew quite a bit of Mexican Spanish.
The other lobe of these gang codes is the system of symbols used in tattoos. It is dizzyingly complex. Most of them are about gang membership as a form of signalling to avoid being aggressed.