[telephonetemp]

On the other hand a substitution cipher is more resistant to user error when encrypting or decrypting since such an error only affects a single character. In secret prison communication you might not have the time to double-check your encryption.

I wonder if imprisoned gang members have access to cell phones, though. They probably do. You could run actual strong crypto even on a J2ME device (or a gaming console, or a programmable calculator, or an MP3 player etc.) and I'm sure there's plenty of people you could hire to write a custom-designed, one-of-a-kind bespoke cryptographic app for a gang.

[mistercow]

I was under the impression that in prison, time is the one thing you have plenty of.

[telephonetemp]

Time when you can fiddle with cards and write down cryptic series of numbers without attracting undue attention is probably scarce.

[EdiX]

> I wonder if imprisoned gang members have access to cell phones, though. They probably do.

No, they don't. Most countries (including USA, AFAIK) restrict inmates phone communication with the external world.

[panarky]

Cell phones and other contraband may be prohibited, but they still exist in prisons.

  State prison records show that for the first seven months of
  this year, corrections officers have seized 319 cell phones
  from inmates [in Ohio's] 28 prisons.

http://www.cleveland.com/metro/index.ssf/2013/08/ohio_prison...