This is a version of a question I asked on Security.SE, this time with a focus on the TV side of it.
In S01E05 of The Wire, at around the 45:00 mark, it's revealed that:
Avon Barksdale's crew encrypt their phone numbers by using a substitution cipher based on the layout of a T9 mobile phone keypad.
In more detail:
Each digit of the number, with the exception of 5 and 0, is replaced with the corresponding digit on the other side of the 5 key ("jump the five''); 5 and 0 are exchanged. Thus, 555-867-5309 becomes 000-243-0751. Devise a one-line statement to encrypt and decrypt numbers encoded in this way.
The in-universe reason for the cipher was that it had to be both secure but simple enough for low-level dealers to understand, but I've been able to find basically nothing for an out-of-universe reason.
Is there any extra material that details how the idea was devised or what the inspiration was?