Forms of encryption are frequently used in prison. More so by the 'hardcore' gangbangers in the pen. When writing kites, they can be intercepted easily, and read by other inmates, or CO's if they're interested in looking at what one inmate may be saying to another.
There are two primary ways that you'll be able to learn these forms of encryption on the inside. Through either A) Reading about them in the library, if they are documented. Or B) Through having another inmate teach you about the encryption methods. If you're roleplaying on the outside, you may take your time to learn these forms of encryption. It will be easier, and could be useful if you're frequently smuggling in messages to inmates.
There are a few forms of encryption used.
Whites in prison usually use nordic runes to communicate. At first glance, the untrained eye wouldn't know what to look for when reading the runes.
This means 'SEX OFFENDER ON THE TIER'.
You can use this website to easily translate these. Nobody really expects you to understand how to speak with runes ooc'ly. https://www.nordicrunes.info/runeconverter.php.
Hispanic inmates are more likely to use Spanish or Aztec languages. The issue with Spanish is that it can be easily decoded. The issue with understanding one of the Aztec languages is that realistically few people can understand them. A small portion of people in Mexico (roughly 1.6m) know one of the Aztec languages. Even less of the youth on the streets.
One of the easiest ways to communicate is through the Roman alphabet tap code. The tap code can be used through either kites, or through actual tapping. It is comparable to morse code.
1,3 2,4 3,1 3,1 1,3 2,3 4,2 2,4 4,3 -> 13243131 1323422443
If you took the time to decipher this it would read 'KILL CHRIS'. Another way to get a message to inmates.
The last thing inmates may take up is creating their own codes/languages. This makes it near enough impossible to decipher - although it has happened in the past - as the codes would be completely private, leaving the I.G.I's (Institutional Gang Inspectors - usually there to validate you) or whoever's trying to decipher the codes unable.
If you ever come into a roleplay situation where you decide you want to use this, just remember that your character might not be sophisticated enough, or have learnt enough to do this. From my understanding, it's usually dedicated people, such as those in the Aryan Brotherhood, the Mexican Mafia or the Black Guerrilla Family who are more likely to use these methods. More so because they've been housed in gang housing units where there'll be more of a chance I.G.I's or CO's will try to intercept the messages.