e.g. the character $a = 97$ (it's computer decimal format, commonly known)
and then using a pattern/key like $y = 31 x + 5$ to get $3012$ (substitute $97$ into $x, y$ is now the encrypted code).
How easy/hard would it be for someone to crack a code like this? 3012 3197 3135 (without them knowing the pattern)
If someone had these encrypted codes, are there any methods they could use besides trial and error to figure out the pattern and then the code?