Unity random characters in string

You can use Linq to generate any number of random characters from an enumerable:

int howManyChars = 5;
var newString = String.Join("", Enumerable.Range(0, howManyChars).Select(k => st[Random.Range(0, st.Length)]));
textbox.text = newString;

If you want completely glitchy strings, go the way of Mojibake. Take a string, convert it to one encoding and then decode it differently. Most programmers end up familiar with this kind of glitch eventually. For example this code:

const string startWith = "Now is the time for all good men to come to the aid of the party";
var asBytes = Encoding.UTF8.GetBytes(startWith);
var glitched = Encoding.Unicode.GetString(asBytes);
Debug.WriteLine(glitched);

consistently results in:

潎⁷獩琠敨琠浩⁥潦⁲污潧摯洠湥琠潣敭琠桴⁥楡⁤景琠敨瀠牡祴

But, if you just want some text with some random glitches, how about something like this. It uses a set of characters that should be glitched in, a count of how many glitches there should be in a string (based on the string length) and then randomly inserts glitches:

private static Random _rand = new Random();
private const string GlitchChars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890!@#$%&";
public static string GlitchAString(string s)
{
    var glitchCount = s.Length / 5;
    var buffer = s.ToCharArray();
    for (var i = 0; i < glitchCount; ++i)
    {
        var position = _rand.Next(s.Length);
        buffer[position] = GlitchChars[_rand.Next(GlitchChars.Length)];
    }

    var result = new string(buffer);
    Debug.WriteLine(result);
    return result;
}

The first two times I ran this (with that same Now is the time... string), I got:

Original String:
    Now is the time for all good men to come to the aid of the party
First Two Results:
    LowmiW HhZ7time for all good mea to comX to ths aid oV the p1ray
    Now is fhO P!me forjall gKod men to @ome to the a@d of F5e Nawty

The algorithm is, to some extent, tunable. You can have more or fewer glitches. You can change the set of glitch chars - whatever you'd like.

Could use something similar:

 public static readonly char[] CHARS = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z' };
    static void Main(string[] args)
    {
        
        static string GenerateGlitchedString(uint wordCount,uint wordslength)
        {
            string glitchedString = "";

            for (int i = 0; i < wordCount; i++) 
            {
                for (int j = 0; j < wordslength; j++) 
                {
                    Random rnd = new Random();
                    glitchedString += CHARS[rnd.Next(CHARS.Length)];
                }
                glitchedString += " "; //Add spaces
            }
            return glitchedString;
        }

        string output = GenerateGlitchedString(5, 5);
        Console.WriteLine(output);
    }