生成一个随机字符串，其长度从变量继承

Question

I am working on a cryptography console application (C#) and want to include an option for the Vigenere Cipher during the encryption process to generate a One-Time Pad key that is completely random letters(AZ) but the same length (in characters) of the message to be encrypted. So the question is how do I generate a random string with the same length of previous user input (Stringbuilder s)? I apologize in advance if this is an obvious solution. The following is my code for initializing and displaying the results of the encryption process. The nested "if" is where I want to generate the One-Time Pad key if the user enters "gen-otp":

//Encrypt
if ((selection == 1) && (type == 1))
{
    Clear();
    WriteLine("\nPlease enter the message you wish to encrypt(please exclude spaces from the message):\n");
    StringBuilder s = new StringBuilder(ReadLine());
    WriteLine("\nPlease enter your key word or phrase:\n");
    string key = ReadLine();
    VigenereEncryptDecrypt.VigenereEncrypt(ref s, key);
    if (ReadLine() == "gen-otp") 
    { 
    }
    Clear();
    WriteLine("\nEncrypted message:\n{0}\n\nKey: {1}\n\nPlease press ENTER to continue:", s, key);
    ReadLine();
    Clear();
    CipherSelection();
}

Answer 1

So the question is how do I generate a random string with the same length of previous user input(Stringbuilder s)

For random letter AZ

Random rnd = new Random();
var otp = string.Concat(Enumerable.Range(0, s.Length)
                                  .Select(i => (char)(rnd.Next(26) + 'A')));