Trying to encrypt a sentence

Question

I'm doing a project where I am supposed to implement the Vigenere cipher.
I am supposed to read in a file that has the sentence/text to be encrypted, and then have the person enter a keyword to shift the letters with...but what's confusing me is trying to get the nested loops right to read through the plaintext and the keyword and then adding them together, so here the String plain is going to be the plaintext.
public static void encrypt(String plain, String keyWord)
for(int i=0; i<=plain.length(); i++)
char c = plain.charAt(i);
int z = (int) c;
     while(i<=plain.length()) //while it's looping through the plaintext i want it to loop through keyWord
          for(int j=0; j<=keyWord.length(); j++)
          char k = keyWord.charAt(j);
          int y = (int) k;
          int alg = (y + z) % 26;
     //need to convert back to letters
Part of the problem is that I can't figure out how to make it loop through the characters of the keyword over and over...I assumed I needed to use a while loop but like here I've got for(int j=0; j<=keyword.length; j++) nested in a while loop. With what I have here it only loops through it once.
So can someone please point me in the right direction or tell me if I should be doing this a completely different way(and, if I need to use an array or something please tell me because we haven't learned those yet, but something silly like us not having yet learned something has not stopped my professor from requiring us to use it)
When I just test what I've got here I also get an error about my string index being out of bounds, and I've searched and searched and cannot figure out what I've done here to get that error.

Answer 1

Sorry about the above I never saw that it was the Vigenre cipher
public static void encrypt(String plain, String keyWord){
  for(int i=0; i<=plain.length(); i++){
    char c = plain.charAt(i);
    int z = (int) c;
    while(i<=plain.length()) //while it's looping through the plaintext i want it to loop through keyWord{
      for(int j=0; j<=keyWord.length(); j++){
        char k = keyWord.charAt(j);
        int y = (int) k;
        int alg = (y + z) % 26;
        //need to convert back to letters
}A few things you will need to remember:
Because in java A starts at 65 and a at 97 you can only work in either uppercase or lowercase letters without mixing.
To display the text you need to know if it was originally upper or lowercase.
Only characters from a-z or A-Z can be used.I don't like to simply give code but here is a method that only uses 1 loop:
//remove all chars that are not a-zA-Z and make all chars uppercase in bot value and key before mkKet or encrypt is called
//used to make the key the same length as the value
  public String mkKey(String plain, String key){
    if(plain.length() == key.length())
      return key;
    if(plain.length() < key.length())
      return key.substring(0,plain.length());
    String out = "";
    for(int i=0;i<plain.length();i++){
      out += key.charAt(i%key.length());
    return out;
  public String encrypt(String value, String key){
    int v;
    int k;
    String out = "";
    for(int i=0;i<value.length();i++){
      v = value.charAt(i);
      k = key.charAt(i);
      out += (char)(((v + k) % 26) + 65);
    return out;
The Vigenre cipher rule says:
C ~i~ = (P ~i~ + K ~i~ ) MOD 26
P ~i~ = (C ~i~ - K ~i~ ) MOD 26but when C ~i~ - K ~i~ returns a negitive it must change to:
P ~i~ = (26 + (C ~i~ - K ~i~ )) MOD 26