Vigenere shifts ALL letters by 1 keys no matter the keyword

Question

This is my code for the cipher itself. No mater what key I use it shifts all the letters by only 1, always.

Please HELP!

for (i = 0; i < m; i++) //Loop through user's input's length.
      {
          for (j = 0; j < 26; j++)
              {
                 key[j] = j; //Assign numbers 0 to 25 to each letter of the key.
              }

              if (isalpha(userMessage[i])) //If string index has letter...
              {
                  if (islower(userMessage[i])) //...and if letter is lowercase...
                  {
                      /*...shift characters by key amount inside lowercase ASCII table...*/
                      userMessage[i] = (((userMessage[i] - 97) + key[j % n]) % 26) + 97; 
                  }

                  else if (isupper(userMessage[i])) //...or if letter is uppercase...
                  { 
                      /*...shift characters by key amount inside uppercase ASCII table */
                      userMessage[i] = (((userMessage[i] - 65) + key[j % n]) % 26) + 65;
                  }
              }

              else if (isdigit(userMessage[i]) || isspace(userMessage[i])) //And if string index has non-alphabetical value... 
              {
                  userMessage[i] = userMessage[i]; //...keep value unchanged.
              }

              printf("%c", userMessage[i]); //Print ciphered text.
}

Answer 1

The code has several problems. First and most important, it appears to be completely ignoring the key that is supposed to be entered as a parameter when the program is executed at the command line. Look at the following:

      for (j = 0; j < 26; j++)
          {
             key[j] = j; //Assign numbers 0 to 25 to each letter of the key.
          }

All this does is assign numbers 0 to 26 sequentially to the key array. It does nothing with the key that is supposed to be used.

Next, what happens if the plain text contains a number or punctuation?

There may or may not be more issues, but that will depend on the fixes implemented for these problems.

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