A general explanation of the Vigenere Cipher:

The Vigenere Cipher is a method of encryption that is similar to the Caesar Cipher. This Cipher takes in a word as an argument and interprets the alphabets of the word as follows- a as 0, b as 1, c as 2 and so on.

So if your input key is abc and you want something like "hi hello" to be encrypted, the output would entail h remaining the same, i shifting by 1 place, h shifting by 2 places, e again remaining the same (as it is being shifted by 0), l shifting by 1 place, the other l by 2 and so on so forth.

The basic idea is that each letter shifts by the corresponding letter in the argument and the spaces and other punctuation marks are ignored. If the argument is shorter than the message (as it is in most cases), the argument simply loops around the message.

My problem:

My message is just being encrypted by the first alphabet of the Vignere Cipher.

For example, ./vc bc ----> message: ABCDE ABCDE becomes BCDEF BCDEF. In other words, the entire message is just being shifted by the value of b when it should instead be shifted by the value of bc (+1 for the first alphabet and +2 for every other alphabet.)

I don't understand why is this happening despite being within a loop.

CODE:

# include  
# include  
# include 
# include 
# include 

int main(int argc, string argv[])
{
    string word = argv[1];
    int i = 0;
    int j = 0;

    if (argc != 2 || isalpha(word[j]) == false)
    {
        printf("Please enter a valid command line argument! \n");
        return 1;
    }

    else if (isalpha(word[j]))
    {
        printf("Message: "); 
        string message = GetString();

        for (int n = strlen(message); i < n; i++)
        {
               int plaintext = message[i];
               int ciphertext = word[j]; 
               int uppcb = (plaintext - 65 + ciphertext - 65);
               int upcipher1 = (uppcb) % 26;
               int uplc = (plaintext - 65 + ciphertext - 97);
               int upcipher2 = (uplc) % 26;
               int lopcb = (plaintext - 97 + ciphertext - 97);
               int locipher1 = (lopcb) % 26;
               int lolp = (plaintext - 97 + ciphertext - 65);
               int locipher2 = (lolp) % 26;

                if (isupper(word[j]) && isupper(message[i])) 
                {
                    j = (j+1)%strlen(word);
                    int upcode = (upcipher1 + 65); 
                    printf("%c", upcode);
                }
                else if (isupper(message[i]) && islower(word[j]))
                {
                    j = (j+1)%strlen(word);
                    int upcode1 = (upcipher2 + 65);
                    printf("%c", upcode1);
                }
                else if (islower(message[i]) && islower(word[j]))
                {
                    j = (j+1)%strlen(word);
                    int locode = (locipher1 + 97); 
                    printf("%c", locode);
                }
                else if (islower(message[i]) && isupper(word[j]))
                {
                    j = (j+1)%strlen(word);
                    int locode1 = (locipher2 +97);
                    printf("%c", locode1);
                }
                else 
                {
                    printf("%c", message[i]);
                }          
        }
        printf("\n");     
    }

}