Problem enciphering in vigenere

Question

Colleagues,

In vigenere, I can obtain and error check the required user inputs and I have a conceptual understanding of the steps required to encipher the plaintext with the keyword, looping where necessary.

However, I am struggling to achieve the correct enciphering and at this point have gone somewhat code-blind..

If anyone can take a look at the code and give me a hint as to where I am going wrong it would be much appreciated.

Relevant code:

for (int i = 0, k = 0; i < plaintext_length; i++)
   {
    if ( isalpha(plaintext[i]) && isupper(plaintext[i]))
    {
        encrypted_char = ((plaintext[i] - 65) + (keyword[k %   keyword_length]) % 26 + 65);
        printf("%c", encrypted_char);
        k++;
    }
    else if ( isalpha(plaintext[i]) && islower(plaintext[i]))
    {
        encrypted_char = ((plaintext[i] - 97) + (keyword[k % keyword_length]) % 26 + 97);
        printf("%c", encrypted_char);
        k++;
    }
    else if (isspace(plaintext[i]))
    {
        printf("%c", encrypted_char);
    }
    else
    {
        printf("%c", plaintext[i]);
    }
 }

Many thanks in advance!

Answer 1

This problem was one of ASCII math(s) and looping. It is now solved. Other problems remain but I am confident they != insurmountable.

I absolutely love CS50, just saying.

Answer 2

I think your way of enciphering is correct and I want to know why you didn't convert the keyword-ASCII into Alphabet-index ( subtracting 65 ). ?

Answer 3

encrypted_char = ((plaintext[i] - 65) + (keyword[k %   keyword_length]) % 26 + 65);

why do you use the % operator here? you should not be dividing but instead just be using k to do the indexing into the keyword, assuming that you first repeated the keyword such that is equal to or longer than the string that requires enciphering (otherwise you need to check the number of k such that it is never longer than the length of the keyword).

else if (isspace(plaintext[i]))
{
    printf("%c", encrypted_char);
}*/ 

not required and causes problems because encrypted_char not defined here.