user3342048 - 4 months ago 49

C Question

I have been working on this Vigenere cipher for about 8 hours straight. Can you please help me? I think that main problem is around the algorithm - I don't know how to utilize the keylength (I know I need to get the mod of it somehow).

`#include <stdio.h>`

#include <cs50.h>

#include <stdlib.h>

#include <string.h>

#include <ctype.h>

int main(int argc, char *argv[])

{

if(argc != 2)

{

printf("Bad!");

return 1;

}

int keylen = strlen(argv[1]);

char *key = argv[1];

for (int i = 0; i < keylen; i++)

{

if(isupper(key[i]))

{

key[i] = key[i]-65;

}

else if(islower(key[i]))

{

key[i] = key[i]-97;

}

}

printf("Plaintext: ");

string p = GetString();

int k = 0;

for (int i = 0; i < strlen(p); i++)

{

if(isalpha(p[i]))

{

if(isupper(p[i]))

{

p[i] = ((p[i]-65)+(key[k % keylen]))%26 + 65;

}

else if(islower(p[i]))

{

p[i] = ((p[i]-97)+(key[k % keylen]))%26 + 97;

}

k++;

}

else

{

//I need to skip over antyhing that isn't a letter

p[i] = p[i];

}

}

printf("Ciphertext: %s\n", p);

}

Answer

The first `for`

loop has a problem. The condition is checking for `i > keylen`

when it should be checking for `i < keylen`

.

Also when computing the next output value, the steps should be

```
(p[i]-65) results in a number between 0 and 25
adding (key[i % keylen]) results in a number between 0 and 50
apply modulo 26 so the number is between 0 and 25 (this is the missing step)
then add 65 to get the output
```

Source (Stackoverflow)

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