CS50 Substitution - c

Question

This problem set requires us to code a programme that takes a key from the user from the CLI and then an input (plaintext) and return a ciphertext version that is scrambled based on the key provided.
My code returns the correct ciphertext given any key and plaintext, however, the apparent output when using the in-built check50 module from cs50 is "", an empty string.
#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>
string encrypt(string keys, string inputs);
char newkey;
string ciphertext;
int main(int argc, string argv[])
{
if (argc == 1)
{
printf("Please enter a key!\n");
return 1;
}
if (argc > 2)
{
printf("You can only have one key. The key must not have any spaces!\n");
return 1;
}
if (0 < strlen(argv[1]) && strlen(argv[1]) < 26)
{
printf("Key must contain 26 characters!\n");
return 1;
}
for (int j = 0; j < strlen(argv[1]); j++)
{
if (!((argv[1][j] >= 'a' && argv[1][j] <= 'z') || (argv[1][j] >= 'A' && argv[1][j] <= 'Z')))
{
printf("Key must contain alphabets only!");
return 1;
}
}
for (int k = 0; k < strlen(argv[1]); k++)
{
for (int l = (k + 1); l < strlen(argv[1]); l++)
{
if (argv[k] == argv[l])
{
printf("There can be no duplicate alphabets in the key!");
return 1;
}
}
}
string key = argv[1];
string input = get_string("plaintext: ");
encrypt(key, input);
printf("ciphertext: %s\n", ciphertext);
}
string encrypt(string keys, string inputs)
{
char ciphertexts[strlen(inputs)];
for (int i = 0; i < strlen(inputs); i++)
{
if (islower(inputs[i]))
{
int index = inputs[i] - 97;
newkey = keys[index];
ciphertexts[i] = tolower(newkey);
}
else if (isupper(inputs[i]))
{
int index = inputs[i] -65;
newkey = keys[index];
ciphertexts[i] = toupper(newkey);
}
else
{
ciphertexts[i] = inputs[i];
}
}
ciphertext = ciphertexts;
printf("%s\n",ciphertexts);
printf("%s\n",ciphertext);
return ciphertext;
}
The errors are as follows:
:( encrypts "A" as "Z" using ZYXWVUTSRQPONMLKJIHGFEDCBA as key
expected "ciphertext: Z\...", not ""
This means that using a key of ZYXWVUTSRQPONMLKJIHGFEDCBA, and a plaintext of "A", "Z" is expected, but my programme outputs "".
I have a printf statement printing the cipher text, but it is somehow not captured

Answer 1

Memory management
Rather than return a pointer to a local variable (which is no longer valid after function ends #kaylum), pass into encrypt() a pointer to where to form the encrypted string.
Don't forget about '\0'
In forming the encrypted string account for the null character. #kaylum
Avoid repeated calls to strlen(inputs)
Advanced: Avoid negative ch with is...(ch)
Along with some other ideas, perhaps:
#include <ctype.h>
void encrypt(char *ciphertexts, const char *keys, char *inputs) {
// Let's work with unsigned char to avoid trouble negative char
const unsigned char *ukeys = (const unsigned char*) keys;
const unsigned char *uinputs = (const unsigned char*) inputs;
unsigned char *uciphertexts = (unsigned char*) ciphertexts;
size_t i = 0;
// Code uses a do loop to catch the null character.
do {
if (islower(uinputs[i])) {
int index = uinputs[i] - 'a';
unsigned char newkey = ukeys[index];
uciphertexts[i] = tolower(newkey);
} else if (isupper(uinputs[i])) {
int index = uinputs[i] - 'A';
unsigned char newkey = ukeys[index];
uciphertexts[i] = toupper(newkey);
} else {
uciphertexts[i] = uinputs[i];
}
} while (uinputs[i++]);
}
Code still has trouble when the locale is not "C" or with non-ASCII, but we can leave those as advanced issues.
Advanced
Since strings to encrypt can be lengthy, consider pre-forming a key valid for all char. I.e.
unsigned char keys256[UCHAR_MAX + 1] = { 0 };
for (i = 0; i <= UCHAR_MAX; i++) {
keys256[i] = i;
}
for (i = 0; key[i]; i++) {
keys256['a' + i] = tolower(key[i]);
keys256['A' + i] = toupper(key[i]);
}
and then in encrypt(..., keys256, ...), use a greatly simplified loop.
...
do {
uciphertexts[i] = keys256(uinputs[i]);
} while (uinputs[i++]);
...