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substitution.c
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substitution.c
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#include <ctype.h>
#include <cs50.h>
#include <stdio.h>
#include <string.h>
bool valid_key(string key);
void encrypt_message(string key, string message);
int main(int argc, string argv[])
{
// Check arguments
if (argc != 2)
{
printf("Usage: ./substitution key\n");
return 1;
}
// Check is a valid key
string key = argv[1];
bool valid = valid_key(key);
if (!valid)
{
printf("Key must contain 26 characters.\n");
return 1;
}
// Do Encrypt Message
string message = get_string("plaintext: ");
encrypt_message(key, message);
return 0;
}
bool valid_key(string key)
{
int length = strlen(key);
int a = (int) 'A';
int z = (int) 'Z';
// containing each letter exactly once
int total_ascii = ((float)(a + z) / 2) * (z - a + 1);
int sum_key = 0;
// containing 26 characters
if (length != 26)
{
return false;
}
for (int i = 0, n = length; i < n; i++)
{
// any character is an alphabetic character
char c = key[i];
if (isalpha(c))
{
// uppercase characters and add to sum_key
sum_key += (int) toupper(c);
}
else
{
return false;
}
}
// check sum_key equal to total_ascii, mean all alphabetic character exactly once
return sum_key == total_ascii;
}
void encrypt_message(string key, string message)
{
printf("ciphertext: ");
// Encrypting
for (int i = 0, n = strlen(message); i < n; i++)
{
char c = message[i];
if (isalpha(c))
{
int index_in_key = (int) toupper(c) - (int) 'A';
char r = key[index_in_key];
printf("%c", isupper(c) ? toupper(r) : tolower(r));
}
else
{
printf("%c", c);
}
}
printf("\n");
}