#include #include #include #include #include const int key_length = 26; int string_length = 0; int main(int argc, string argv[]) { if (argc != 2) { printf("Usage: ./caesar key\n"); return 1; } for (int i = 0, n = strlen(argv[1]); i < n; i++) { if (!isalpha(argv[1][i])) { printf("Key must contain only alphabetic characters.\n"); return 2; } else { string_length += 1; } } if (string_length != key_length) { printf("Key must contain 26 characters.\n"); return 3; } string key = malloc(strlen(argv[1] + 1)); strcpy(key, argv[1]); for (int a = 0; a < strlen(key); a++) { if (!isupper(key[a])) { key[a] = toupper(key[a]); } } for (int l = 0; l < strlen(key); l++) { for (int h = l + 1; h < strlen(key); h++) { if (key[l] == key[h]) { printf("No duplicates in key: %c\n", key[l]); return 4; } } } string plaintext = get_string("plaintext: "); int charcount = strlen(plaintext); char ciphertext[charcount]; string abc = "abcdefghijklmnopqrstuvwxyz"; for (int c = 0; c < charcount; c++) { if (isupper(plaintext[c]) != 0) { for (int j = 0; j < 26; j++) { if (abc[j] == tolower(plaintext[c])) { ciphertext[c] = toupper(key[j]); break; } } } else if (islower(plaintext[c]) != 0) { for (int j = 0; j < 26; j++) { if (abc[j] == plaintext[c]) { ciphertext[c] = tolower(key[j]); break; } } } else { ciphertext[c] = plaintext[c]; } } printf("ciphertext: %s\n", ciphertext); free(key); //freeing malloc space. }

substitute.c