Task: Vigenère cipher
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>
#include <ctype.h>
#include <getopt.h>

#define NUMLETTERS 26
#define BUFSIZE 4096

char *get_input(void);

int main(int argc, char *argv[])
{
    char const usage[] = "Usage: vinigere [-d] key";
    char sign = 1; 
    char const plainmsg[] = "Plain text:  ";
    char const cryptmsg[] = "Cipher text: ";
    bool encrypt = true;
    int opt;

    while ((opt = getopt(argc, argv, "d")) != -1) {
        switch (opt) {
        case 'd': 
            sign = -1;
            encrypt = false;
            break;
        default: 
            fprintf(stderr, "Unrecogized command line argument:'-%i'\n", opt);
            fprintf(stderr, "\n%s\n", usage);
            return 1;
        }
    }

    if (argc - optind != 1) {
        fprintf(stderr, "%s requires one argument and one only\n", argv[0]);
        fprintf(stderr, "\n%s\n", usage);
        return 1;
    }


    
    char const *const restrict key = argv[optind];
    size_t const keylen = strlen(key);
    char shifts[keylen];

    char const *restrict plaintext = NULL; 
    for (size_t i = 0; i < keylen; i++) {
        if (!(isalpha(key[i]))) {
            fprintf(stderr, "Invalid key\n");
            return 2;
        }
        char const charcase = (isupper(key[i])) ? 'A' : 'a';
        
        
        shifts[i] = (key[i] - charcase) * sign;
    }

    do {
        fflush(stdout);
        
        
        printf("%s", (encrypt) ? plainmsg : cryptmsg);
        plaintext = get_input();
        if (plaintext == NULL) {
            fprintf(stderr, "Error getting input\n");
            return 4;
        }
    } while (strcmp(plaintext, "") == 0); 

    size_t const plainlen = strlen(plaintext);

    char* const restrict ciphertext = calloc(plainlen + 1, sizeof *ciphertext);
    if (ciphertext == NULL) {
        fprintf(stderr, "Memory error\n");
        return 5;
    }

    for (size_t i = 0, j = 0; i < plainlen; i++) {
        
        if (!(isalpha(plaintext[i]))) {
            ciphertext[i] = plaintext[i];
            continue;
        }
        
        char const charcase = (isupper(plaintext[i])) ? 'A' : 'a';
        
        ciphertext[i] = ((plaintext[i] + shifts[j] - charcase + NUMLETTERS) % NUMLETTERS) + charcase;
        j = (j+1) % keylen;
    }
    ciphertext[plainlen] = '\0';
    printf("%s%s\n", (encrypt) ? cryptmsg : plainmsg, ciphertext);

    free(ciphertext);
    
    free((char*) plaintext);
    return 0;
}
char *get_input(void) {

    char *const restrict buf = malloc(BUFSIZE * sizeof (char));
    if (buf == NULL) {
        return NULL;
    }

    fgets(buf, BUFSIZE, stdin);

    
    size_t const len = strlen(buf);
    if (buf[len - 1] == '\n') buf[len - 1] = '\0';

    return buf;
}

C++: #include <iostream>
#include <string>
using namespace std;

class Vigenere
{
public:
  string key;

  Vigenere(string key)
  {
    for(int i = 0; i < key.size(); ++i)
    {
      if(key[i] >= 'A' && key[i] <= 'Z')
        this->key += key[i];
      else if(key[i] >= 'a' && key[i] <= 'z')
        this->key += key[i] + 'A' - 'a';
    }
  }

  string encrypt(string text)
  {
    string out;

    for(int i = 0, j = 0; i < text.length(); ++i)
    {
      char c = text[i];
      
      if(c >= 'a' && c <= 'z')
        c += 'A' - 'a';
      else if(c < 'A' || c > 'Z')
        continue;

      out += (c + key[j] - 2*'A') % 26 + 'A'; 
      j = (j + 1) % key.length();
    }

    return out;
  }

  string decrypt(string text)
  {
    string out;

    for(int i = 0, j = 0; i < text.length(); ++i)
    {
      char c = text[i];
      
      if(c >= 'a' && c <= 'z')
        c += 'A' - 'a';
      else if(c < 'A' || c > 'Z')
        continue;

      out += (c - key[j] + 26) % 26 + 'A'; 
      j = (j + 1) % key.length();
    }

    return out;
  }
};

int main()
{
  Vigenere cipher("VIGENERECIPHER");

  string original = "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!";
  string encrypted = cipher.encrypt(original);
  string decrypted = cipher.decrypt(encrypted);

  cout << original << endl;
  cout << "Encrypted: " << encrypted << endl;
  cout << "Decrypted: " << decrypted << endl;
}