Vigenere Chiper

main.c

/**
 * Author:    Siv Sugaman
 * Created:   13-01-2021
 * 
 * (c) Copyright by Siv S<sivsivsree@gmail.com>.
 **/

#include<stdio.h>
#include<string.h>
#include<stdlib.h>

// The key hash will buffer the key to the length of 
// the Message
const char* keyHash(int len, char key[]){
  int i, j;
  int keyLen = strlen(key);
  char *newKey = malloc(len+1);

  for(i = 0, j = 0; i < len; ++i, ++j){
    if(j == keyLen) { 
        j = 0;
    }
    newKey[i] = key[j];
  }
  newKey[i] = '\0';
  // printf("key.. %s\n", newKey);  
  return newKey;
}

// Encrypt routine will encrypt by using the below
// formula, I have used the algorithm
// E(i) = { [ msg(i) + keyBuffer(i)] % 26 } + 'A' -> for char encode
const char* encrypt(char msg[], char key[]){
  int msgLen = strlen(msg);
  int i, j;
  char *encryptedMsg = malloc(msgLen);
  char *newKey = keyHash(msgLen, key);
 
  for(i = 0; i < msgLen; ++i){
    char encode = ((msg[i] + newKey[i]) % 26) + 'A'; 
    encryptedMsg[i] = encode;
    }
  encryptedMsg[i] = '\0';  
  return encryptedMsg;
}

// Decrypt routine has the logic 
// to decrypt the message using the key
const char* decrypt(char message[], char key[]) {
  int msgLen = strlen(message), i, j;
  char *dMsg = malloc(msgLen);
  char *newKey = keyHash(msgLen, key);

  for(i = 0; i < msgLen; ++i){
    dMsg[i] = (((message[i] - newKey[i]) + 26) % 26) + 'A';
  }
  dMsg[i] = '\0';
  printf("%s", dMsg);
  return dMsg;
}


int main(){

  int sel, defer;
  char passphrase[254];
  char key[35];

   while (1){
     
     printf(" 1. Encryption \n 2. Decryption \n 3. Clear Screen \n 4. Exit \n");
     scanf("%d", &sel);
     switch(sel) {
       case 1:
          printf("Please enter MESSSAGE to ENCRYPT: ");
          scanf("%s", passphrase);
          printf("Please enter KEY: ");
          scanf("%s", key);

          // Encryption Logic call
          char *encryptedMsg = encrypt(passphrase, key);
          printf("\nEncrypted Message: '%s' \n", encryptedMsg);
          
          printf("\npress any key to continue...");
          scanf("%d", &defer);
          
       break;
       case 2: 
          printf("Please enter MESSSAGE to DECRYPT: ");
          scanf("%s", passphrase);
          printf("Please enter KEY: ");
          scanf("%s", key);

          // Decription Logic call
          char *decryptedMsg = decrypt(passphrase, key);
          printf("\nDecrypted Message: '%s'\n", decryptedMsg);

          printf("\npress any key to continue...");
          scanf("%d", &defer);
       break;
       case 3: 
         // ASCII clear screen
          printf("\e[1;1H\e[2J");
       break;
       case 4:
          exit(0);
     }
   }

 
   return 0;
}