Vigenere

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <process.h>

void vigenereCipher(char *,char *);
void encipher();
void decipher();

void main()
{
     int choice;
     //loop takes choice from user and calles appropriate function
     while(1)
     {
          printf("\n1. Encrypt Text\n");
          printf("2. Decrypt Text\n");
          printf("3. Exit\n");
          printf("Enter Your Choice : ");
          scanf("%d",&choice);
          fflush(stdin);
          if(choice == 3)
               exit(0);
          else if(choice == 1)
               encipher();
          else if(choice == 2)
               decipher();
          else
               printf("Please Enter Valid Option.");
     }
}

void encipher()
{
     unsigned int i,j;
     char input[257],key[33];
     printf("Enter Text to be Encrypted [Max. 256 characters/ only alphabets]:\n ");
     gets(input);
     printf("Enter Encryption Key [Max. 32 Characters/ only aphabets]: ");
     gets(key);
     for(i=0,j=0;i<strlen(input);i++,j++)
     {
          //repeat the key if you are at end of it.
          if(j>=strlen(key))
          {
               j=0;
          }
          //actual logic -> character from input + character from key % 26 is encrypted charater
          printf("%c",65+(((toupper(input[i])-65)+(toupper(key[j])-65))%26));
     }

}

void decipher()
{
     unsigned int i,j;
     char input[257],key[33];
     int value;
     printf("Enter Text to be Decrypted [Max. 256 characters/ only alphabets]:\n ");
     gets(input);
     printf("Enter Decryption Key [Max. 32 Characters/ only aphabets]: ");
     gets(key);
     for(i=0,j=0;i<strlen(input);i++,j++)
     {
          //repeat the key if you are at end of it.
          if(j>=strlen(key))
          {
               j=0;
          }
          //similar to encipher only difference is you need to subtract
          value = (toupper(input[i])-64)-(toupper(key[j])-64);
          //if value is negative. We have to rotate it backwards (like backwards from z,y,x)
          //so add it to 26 (it's a negative value to adding will actually cause subtraction) to get original character.
          if( value < 0)
          {
               value = 26 + value;
          }
          printf("%c",65 + (value % 26));
     }

}