Program 231: Decryption of Viginere Cipher using C

Program 231:
Click here to know Encryption of Viginere Cipher 
 

#include<stdio.h>
#include<string.h>
#include<ctype.h>
#include<stdlib.h>
main()
{
 int i,j,k,numstr[100],numkey[100],numcipher[100];
 char str[100],key[100];
 printf("Enter a string to Decrypt\n");
 gets(str);
 //converting entered string to Capital letters
 for(i=0,j=0;i<strlen(str);i++)
 {
  if(str[i]!=' ')
  {
   str[j]=toupper(str[i]);   
   j++;
  }
 }
 str[j]='\0';
 printf("Entered string is : %s \n",str);
 //Storing string in terms of ascii
 for(i=0;i<strlen(str);i++)
 {
  numstr[i]=str[i]-'A';
 }
 printf("Enter a key\n");
 gets(key);
    //converting entered key to Capital letters
 for(i=0,j=0;i<strlen(key);i++)
 {
  if(key[i]!=' ')
  {
   key[j]=toupper(key[i]);   
   j++;
  }
 }
 key[j]='\0';
     //Assigning key to the string
    for(i=0;i<strlen(str);)
    {
     for(j=0;(j<strlen(key))&&(i<strlen(str));j++)
     {
      numkey[i]=key[j]-'A';
      i++;
     }
     
    }
    
    for(i=0;i<strlen(str);i++)
    {
     numcipher[i]=numstr[i]-numkey[i];//changed from + to - for decryption 
     if(numcipher[i]<0)
     {
      numcipher[i]+=26;
     }
    }

    printf("Decrypted Vigenere Cipher text is\n");   
    for(i=0;i<strlen(str);i++)
    {
      printf("%c",(numcipher[i]+'A')); 
    }
    
 printf("\n");
}

Explanation:

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Output: