Vigenere Cipher

Vigenere Cipher:

Vigenere Cipher is a method of encrypting alphabetic text. It uses a simple form of polyalphabetic substitution. A polyalphabetic cipher is any cipher based on substitution, using multiple substitution alphabets.

This is the code in C:

// C program to implement vigenere cipher
#include<stdio.h>
#include<string.h>

//function for encryption
void encryption(const char *text,const char *keyword)
{
    int len=strlen(text);
    char entext[len+1];
    memset(entext, '\0', sizeof(entext));
    for(int i=0;i<len;i++)
    {
        entext[i]= (text[i] + keyword[i])%26 +'A';
    }
    printf("%s", entext);
}

//function for decryption
void decryption(const char *cipher,const char *keyword)
{
    int len=strlen(cipher);
    char detext[len+1];
    memset(detext, '\0', sizeof(detext));
    for(int i=0;i<len;i++)
    {
        detext[i]= (cipher[i]- keyword[i] +26)%26 +'A';
    }
    printf("%s", detext);
}

int main()
{
    char text[100], key[100], cipher[100];
    int textLen, keyLen, ciphLen; //variables to store length of plain text, key and cipher text respectively
    int n;
    int option;
    printf("This is a C program to implement Vigenere Cipher. \n\nEnter: 1. Encryption \n2. Decryption\n");
    scanf("%d",&option);

    if(option==1)
    {
        printf("You have chosen Encryption. \nEnter the plain text: ");
        scanf("%s", text);  //user input of only uppercase characters without white spaces
        textLen= strlen(text);
        n=textLen;
        printf("Enter the key: ");
        scanf("%s", key);
        keyLen= strlen(key);
    }

    else if(option==2)
    {
        printf("You have chosen Decryption. \nEnter the Cipher text(Encrypted text): ");
        scanf("%s", cipher);
        ciphLen=strlen(cipher);
        n=ciphLen;
        printf("Enter the key: ");
        scanf("%s", key);
        keyLen= strlen(key);
    }

    else
    {
        printf("Wrong option entered! Enter either 1(for encryption) or 2(for Decryption)");
        return 0;
    }

    //Generating the keyword(new key)
    int i,j;
    char keyword[100];
    for(i=0,j=0;i<n;i++,j++)
    {
        if(j==keyLen)
            j=0;
        keyword[i]=key[j];
    }

    if(option==1)
    {
        printf("The encrypted text(cipher text) is: ");
        encryption(text,keyword);
    }

    else
    {
        printf("The decrypted text(plain text) is: ");
        decryption(cipher,keyword);
    }
    printf("\n");
    return 0;
}

Built With

• c