Question

# shift and affine cipher program in Java ND c

shift and affine cipher program in Java ND c

Affine Cipher problem :

Affine cipher is a type of monoalphabetic substitution cipher,

wherein each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which.

The whole process relies on working modulo m (the length of the alphabet used).

In the affine cipher, the letters of an alphabet of size m are first mapped to the integers in the range 0 … m-1.
The ‘key’ for the Affine cipher consists of 2 numbers, we’ll call them a and b. The following discussion assumes the use of a 26 character alphabet (m = 26).

A should be chosen to be relatively prime to m (i.e. a should have no factors in common with m).

Instructions while running code :

Please save file name as AffineCipher.java

Compile it using command javac AffineCipher.java

After successful compilation execute it using java AffineCipher

Java Source Code :

`import java.util.Scanner;`
`public class AffineCipher`
`{`
`    public static String encryptionMessage(String Msg,int a,int b)`
`    {`
`        String CTxt = "";        `
`        for (int i = 0; i < Msg.length(); i++)`
`        {`

// encryption going on here

`            CTxt = CTxt + (char) ((((a * Msg.charAt(i)) + b) % 26) + 65);`
`        }`
`        return CTxt;`
`    }`
` `
`    public static String decryptionMessage(String CTxt,int a,int b)`
`    {`
`        String Msg = "";        `
`        int a_inv = 0;`
`        int flag = 0;`
`        for (int i = 0; i < 26; i++)`
`        {`
`            flag = (a * i) % 26;`
`            if (flag == 1)`
`            {`
`                a_inv = i;`
`                System.out.println(i);`
`            }`
`        }`
`        for (int i = 0; i < CTxt.length(); i++)`
`        {`

// message decryption going on here

`            Msg = Msg + (char) (((a_inv * ((CTxt.charAt(i) - b)) % 26)) + 65);`
`        }`
`        return Msg;`
`    }`
` `
`    public static void main(String[] args)`
`    {`
`        Scanner sc = new Scanner(System.in);`

int a,b;

`        System.out.println("Enter the message: ");`
`        String message = sc.next();`
```        System.out.println("Enter values of a and b :");
a=sc.nextInt();
b=sc.nextInt();
System.out.println("Message is :" + message);```
`        System.out.println("Encrypted Message is : "`
`                + encryptionMessage(message,a,b));`
`        System.out.println("Decrypted Message is: "`
`                + decryptionMessage(encryptionMessage(message,a,b)));`
`        sc.close();`
`    }`
`}`

// end of java source code

C source code :

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

int CalcGCD(int);
void main()
{
int i,j,k,gcd,a,b,numstr[100],numcipher[100];
char str[100],cipher[100];
printf("Enter a string\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++;
}
else
{
str[j]=' ';
j++;
}
}
str[j]='\0';
printf("Entered string is : %s \n",str);
printf("Enter a value and must be between 1 and 25 both included\n");
scanf("%d",&a);
//Checking consitions
if(a<1 || a>25)
{
printf("a should lie in between 1 and 25\nSorry Try again !\n");
exit(0);
}
gcd=CalcGCD(a);
if(gcd!=1)
{
printf("gcd(a,26)=1 but \n gcd(%d,26)=%d\nSorry Try again !\n",a,gcd);
exit(0);
}
printf("Enter b value and must be between 0 and 25 both included\n");
scanf("%d",&b);
if(b<0 || b>25)
{
printf("b value should lie between 0 and 25\nSorry Try again !\n");
exit(0);
}
//Conditions Over
//Program Starts
//Storing string in terms of ascii and to restore spaces I used -20
for(i=0;i<strlen(str);i++)
{
if(str[i]!=' ')
numstr[i]=str[i]-'A';
else
numstr[i]=-20;
}
//Ciphering Process
//If numcipher is more than 25 .We need to convert and ensure that lie in between 0 and 25.(indicating Alphabets)
//A-0,B-1,C-2,.....Y-24,Z-25
printf("Affine Cipher text is\n");
for(i=0;i<strlen(str);i++)
{
if(numstr[i]!=-20)
{
numcipher[i]=((alpha*numstr[i])+b)%26;
printf("%c",(numcipher[i]+'A'));
}
else
{
printf(" ");
}
}
printf("\n");

}

int CalcGCD(int a)
{
int x;
int temp1=a;
int temp2=26;

while(temp2!=0)
{
x=temp2;
temp2=temp1%temp2;
temp1=x;
}
return(temp1);
}```

// end of cpp source code

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