C Program to Implement Affine Cipher

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This is a C Program to implement Affine Cipher. Affine Cipher is a type of Monoalphabetic Substitution Cipher, where each letter is mapped to its numeric value and it is encrypted using mathematical function and converted back to alphabets.
Each letter is enciphered with the function (ax+b)mod(26), where b is the magnitude of the shift.

Here is source code of the C Program to Implement Affine Cipher. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

  1. #include <stdlib.h>
  2. #include <string.h>
  3. #include <stdio.h>
  4. #include <math.h>
  5.  
  6. main(int argc, char *argv[])
  7. {  
  8.    FILE *fin, *fout;
  9.    char c, A[80], B[80];
  10.    int  j,k,nc, a,b,s,t, x,y, ptr[26];
  11.  
  12.    sscanf(argv[1],"%d", &a); 
  13.    sscanf(argv[2],"%d", &b); 
  14.    fin =fopen(argv[3],"r"); 
  15.    /* fout=fopen(argv[4],"w"); */
  16.  
  17.    for (j=0; j<26; j++) ptr[j]=0;
  18.    ptr[ 1]= 1;  ptr[ 3]= 9;  ptr[ 5]=21;  ptr[ 7]=15;  
  19.    ptr[ 9]= 3;  ptr[11]=19;  ptr[15]= 7;  ptr[17]=23;
  20.    ptr[19]=11;  ptr[21]= 5;  ptr[23]=17;  ptr[25]=25;
  21.  
  22.    nc=0;  
  23.    fscanf(fin,"%c",&c); A[0]=c;
  24.    while (c != '\n') {fscanf(fin,"%c",&c); A[++nc]=c;}
  25.    /* A[++nc]=c; */
  26.    printf("y=%2d x + %d (mod 26)\n", a,b); 
  27.    printf("plaintext:  "); 
  28.    for (j=0; j<nc; j++) printf("%c",A[j]); printf("\n");
  29.  
  30.    if (a%2==0 || a==13 || a>25 || b>25) 
  31.      printf("(a,b)=(%2d,%2d) is invalid\n",a,b);
  32.    else {s=ptr[a]; t=(s*(26-b))%26;}
  33.  
  34.    for (k=0; k<nc; k++) {
  35.      x=(int) A[k] - (int) 'a';
  36.      y=(a*x+b)%26; 
  37.      B[k]=(char) (y+65);
  38.    }
  39.    printf("ciphertext: "); 
  40.    for (j=0; j<nc; j++) printf("%c",B[j]); printf("\n");
  41.  
  42.    printf("Decipher: y=%2dx + %2d\n",s,t);
  43.    for (k=0; k<nc; k++) {
  44.      y=(int) B[k] - (int) 'A';
  45.      x=(s*y+t)%26; 
  46.      A[k]=((x==23)? ' ' : (char) (x+97));
  47.    }
  48.    printf("decrypted message: "); 
  49.    for (j=0; j<nc; j++) printf("%c",A[j]); printf("\n");
  50.    printf("\n");
  51. }

Output:

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$ gcc AffineCipher.c
$ ./a.out
 
Enter the message: SANFOUNDRY
Message is :SANFOUNDRY
Encrypted Message is : VTGIJBGCSN
Decrypted Message is: SANFOUNDRY

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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