#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef unsigned char uc;
uc PC[256], CP[256];
uc const iv = 42;

// Encrypt:
uc CBCc(uc const * const p, uc * const c, int l){
   uc v = iv;
   {int j; for(j=0; j<l; ++j) v = c[j] = PC[v ^ p[j]];}
   return v;}

// Decrypt:
void CBCd(uc const * const c, uc * const p, int l){
   uc v = iv;
   {int j; for(j=0; j<l; ++j) {p[j] = v ^ CP[c[j]]; v = c[j];}}}

// Display ciphertext
void pct(char * whch, uc * ct, int l){
   int j;
   for(j=0; j<l; ++j) printf("%02X ", ct[j]);
   printf(" is %s\n", whch);}

int main(){

   // Generate the trivial permutation.
  {int j; for(j=0; j<256; ++j) PC[j]=j;}

   // Scramble it a bit.
  {int j = 5000; while(j--){
     uc k = rand(), l = rand();
     uc t = PC[k]; PC[k] = PC[l]; PC[l] = t;}}

   // Compute the inverse permutation.
  {int j; for(j=0; j<256; ++j) CP[PC[j]] = j;}

  {
   uc ct[250];
   uc * pt = "Target of corruption";
   int len = strlen(pt);
  
   // Alice computes MAC and encrypts:
   int MAC = PC[CBCc(pt, ct, len)];
   pct("good", ct, len);

   // Mallet changes ciphertext;
   ct[0] ^= 8;
   pct("corrupted", ct, len);

   // Bob decrypts:
   {uc pp[250];
    CBCd(ct, pp, len);

    // and computes MAC:
    {
      uc dump[250];
      uc MACp = PC[CBCc(pp, dump, len)];
      pct("reciphered", dump, len);
      printf("MAC = %02X, MACp = %02X\n", MAC, MACp);
      if(MAC == MACp) printf("The MAC is good!\n");}

    pp[len] = 0; // terminate string for printing.
    printf ("Transmitted string = \"%s\"\n", pp);}
    }
 return 0;}