implement S-AES BLOCK cipher encryption and decryption technique

#include<stdio.h>
#include<conio.h>
#include<string.h>

unsigned char mssg[100];
unsigned char ch[2];
unsigned char rcon[] = {0x80,0x30};
int ind;
int k[3];

unsigned char nib[2][2];

unsigned char M[][2] = { {0x1,0x4},
                        {0x4,0x1}};

unsigned char IM[][2] = { {0x9,0x2},
                          {0x2,0x9}};

unsigned char SBox[][4] = { {0x9,0x4,0xA,0xB},
                           {0xD,0x1,0x8,0x5},
                           {0x6,0x2,0x0,0x3},
                           {0xC,0xE,0xF,0x7}};

unsigned char ISBox[][4] = { {0xA,0x5,0x9,0xB},
                            {0x1,0x7,0x8,0xF},
                            {0x6,0x0,0x2,0x3},
                            {0xC,0x4,0xD,0xE}};
               
                              
void get_mssg()
{
   printf("\n Enter the Plaintext : ");
   gets((char *)mssg);
}

void print_nib()
{
  printf("\n %x %x",nib[0][0],nib[0][1]);
  printf("\n %x %x",nib[1][0],nib[1][1]);  
}

void Nib_Sub(int ind)
{
  for(int i=0;i<2;i++)
  {
    for(int j=0;j<2;j++)
    {
      if(ind == 0)
        nib[i][j] = SBox[(nib[i][j] & 0xC)>>2][nib[i][j] & 0x3];
      else
        nib[i][j] = ISBox[(nib[i][j] & 0xC)>>2][nib[i][j] & 0x3];            
    }      
  } 
}

void Shift_Row()
{
  unsigned char tmp;
  tmp = nib[1][0];
  nib[1][0] = nib[1][1];
  nib[1][1] = tmp;     
}

void Mix_Col(int ind)
{
  unsigned char a,b;
  a = (ind == 0)? 0x1 : 0x9; 
  b = (ind == 0)? 0x4 : 0x2; 
 




  unsigned char tmp;
  tmp = nib[0][0];
  nib[0][0] = (a*tmp) xor (b*nib[1][0]);
  nib[1][0] = (b*tmp) xor (a*nib[1][0]);

  tmp = nib[0][1];
  nib[0][1] = (a*tmp) xor (b*nib[1][1]);
  nib[1][1] = (b*tmp) xor (a*nib[1][1]); 
}

void Make_Key(int& i)
{
  unsigned char cho[2],tcho;
 
  cho[0] = (k[i] & 0xFF00) >> 8;
  cho[1] = k[i] & 0x00FF;
 
  tcho = (SBox[(cho[1]&0xC0)>>6][(cho[1]&0x30)>>4]) |
((SBox[(cho[1]&0x0C)>>2][cho[1]&0x03])<< 4);
 
  cho[0] = cho[0] xor rcon[i] xor tcho;
  cho[1] = cho[0] xor cho[1];
 
  k[++i] = (cho[0]<<8) | cho[1]; 
}


void Add_Key(int i)
{
  nib[0][0] = nib[0][0] xor ((k[i] & 0xF000)>>12);
  nib[1][0] = nib[1][0] xor ((k[i] & 0x0F00)>>8);
  nib[0][1] = nib[0][1] xor ((k[i] & 0x00F0)>>4);
  nib[1][1] = nib[1][1] xor (k[i] & 0x000F);  
}

void enc_S_AES()
{
   int i = 0;

   nib[0][0] = (ch[0] & 0xF0)>>4;
   nib[1][0] = ch[0] & 0x0F;
   nib[0][1] = (ch[1] & 0xF0)>>4;
   nib[1][1] = ch[1] & 0x0F;
 
   k[0] = 0x2D55;
 
   Add_Key(i);
   while(i<2)
   {
     Shift_Row();
     Nib_Sub(0);   
     Mix_Col(0);
     Make_Key(i);
     Add_Key(i);        
   }
   
   ch[0] = (nib[0][0] << 4)| nib[1][0];
   ch[1] = (nib[0][1] << 4)| nib[1][1]; 
}

void dec_S_AES()
{
   int i = 2;
 
   nib[0][0] = (ch[0] & 0xF0)>>4;
   nib[1][0] = ch[0] & 0x0F;
   nib[0][1] = (ch[1] & 0xF0)>>4;
   nib[1][1] = ch[1] & 0x0F;
  
   Add_Key(i);
 
   while(i>0)
   {
     Nib_Sub(1);
     Shift_Row();
     Add_Key(--i);
     Mix_Col(1);             
   }
   


   ch[0] = (nib[0][0] << 4)| nib[1][0];
   ch[1] = (nib[0][1] << 4)| nib[1][1]; 
}

void en_mssg()
{
  int i = 0;
  int j = strlen((char *)mssg);
  ind = 0;

  while(i < j)
  {        
    ch[0] = mssg[i];

    if(mssg[i+1] == '\0')
    {
      ind = 1;
      ch[1] = ((mssg[i]- 65 + 12)% 26)+65;           
    }
    else
     ch[1] = mssg[i+1];
    enc_S_AES();
  
    mssg[i] = ch[0];
    mssg[i+1] = ch[1];          
    i += 2;
  }

  if(i > j)
   mssg[i] = '\0';
 
  printf("\n The Ciphertext is : ");
  puts((char *)mssg);
}

void de_mssg()
{
  int i = 0;
  int j = strlen((char *)mssg);
 
  while(i < j)
  {        
    ch[0] = mssg[i];
    ch[1] = mssg[i+1];
  
    dec_S_AES();
  
    mssg[i] = ch[0];
    mssg[i+1] = ch[1];          
    i += 2;
  }

  if(ind == 1)
   mssg[i-1] = '\0';
 
  printf("\n The Decrypted Plaintext is : ");
  puts((char *)mssg);
}

int main()
{
  get_mssg();
  en_mssg();
  de_mssg();
  getch();
  return 0;
}