adaptation d'un code de 32 bits à une plateforme 8 bits

#include <stdio.h>
#include <conio.h>
/* RC6 is parameterized for w-bit words, b bytes of key, and
 * r rounds.  The AES version of RC6 specifies b=16, 24, or 32;
 * w=32; and r=20;
 */
  
#define w 32	/* word size in bits */
#define r 20	/* based on security estimates */

#define P32 0xB7E15163	/* Magic constants for key setup */
#define Q32 0x9E3779B9

/* derived constants */
#define bytes   (w / 8)				/* bytes per word */
#define c       ((b + bytes - 1) / bytes)	/* key in words, rounded up */
#define R24     (2 * r + 4)
#define lgw     5                       	/* log2(w) -- wussed out */
#define b        16             /* number of bytes in key            */
/* Rotations */
#define ROTL(x,y) (((x)<<(y&(w-1))) | ((x)>>(w-(y&(w-1)))))
#define ROTR(x,y) (((x)>>(y&(w-1))) | ((x)<<(w-(y&(w-1)))))

unsigned int S[R24 - 1];		/* Key schedule */

void rc6_key_setup(unsigned char *K)
{
	int i, j, s, v;
	unsigned int L[(32 + bytes - 1) / bytes]; /* Big enough for max b */
	unsigned int A, B;

	L[c - 1] = 0;
	for (i = b - 1; i >= 0; i--)
		L[i / bytes] = (L[i / bytes] << 8) + K[i];
     //printf("L= \n",L[i/bytes]);}
	
    S[0] = P32;
	for (i = 1; i <= 2 * r + 3; i++)
		S[i] = S[i - 1] + Q32;
       // printf("Si= \n",S[i]);}
	A = B = i = j = 0;
	v = R24;
	if (c > v) v = c;
	v *= 3;

	for (s = 1; s <= v; s++)
	{
		A = S[i] = ROTL(S[i] + A + B, 3);
		B = L[j] = ROTL(L[j] + A + B, A + B);
		i = (i + 1) % R24;
		j = (j + 1) % c;
	}
}

void rc6_block_encrypt(unsigned int *pt, unsigned int *ct)
{
	unsigned int A, B, C, D, t, u, x;
	int i, j;

	A = pt[0];
	B = pt[1];
	C = pt[2];
	D = pt[3];
	B += S[0];
	D += S[1];
	for (i = 2; i <= 2 * r; i += 2)
	{
		t = ROTL(B * (2 * B + 1), lgw);
		u = ROTL(D * (2 * D + 1), lgw);
		A = ROTL(A ^ t, u) + S[i];
		C = ROTL(C ^ u, t) + S[i + 1];
		x = A;
		A = B;
		B = C;
		C = D;
		D = x;
	}
	A += S[2 * r + 2];
	C += S[2 * r + 3];
	ct[0] = A;
	ct[1] = B;
	ct[2] = C;
	ct[3] = D;
}

void rc6_block_decrypt(unsigned int *ct, unsigned int *pt)
{
	unsigned int A, B, C, D, t, u, x;
	int i, j;

	A = ct[0];
	B = ct[1];
	C = ct[2];
	D = ct[3];
	C -= S[2 * r + 3];
	A -= S[2 * r + 2];
	for (i = 2 * r; i >= 2; i -= 2)
	{
		x = D;
		D = C;
		C = B;
		B = A;
		A = x;
		u = ROTL(D * (2 * D + 1), lgw);
		t = ROTL(B * (2 * B + 1), lgw);
		C = ROTR(C - S[i + 1], t) ^ u;
		A = ROTR(A - S[i], u) ^ t;
	}
	D -= S[1];
	B -= S[0];
	pt[0] = A;
	pt[1] = B;
	pt[2] = C;
	pt[3] = D;	
}

int main()
{
	int  j;
	unsigned char K[b];
	unsigned int pt[4] = {0,0,0,0};
	unsigned int ct[4] = {0,0,0,0};
     printf("Key is :");
     for (j=0;j<b;j++) 
         { K[j] = ct[0]%(255-j);
     printf("%.2X ",K[j]);
     }
  rc6_key_setup(K);
  rc6_block_encrypt(pt,ct); 
  printf("\n plaintext is   %.8lX %.8lX %.8lX %.8lX",pt[0], pt[1],  pt[2], pt[3]);
  rc6_block_decrypt(ct,pt);
  printf("\n ciphertext is  %.8lX %.8lX %.8lX %.8lX ",ct[0], ct[1],ct[2], ct[3]);
  getch();
}