[Hardware] RC5 algorithm

[Martin Klingensmith]

I'm using the test code here:
<http://www.rsasecurity.com/rsalabs/node.asp?id=2105>
But I can't get it to match the output. Seeing as how it's an encryption
algorithm and all, it's pretty hard to tell what's wrong when I don't
have a working reference (and the output would be completely different
anyway)
I think I'm having issues with endianness. The RC5 algorithm implies
little-endian. I'm confused regarding the order of the key bytes and the
plaintext bytes (thus I cannot get the RSA test to match my code).
This is the modified code if anyone doesn't mind looking at it:
---

/* RC5REF.C -- Reference implementation of RC5-32/12/16 in C.        */
/* Copyright (C) 1995 RSA Data Security, Inc.                        */
#include <stdio.h>
typedef unsigned long int WORD; /* Should be 32-bit = 4 bytes        */
#define w        32             /* word size in bits                 */
#define r        12             /* number of rounds                  */
#define b        9             /* number of bytes in key            */
#define c         4             /* number  words in key = ceil(8*b/w)*/
#define t        26             /* size of table S = 2*(r+1) words   */
WORD S[t];                      /* expanded key table                */
WORD P = 0xb7e15163, Q = 0x9e3779b9;  /* magic constants             */
/* Rotation operators. x must be unsigned, to get logical right shift*/
#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)))))

void RC5_ENCRYPT(WORD *pt, WORD *ct);
void RC5_DECRYPT(WORD *ct, WORD *pt);
void RC5_SETUP(unsigned char *K);

int main()
{
    WORD i, j, pt1[2], pt2[2], ct[2] = {0,0};
    unsigned char key[b];

      // plaintext: "The Unkn"
      pt1[0]=0x54686520;
      pt1[1]=0x756e6b6e;

      key[8] = 0x85; // c9 0c 03 53 c0 d4 e1 fe 85
      key[7] = 0xfe;
      key[6] = 0xe1;
      key[5] = 0xd4;
      key[4] = 0xc0;
      key[3] = 0x53;
      key[2] = 0x03;
      key[1] = 0x0c;
      key[0] = 0xc9;

      /* Setup, encrypt, and decrypt */
      RC5_SETUP(key);
      RC5_ENCRYPT(pt1,ct);
      RC5_DECRYPT(ct,pt2);
      /* Print out results, checking for decryption failure */

      printf("key = ",i);
      for (j=0; j<b; j++) printf("%.2X ",key[j]);
      printf("\n   plaintext %.8lX %.8lX  --->  ciphertext %.8lX %.8lX  \n",
             pt1[0], pt1[1], ct[0], ct[1]);
      if (pt1[0]!=pt2[0] || pt1[1]!=pt2[1])
        printf("Decryption Error!");

  return 0;
}

void RC5_ENCRYPT(WORD *pt, WORD *ct) /* 2 WORD input pt/output ct    */
{
    WORD i, A=pt[0]+S[0], B=pt[1]+S[1];
    for (i=1; i<=r; i++) {
        A = ROTL(A^B,B)+S[2*i];
        B = ROTL(B^A,A)+S[2*i+1];
    }
    ct[0] = A; ct[1] = B;
}

void RC5_DECRYPT(WORD *ct, WORD *pt) /* 2 WORD input ct/output pt    */
{
    WORD i, B=ct[1], A=ct[0];
    for (i=r; i>0; i--) {
        B = ROTR(B-S[2*i+1],A)^A;
        A = ROTR(A-S[2*i],B)^B;
    }
    pt[1] = B-S[1]; pt[0] = A-S[0];
}

void RC5_SETUP(unsigned char *K) /* secret input key K[0...b-1]      */
{
    WORD i, j, k, u=w/8, A, B, L[c];
    /* Initialize L, then S, then mix key into S */
    for (i=b-1,L[c-1]=0; i!=-1; i--)
        L[i/u] = (L[i/u]<<8)+K[i];
    for (S[0]=P,i=1; i<t; i++)
        S[i] = S[i-1]+Q;
    for (A=B=i=j=k=0; k<3*t; k++,i=(i+1)%t,j=(j+1)%c)   /* 3*t > 3*c */
    {
        A = S[i] = ROTL(S[i]+(A+B),3);
        B = L[j] = ROTL(L[j]+(A+B),(A+B));
    }
}



-- 
---
Martin Klingensmith
nnytech.net
infoarchive.net