--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tomsfastmath/src/exptmod/fp_exptmod.c	Wed Nov 23 18:10:20 2011 +0700
@@ -0,0 +1,276 @@
+/* TomsFastMath, a fast ISO C bignum library.
+ * 
+ * This project is meant to fill in where LibTomMath
+ * falls short.  That is speed ;-)
+ *
+ * This project is public domain and free for all purposes.
+ * 
+ * Tom St Denis, [email protected]
+ */
+#include <tfm.h>
+
+#ifdef TFM_TIMING_RESISTANT
+
+/* timing resistant montgomery ladder based exptmod 
+
+   Based on work by Marc Joye, Sung-Ming Yen, "The Montgomery Powering Ladder", Cryptographic Hardware and Embedded Systems, CHES 2002
+*/
+static int _fp_exptmod(fp_int * G, fp_int * X, fp_int * P, fp_int * Y)
+{
+  fp_int   R[2];
+  fp_digit buf, mp;
+  int      err, bitcnt, digidx, y;
+
+  /* now setup montgomery  */
+  if ((err = fp_montgomery_setup (P, &mp)) != FP_OKAY) {
+     return err;
+  }
+
+  fp_init(&R[0]);   
+  fp_init(&R[1]);   
+   
+  /* now we need R mod m */
+  fp_montgomery_calc_normalization (&R[0], P);
+
+  /* now set R[0][1] to G * R mod m */
+  if (fp_cmp_mag(P, G) != FP_GT) {
+     /* G > P so we reduce it first */
+     fp_mod(G, P, &R[1]);
+  } else {
+     fp_copy(G, &R[1]);
+  }
+  fp_mulmod (&R[1], &R[0], P, &R[1]);
+
+  /* for j = t-1 downto 0 do
+        r_!k = R0*R1; r_k = r_k^2
+  */
+  
+  /* set initial mode and bit cnt */
+  bitcnt = 1;
+  buf    = 0;
+  digidx = X->used - 1;
+
+  for (;;) {
+    /* grab next digit as required */
+    if (--bitcnt == 0) {
+      /* if digidx == -1 we are out of digits so break */
+      if (digidx == -1) {
+        break;
+      }
+      /* read next digit and reset bitcnt */
+      buf    = X->dp[digidx--];
+      bitcnt = (int)DIGIT_BIT;
+    }
+
+    /* grab the next msb from the exponent */
+    y     = (fp_digit)(buf >> (DIGIT_BIT - 1)) & 1;
+    buf <<= (fp_digit)1;
+
+    /* do ops */
+    fp_mul(&R[0], &R[1], &R[y^1]); fp_montgomery_reduce(&R[y^1], P, mp);
+    fp_sqr(&R[y], &R[y]);          fp_montgomery_reduce(&R[y], P, mp);
+  }
+
+   fp_montgomery_reduce(&R[0], P, mp);
+   fp_copy(&R[0], Y);
+   return FP_OKAY;
+}   
+
+#else
+
+/* y = g**x (mod b) 
+ * Some restrictions... x must be positive and < b
+ */
+static int _fp_exptmod(fp_int * G, fp_int * X, fp_int * P, fp_int * Y)
+{
+  fp_int   M[64], res;
+  fp_digit buf, mp;
+  int      err, bitbuf, bitcpy, bitcnt, mode, digidx, x, y, winsize;
+
+  /* find window size */
+  x = fp_count_bits (X);
+  if (x <= 21) {
+    winsize = 1;
+  } else if (x <= 36) {
+    winsize = 3;
+  } else if (x <= 140) {
+    winsize = 4;
+  } else if (x <= 450) {
+    winsize = 5;
+  } else {
+    winsize = 6;
+  } 
+
+  /* init M array */
+  memset(M, 0, sizeof(M)); 
+
+  /* now setup montgomery  */
+  if ((err = fp_montgomery_setup (P, &mp)) != FP_OKAY) {
+     return err;
+  }
+
+  /* setup result */
+  fp_init(&res);
+
+  /* create M table
+   *
+   * The M table contains powers of the input base, e.g. M[x] = G^x mod P
+   *
+   * The first half of the table is not computed though accept for M[0] and M[1]
+   */
+
+   /* now we need R mod m */
+   fp_montgomery_calc_normalization (&res, P);
+
+   /* now set M[1] to G * R mod m */
+   if (fp_cmp_mag(P, G) != FP_GT) {
+      /* G > P so we reduce it first */
+      fp_mod(G, P, &M[1]);
+   } else {
+      fp_copy(G, &M[1]);
+   }
+   fp_mulmod (&M[1], &res, P, &M[1]);
+
+  /* compute the value at M[1<<(winsize-1)] by squaring M[1] (winsize-1) times */
+  fp_copy (&M[1], &M[1 << (winsize - 1)]);
+  for (x = 0; x < (winsize - 1); x++) {
+    fp_sqr (&M[1 << (winsize - 1)], &M[1 << (winsize - 1)]);
+    fp_montgomery_reduce (&M[1 << (winsize - 1)], P, mp);
+  }
+
+  /* create upper table */
+  for (x = (1 << (winsize - 1)) + 1; x < (1 << winsize); x++) {
+    fp_mul(&M[x - 1], &M[1], &M[x]);
+    fp_montgomery_reduce(&M[x], P, mp);
+  }
+
+  /* set initial mode and bit cnt */
+  mode   = 0;
+  bitcnt = 1;
+  buf    = 0;
+  digidx = X->used - 1;
+  bitcpy = 0;
+  bitbuf = 0;
+
+  for (;;) {
+    /* grab next digit as required */
+    if (--bitcnt == 0) {
+      /* if digidx == -1 we are out of digits so break */
+      if (digidx == -1) {
+        break;
+      }
+      /* read next digit and reset bitcnt */
+      buf    = X->dp[digidx--];
+      bitcnt = (int)DIGIT_BIT;
+    }
+
+    /* grab the next msb from the exponent */
+    y     = (fp_digit)(buf >> (DIGIT_BIT - 1)) & 1;
+    buf <<= (fp_digit)1;
+
+    /* if the bit is zero and mode == 0 then we ignore it
+     * These represent the leading zero bits before the first 1 bit
+     * in the exponent.  Technically this opt is not required but it
+     * does lower the # of trivial squaring/reductions used
+     */
+    if (mode == 0 && y == 0) {
+      continue;
+    }
+
+    /* if the bit is zero and mode == 1 then we square */
+    if (mode == 1 && y == 0) {
+      fp_sqr(&res, &res);
+      fp_montgomery_reduce(&res, P, mp);
+      continue;
+    }
+
+    /* else we add it to the window */
+    bitbuf |= (y << (winsize - ++bitcpy));
+    mode    = 2;
+
+    if (bitcpy == winsize) {
+      /* ok window is filled so square as required and multiply  */
+      /* square first */
+      for (x = 0; x < winsize; x++) {
+        fp_sqr(&res, &res);
+        fp_montgomery_reduce(&res, P, mp);
+      }
+
+      /* then multiply */
+      fp_mul(&res, &M[bitbuf], &res);
+      fp_montgomery_reduce(&res, P, mp);
+
+      /* empty window and reset */
+      bitcpy = 0;
+      bitbuf = 0;
+      mode   = 1;
+    }
+  }
+
+  /* if bits remain then square/multiply */
+  if (mode == 2 && bitcpy > 0) {
+    /* square then multiply if the bit is set */
+    for (x = 0; x < bitcpy; x++) {
+      fp_sqr(&res, &res);
+      fp_montgomery_reduce(&res, P, mp);
+
+      /* get next bit of the window */
+      bitbuf <<= 1;
+      if ((bitbuf & (1 << winsize)) != 0) {
+        /* then multiply */
+        fp_mul(&res, &M[1], &res);
+        fp_montgomery_reduce(&res, P, mp);
+      }
+    }
+  }
+
+  /* fixup result if Montgomery reduction is used
+   * recall that any value in a Montgomery system is
+   * actually multiplied by R mod n.  So we have
+   * to reduce one more time to cancel out the factor
+   * of R.
+   */
+  fp_montgomery_reduce(&res, P, mp);
+
+  /* swap res with Y */
+  fp_copy (&res, Y);
+  return FP_OKAY;
+}
+
+#endif
+
+
+int fp_exptmod(fp_int * G, fp_int * X, fp_int * P, fp_int * Y)
+{
+   fp_int tmp;
+   int    err;
+   
+#ifdef TFM_CHECK
+   /* prevent overflows */
+   if (P->used > (FP_SIZE/2)) {
+      return FP_VAL;
+   }
+#endif
+
+   /* is X negative?  */
+   if (X->sign == FP_NEG) {
+      /* yes, copy G and invmod it */
+      fp_copy(G, &tmp);
+      if ((err = fp_invmod(&tmp, P, &tmp)) != FP_OKAY) {
+         return err;
+      }
+      X->sign = FP_ZPOS;
+      err =  _fp_exptmod(&tmp, X, P, Y);
+      if (X != Y) {
+         X->sign = FP_NEG;
+      }
+      return err;
+   } else {
+      /* Positive exponent so just exptmod */
+      return _fp_exptmod(G, X, P, Y);
+   }
+}
+
+/* $Source$ */
+/* $Revision$ */
+/* $Date$ */