--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/bn_mp_invmod.c	Mon May 31 18:23:46 2004 +0000
@@ -0,0 +1,174 @@
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+ *
+ * LibTomMath is a library that provides multiple-precision
+ * integer arithmetic as well as number theoretic functionality.
+ *
+ * The library was designed directly after the MPI library by
+ * Michael Fromberger but has been written from scratch with
+ * additional optimizations in place.
+ *
+ * The library is free for all purposes without any express
+ * guarantee it works.
+ *
+ * Tom St Denis, [email protected], http://math.libtomcrypt.org
+ */
+#include <tommath.h>
+
+/* hac 14.61, pp608 */
+int mp_invmod (mp_int * a, mp_int * b, mp_int * c)
+{
+  mp_int  x, y, u, v, A, B, C, D;
+  int     res;
+
+  /* b cannot be negative */
+  if (b->sign == MP_NEG || mp_iszero(b) == 1) {
+    return MP_VAL;
+  }
+
+  /* if the modulus is odd we can use a faster routine instead */
+  if (mp_isodd (b) == 1) {
+    return fast_mp_invmod (a, b, c);
+  }
+  
+  /* init temps */
+  if ((res = mp_init_multi(&x, &y, &u, &v, 
+                           &A, &B, &C, &D, NULL)) != MP_OKAY) {
+     return res;
+  }
+
+  /* x = a, y = b */
+  if ((res = mp_copy (a, &x)) != MP_OKAY) {
+    goto __ERR;
+  }
+  if ((res = mp_copy (b, &y)) != MP_OKAY) {
+    goto __ERR;
+  }
+
+  /* 2. [modified] if x,y are both even then return an error! */
+  if (mp_iseven (&x) == 1 && mp_iseven (&y) == 1) {
+    res = MP_VAL;
+    goto __ERR;
+  }
+
+  /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
+  if ((res = mp_copy (&x, &u)) != MP_OKAY) {
+    goto __ERR;
+  }
+  if ((res = mp_copy (&y, &v)) != MP_OKAY) {
+    goto __ERR;
+  }
+  mp_set (&A, 1);
+  mp_set (&D, 1);
+
+top:
+  /* 4.  while u is even do */
+  while (mp_iseven (&u) == 1) {
+    /* 4.1 u = u/2 */
+    if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
+      goto __ERR;
+    }
+    /* 4.2 if A or B is odd then */
+    if (mp_isodd (&A) == 1 || mp_isodd (&B) == 1) {
+      /* A = (A+y)/2, B = (B-x)/2 */
+      if ((res = mp_add (&A, &y, &A)) != MP_OKAY) {
+         goto __ERR;
+      }
+      if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
+         goto __ERR;
+      }
+    }
+    /* A = A/2, B = B/2 */
+    if ((res = mp_div_2 (&A, &A)) != MP_OKAY) {
+      goto __ERR;
+    }
+    if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
+      goto __ERR;
+    }
+  }
+
+  /* 5.  while v is even do */
+  while (mp_iseven (&v) == 1) {
+    /* 5.1 v = v/2 */
+    if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
+      goto __ERR;
+    }
+    /* 5.2 if C or D is odd then */
+    if (mp_isodd (&C) == 1 || mp_isodd (&D) == 1) {
+      /* C = (C+y)/2, D = (D-x)/2 */
+      if ((res = mp_add (&C, &y, &C)) != MP_OKAY) {
+         goto __ERR;
+      }
+      if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
+         goto __ERR;
+      }
+    }
+    /* C = C/2, D = D/2 */
+    if ((res = mp_div_2 (&C, &C)) != MP_OKAY) {
+      goto __ERR;
+    }
+    if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
+      goto __ERR;
+    }
+  }
+
+  /* 6.  if u >= v then */
+  if (mp_cmp (&u, &v) != MP_LT) {
+    /* u = u - v, A = A - C, B = B - D */
+    if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
+      goto __ERR;
+    }
+
+    if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) {
+      goto __ERR;
+    }
+
+    if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
+      goto __ERR;
+    }
+  } else {
+    /* v - v - u, C = C - A, D = D - B */
+    if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
+      goto __ERR;
+    }
+
+    if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) {
+      goto __ERR;
+    }
+
+    if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
+      goto __ERR;
+    }
+  }
+
+  /* if not zero goto step 4 */
+  if (mp_iszero (&u) == 0)
+    goto top;
+
+  /* now a = C, b = D, gcd == g*v */
+
+  /* if v != 1 then there is no inverse */
+  if (mp_cmp_d (&v, 1) != MP_EQ) {
+    res = MP_VAL;
+    goto __ERR;
+  }
+
+  /* if its too low */
+  while (mp_cmp_d(&C, 0) == MP_LT) {
+      if ((res = mp_add(&C, b, &C)) != MP_OKAY) {
+         goto __ERR;
+      }
+  }
+  
+  /* too big */
+  while (mp_cmp_mag(&C, b) != MP_LT) {
+      if ((res = mp_sub(&C, b, &C)) != MP_OKAY) {
+         goto __ERR;
+      }
+  }
+  
+  /* C is now the inverse */
+  mp_exch (&C, c);
+  res = MP_OKAY;
+__ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL);
+  return res;
+}