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 #include #ifdef BN_MP_INVMOD_SLOW_C /* 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, tomstdenis@gmail.com, http://math.libtomcrypt.com */ /* hac 14.61, pp608 */ int mp_invmod_slow (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; } /* 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_mod(a, b, &x)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_copy (b, &y)) != MP_OKAY) { goto LBL_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 LBL_ERR; } /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ if ((res = mp_copy (&x, &u)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_copy (&y, &v)) != MP_OKAY) { goto LBL_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 LBL_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 LBL_ERR; } if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) { goto LBL_ERR; } } /* A = A/2, B = B/2 */ if ((res = mp_div_2 (&A, &A)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_div_2 (&B, &B)) != MP_OKAY) { goto LBL_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 LBL_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 LBL_ERR; } if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) { goto LBL_ERR; } } /* C = C/2, D = D/2 */ if ((res = mp_div_2 (&C, &C)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_div_2 (&D, &D)) != MP_OKAY) { goto LBL_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 LBL_ERR; } if ((res = mp_sub (&A, &C, &A)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) { goto LBL_ERR; } } else { /* v - v - u, C = C - A, D = D - B */ if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_sub (&C, &A, &C)) != MP_OKAY) { goto LBL_ERR; } if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) { goto LBL_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 LBL_ERR; } /* if its too low */ while (mp_cmp_d(&C, 0) == MP_LT) { if ((res = mp_add(&C, b, &C)) != MP_OKAY) { goto LBL_ERR; } } /* too big */ while (mp_cmp_mag(&C, b) != MP_LT) { if ((res = mp_sub(&C, b, &C)) != MP_OKAY) { goto LBL_ERR; } } /* C is now the inverse */ mp_exch (&C, c); res = MP_OKAY; LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &A, &B, &C, &D, NULL); return res; } #endif /* \$Source: /cvs/libtom/libtommath/bn_mp_invmod_slow.c,v \$ */ /* \$Revision: 1.3 \$ */ /* \$Date: 2006/03/31 14:18:44 \$ */