diff options
Diffstat (limited to 'libtommath/bn_mp_invmod_slow.c')
| -rw-r--r-- | libtommath/bn_mp_invmod_slow.c | 171 |
1 files changed, 171 insertions, 0 deletions
diff --git a/libtommath/bn_mp_invmod_slow.c b/libtommath/bn_mp_invmod_slow.c new file mode 100644 index 0000000..c1884c0 --- /dev/null +++ b/libtommath/bn_mp_invmod_slow.c @@ -0,0 +1,171 @@ +#include <tommath.h> +#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@iahu.ca, http://math.libtomcrypt.org + */ + +/* 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_copy (a, &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 |