diff options
Diffstat (limited to 'libtommath/bn_mp_gcd.c')
-rw-r--r-- | libtommath/bn_mp_gcd.c | 101 |
1 files changed, 101 insertions, 0 deletions
diff --git a/libtommath/bn_mp_gcd.c b/libtommath/bn_mp_gcd.c new file mode 100644 index 0000000..68cfa03 --- /dev/null +++ b/libtommath/bn_mp_gcd.c @@ -0,0 +1,101 @@ +#include <tommath.h> +#ifdef BN_MP_GCD_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 + */ + +/* Greatest Common Divisor using the binary method */ +int mp_gcd (mp_int * a, mp_int * b, mp_int * c) +{ + mp_int u, v; + int k, u_lsb, v_lsb, res; + + /* either zero than gcd is the largest */ + if (mp_iszero (a) == MP_YES) { + return mp_abs (b, c); + } + if (mp_iszero (b) == MP_YES) { + return mp_abs (a, c); + } + + /* get copies of a and b we can modify */ + if ((res = mp_init_copy (&u, a)) != MP_OKAY) { + return res; + } + + if ((res = mp_init_copy (&v, b)) != MP_OKAY) { + goto LBL_U; + } + + /* must be positive for the remainder of the algorithm */ + u.sign = v.sign = MP_ZPOS; + + /* B1. Find the common power of two for u and v */ + u_lsb = mp_cnt_lsb(&u); + v_lsb = mp_cnt_lsb(&v); + k = MIN(u_lsb, v_lsb); + + if (k > 0) { + /* divide the power of two out */ + if ((res = mp_div_2d(&u, k, &u, NULL)) != MP_OKAY) { + goto LBL_V; + } + + if ((res = mp_div_2d(&v, k, &v, NULL)) != MP_OKAY) { + goto LBL_V; + } + } + + /* divide any remaining factors of two out */ + if (u_lsb != k) { + if ((res = mp_div_2d(&u, u_lsb - k, &u, NULL)) != MP_OKAY) { + goto LBL_V; + } + } + + if (v_lsb != k) { + if ((res = mp_div_2d(&v, v_lsb - k, &v, NULL)) != MP_OKAY) { + goto LBL_V; + } + } + + while (mp_iszero(&v) == 0) { + /* make sure v is the largest */ + if (mp_cmp_mag(&u, &v) == MP_GT) { + /* swap u and v to make sure v is >= u */ + mp_exch(&u, &v); + } + + /* subtract smallest from largest */ + if ((res = s_mp_sub(&v, &u, &v)) != MP_OKAY) { + goto LBL_V; + } + + /* Divide out all factors of two */ + if ((res = mp_div_2d(&v, mp_cnt_lsb(&v), &v, NULL)) != MP_OKAY) { + goto LBL_V; + } + } + + /* multiply by 2**k which we divided out at the beginning */ + if ((res = mp_mul_2d (&u, k, c)) != MP_OKAY) { + goto LBL_V; + } + c->sign = MP_ZPOS; + res = MP_OKAY; +LBL_V:mp_clear (&u); +LBL_U:mp_clear (&v); + return res; +} +#endif |