diff options
Diffstat (limited to 'lib/mpi/mpi-inv.c')
| -rw-r--r-- | lib/mpi/mpi-inv.c | 187 |
1 files changed, 187 insertions, 0 deletions
diff --git a/lib/mpi/mpi-inv.c b/lib/mpi/mpi-inv.c new file mode 100644 index 00000000000..0951f984774 --- /dev/null +++ b/lib/mpi/mpi-inv.c @@ -0,0 +1,187 @@ +/* mpi-inv.c - MPI functions + * Copyright (C) 1998, 1999, 2000, 2001 Free Software Foundation, Inc. + * + * This file is part of GnuPG. + * + * GnuPG is free software; you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation; either version 2 of the License, or + * (at your option) any later version. + * + * GnuPG is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with this program; if not, write to the Free Software + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA + */ + +#include "mpi-internal.h" + +/**************** + * Calculate the multiplicative inverse X of A mod N + * That is: Find the solution x for + * 1 = (a*x) mod n + */ +int mpi_invm(MPI x, const MPI a, const MPI n) +{ + /* Extended Euclid's algorithm (See TAOPC Vol II, 4.5.2, Alg X) + * modified according to Michael Penk's solution for Exercice 35 + * with further enhancement */ + MPI u = NULL, v = NULL; + MPI u1 = NULL, u2 = NULL, u3 = NULL; + MPI v1 = NULL, v2 = NULL, v3 = NULL; + MPI t1 = NULL, t2 = NULL, t3 = NULL; + unsigned k; + int sign; + int odd = 0; + int rc = -ENOMEM; + + if (mpi_copy(&u, a) < 0) + goto cleanup; + if (mpi_copy(&v, n) < 0) + goto cleanup; + + for (k = 0; !mpi_test_bit(u, 0) && !mpi_test_bit(v, 0); k++) { + if (mpi_rshift(u, u, 1) < 0) + goto cleanup; + if (mpi_rshift(v, v, 1) < 0) + goto cleanup; + } + odd = mpi_test_bit(v, 0); + + u1 = mpi_alloc_set_ui(1); + if (!u1) + goto cleanup; + if (!odd) { + u2 = mpi_alloc_set_ui(0); + if (!u2) + goto cleanup; + } + if (mpi_copy(&u3, u) < 0) + goto cleanup; + if (mpi_copy(&v1, v) < 0) + goto cleanup; + if (!odd) { + v2 = mpi_alloc(mpi_get_nlimbs(u)); + if (!v2) + goto cleanup; + if (mpi_sub(v2, u1, u) < 0) + goto cleanup; /* U is used as const 1 */ + } + if (mpi_copy(&v3, v) < 0) + goto cleanup; + if (mpi_test_bit(u, 0)) { /* u is odd */ + t1 = mpi_alloc_set_ui(0); + if (!t1) + goto cleanup; + if (!odd) { + t2 = mpi_alloc_set_ui(1); + if (!t2) + goto cleanup; + t2->sign = 1; + } + if (mpi_copy(&t3, v) < 0) + goto cleanup; + t3->sign = !t3->sign; + goto Y4; + } else { + t1 = mpi_alloc_set_ui(1); + if (!t1) + goto cleanup; + if (!odd) { + t2 = mpi_alloc_set_ui(0); + if (!t2) + goto cleanup; + } + if (mpi_copy(&t3, u) < 0) + goto cleanup; + } + do { + do { + if (!odd) { + if (mpi_test_bit(t1, 0) || mpi_test_bit(t2, 0)) { /* one is odd */ + if (mpi_add(t1, t1, v) < 0) + goto cleanup; + if (mpi_sub(t2, t2, u) < 0) + goto cleanup; + } + if (mpi_rshift(t1, t1, 1) < 0) + goto cleanup; + if (mpi_rshift(t2, t2, 1) < 0) + goto cleanup; + if (mpi_rshift(t3, t3, 1) < 0) + goto cleanup; + } else { + if (mpi_test_bit(t1, 0)) + if (mpi_add(t1, t1, v) < 0) + goto cleanup; + if (mpi_rshift(t1, t1, 1) < 0) + goto cleanup; + if (mpi_rshift(t3, t3, 1) < 0) + goto cleanup; + } +Y4: + ; + } while (!mpi_test_bit(t3, 0)); /* while t3 is even */ + + if (!t3->sign) { + if (mpi_set(u1, t1) < 0) + goto cleanup; + if (!odd) + if (mpi_set(u2, t2) < 0) + goto cleanup; + if (mpi_set(u3, t3) < 0) + goto cleanup; + } else { + if (mpi_sub(v1, v, t1) < 0) + goto cleanup; + sign = u->sign; + u->sign = !u->sign; + if (!odd) + if (mpi_sub(v2, u, t2) < 0) + goto cleanup; + u->sign = sign; + sign = t3->sign; + t3->sign = !t3->sign; + if (mpi_set(v3, t3) < 0) + goto cleanup; + t3->sign = sign; + } + if (mpi_sub(t1, u1, v1) < 0) + goto cleanup; + if (!odd) + if (mpi_sub(t2, u2, v2) < 0) + goto cleanup; + if (mpi_sub(t3, u3, v3) < 0) + goto cleanup; + if (t1->sign) { + if (mpi_add(t1, t1, v) < 0) + goto cleanup; + if (!odd) + if (mpi_sub(t2, t2, u) < 0) + goto cleanup; + } + } while (mpi_cmp_ui(t3, 0)); /* while t3 != 0 */ + /* mpi_lshift( u3, k ); */ + rc = mpi_set(x, u1); + +cleanup: + mpi_free(u1); + mpi_free(v1); + mpi_free(t1); + if (!odd) { + mpi_free(u2); + mpi_free(v2); + mpi_free(t2); + } + mpi_free(u3); + mpi_free(v3); + mpi_free(t3); + + mpi_free(u); + mpi_free(v); + return rc; +} |