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Diffstat (limited to 'lib/mpi/mpi-inv.c')
| -rw-r--r-- | lib/mpi/mpi-inv.c | 143 |
1 files changed, 143 insertions, 0 deletions
diff --git a/lib/mpi/mpi-inv.c b/lib/mpi/mpi-inv.c new file mode 100644 index 000000000000..61e37d18f793 --- /dev/null +++ b/lib/mpi/mpi-inv.c @@ -0,0 +1,143 @@ +/* mpi-inv.c - MPI functions + * Copyright (C) 1998, 2001, 2002, 2003 Free Software Foundation, Inc. + * + * This file is part of Libgcrypt. + * + * Libgcrypt is free software; you can redistribute it and/or modify + * it under the terms of the GNU Lesser General Public License as + * published by the Free Software Foundation; either version 2.1 of + * the License, or (at your option) any later version. + * + * Libgcrypt 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 Lesser General Public License for more details. + * + * You should have received a copy of the GNU Lesser General Public + * License along with this program; if not, see <http://www.gnu.org/licenses/>. + */ + +#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, MPI a, MPI n) +{ + /* Extended Euclid's algorithm (See TAOCP Vol II, 4.5.2, Alg X) + * modified according to Michael Penk's solution for Exercise 35 + * with further enhancement + */ + MPI u, v, u1, u2 = NULL, u3, v1, v2 = NULL, v3, t1, t2 = NULL, t3; + unsigned int k; + int sign; + int odd; + + if (!mpi_cmp_ui(a, 0)) + return 0; /* Inverse does not exists. */ + if (!mpi_cmp_ui(n, 1)) + return 0; /* Inverse does not exists. */ + + u = mpi_copy(a); + v = mpi_copy(n); + + for (k = 0; !mpi_test_bit(u, 0) && !mpi_test_bit(v, 0); k++) { + mpi_rshift(u, u, 1); + mpi_rshift(v, v, 1); + } + odd = mpi_test_bit(v, 0); + + u1 = mpi_alloc_set_ui(1); + if (!odd) + u2 = mpi_alloc_set_ui(0); + u3 = mpi_copy(u); + v1 = mpi_copy(v); + if (!odd) { + v2 = mpi_alloc(mpi_get_nlimbs(u)); + mpi_sub(v2, u1, u); /* U is used as const 1 */ + } + v3 = mpi_copy(v); + if (mpi_test_bit(u, 0)) { /* u is odd */ + t1 = mpi_alloc_set_ui(0); + if (!odd) { + t2 = mpi_alloc_set_ui(1); + t2->sign = 1; + } + t3 = mpi_copy(v); + t3->sign = !t3->sign; + goto Y4; + } else { + t1 = mpi_alloc_set_ui(1); + if (!odd) + t2 = mpi_alloc_set_ui(0); + t3 = mpi_copy(u); + } + + do { + do { + if (!odd) { + if (mpi_test_bit(t1, 0) || mpi_test_bit(t2, 0)) { + /* one is odd */ + mpi_add(t1, t1, v); + mpi_sub(t2, t2, u); + } + mpi_rshift(t1, t1, 1); + mpi_rshift(t2, t2, 1); + mpi_rshift(t3, t3, 1); + } else { + if (mpi_test_bit(t1, 0)) + mpi_add(t1, t1, v); + mpi_rshift(t1, t1, 1); + mpi_rshift(t3, t3, 1); + } +Y4: + ; + } while (!mpi_test_bit(t3, 0)); /* while t3 is even */ + + if (!t3->sign) { + mpi_set(u1, t1); + if (!odd) + mpi_set(u2, t2); + mpi_set(u3, t3); + } else { + mpi_sub(v1, v, t1); + sign = u->sign; u->sign = !u->sign; + if (!odd) + mpi_sub(v2, u, t2); + u->sign = sign; + sign = t3->sign; t3->sign = !t3->sign; + mpi_set(v3, t3); + t3->sign = sign; + } + mpi_sub(t1, u1, v1); + if (!odd) + mpi_sub(t2, u2, v2); + mpi_sub(t3, u3, v3); + if (t1->sign) { + mpi_add(t1, t1, v); + if (!odd) + mpi_sub(t2, t2, u); + } + } while (mpi_cmp_ui(t3, 0)); /* while t3 != 0 */ + /* mpi_lshift( u3, k ); */ + mpi_set(x, u1); + + 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 1; +} +EXPORT_SYMBOL_GPL(mpi_invm); |