From a0b4a14150e26ee5feed60365e335b0953b28b04 Mon Sep 17 00:00:00 2001 From: Adam Langley Date: Tue, 23 Apr 2013 12:13:51 -0400 Subject: [PATCH 07/11] exp_zero_mod_one Ensure that x**0 mod 1 = 0. --- crypto/bn/bn_exp.c | 9 ++++++++- crypto/bn/exptest.c | 45 ++++++++++++++++++++++++++++++++++++++++++++- 2 files changed, 52 insertions(+), 2 deletions(-) diff --git a/crypto/bn/bn_exp.c b/crypto/bn/bn_exp.c index 8441994..d26b0ca 100644 --- a/crypto/bn/bn_exp.c +++ b/crypto/bn/bn_exp.c @@ -980,7 +980,14 @@ int BN_mod_exp_mont_word(BIGNUM *rr, BN_ULONG a, const BIGNUM *p, bits = BN_num_bits(p); if (bits == 0) { - ret = BN_one(rr); + /* x**0 mod 1 is still zero. */ + if (BN_is_one(m)) + { + ret = 1; + BN_zero(rr); + } + else + ret = BN_one(rr); return ret; } if (a == 0) diff --git a/crypto/bn/exptest.c b/crypto/bn/exptest.c index 074a8e8..5fa02a1 100644 --- a/crypto/bn/exptest.c +++ b/crypto/bn/exptest.c @@ -71,6 +71,43 @@ static const char rnd_seed[] = "string to make the random number generator think it has entropy"; +/* test_exp_mod_zero tests that x**0 mod 1 == 0. It returns zero on success. */ +static int test_exp_mod_zero() { + BIGNUM a, p, m; + BIGNUM r; + BN_CTX *ctx = BN_CTX_new(); + int ret = 1; + + BN_init(&m); + BN_one(&m); + + BN_init(&a); + BN_one(&a); + + BN_init(&p); + BN_zero(&p); + + BN_init(&r); + BN_mod_exp(&r, &a, &p, &m, ctx); + BN_CTX_free(ctx); + + if (BN_is_zero(&r)) + ret = 0; + else + { + printf("1**0 mod 1 = "); + BN_print_fp(stdout, &r); + printf(", should be 0\n"); + } + + BN_free(&r); + BN_free(&a); + BN_free(&p); + BN_free(&m); + + return ret; +} + int main(int argc, char *argv[]) { BN_CTX *ctx; @@ -190,7 +227,13 @@ int main(int argc, char *argv[]) ERR_remove_thread_state(NULL); CRYPTO_mem_leaks(out); BIO_free(out); - printf(" done\n"); + printf("\n"); + + if (test_exp_mod_zero() != 0) + goto err; + + printf("done\n"); + EXIT(0); err: ERR_load_crypto_strings(); -- 1.8.2.1