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Validate the DH parameters for correctness.

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We use sensible parameters taken from common sources, and no longer
have dynamic DH groups as an option, but it feels prudent to have
OpenSSL validate p and g at initialization time.
This commit is contained in:
Yawning Angel 2016-02-02 22:03:48 +00:00
parent f087a895d3
commit c625ab9f5a
2 changed files with 74 additions and 8 deletions
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3
changes/bug18221 Normal file
View File

@ -0,0 +1,3 @@
o Minor features (crypto):
- Validate the Diffie-Hellman hard coded parameters and ensure that
p is a safe prime, and g is suitable. Closes ticket 18221.

79
src/common/crypto.c
View File

@ -2084,6 +2084,71 @@ static BIGNUM *dh_param_p_tls = NULL;
/** Shared G parameter for our DH key exchanges. */
static BIGNUM *dh_param_g = NULL;
/** Validate a given set of Diffie-Hellman parameters. This is moderately
* computationally expensive (milliseconds), so should only be called when
* the DH parameters change. Returns 0 on success, * -1 on failure.
*/
static int
crypto_validate_dh_params(const BIGNUM *p, const BIGNUM *g)
{
DH *dh = NULL;
int ret = -1;
/* Copy into a temporary DH object. */
if (!(dh = DH_new()))
goto out;
if (!(dh->p = BN_dup(p)))
goto out;
if (!(dh->g = BN_dup(g)))
goto out;
/* Perform the validation. */
int codes = 0;
if (!DH_check(dh, &codes))
goto out;
if (BN_is_word(dh->g, DH_GENERATOR_2)) {
/* Per https://wiki.openssl.org/index.php/Diffie-Hellman_parameters
*
* OpenSSL checks the prime is congruent to 11 when g = 2; while the
* IETF's primes are congruent to 23 when g = 2.
*/
BN_ULONG residue = BN_mod_word(dh->p, 24);
if (residue == 11 || residue == 23)
codes &= ~DH_NOT_SUITABLE_GENERATOR;
}
if (codes != 0) /* Specifics on why the params suck is irrelevant. */
goto out;
/* Things are probably not evil. */
ret = 0;
out:
if (dh)
DH_free(dh);
return ret;
}
/** Set the global Diffie-Hellman generator, used for both TLS and internal
* DH stuff.
*/
static void
crypto_set_dh_generator(void)
{
BIGNUM *generator;
int r;
if (dh_param_g)
return;
generator = BN_new();
tor_assert(generator);
r = BN_set_word(generator, DH_GENERATOR);
tor_assert(r);
dh_param_g = generator;
}
/** Set the global TLS Diffie-Hellman modulus. Use the Apache mod_ssl DH
* modulus. */
void
@ -2116,6 +2181,8 @@ crypto_set_tls_dh_prime(void)
tor_assert(tls_prime);
dh_param_p_tls = tls_prime;
crypto_set_dh_generator();
tor_assert(0 == crypto_validate_dh_params(dh_param_p_tls, dh_param_g));
}
/** Initialize dh_param_p and dh_param_g if they are not already
@ -2123,18 +2190,13 @@ crypto_set_tls_dh_prime(void)
static void
init_dh_param(void)
{
BIGNUM *circuit_dh_prime, *generator;
BIGNUM *circuit_dh_prime;
int r;
if (dh_param_p && dh_param_g)
return;
circuit_dh_prime = BN_new();
generator = BN_new();
tor_assert(circuit_dh_prime && generator);
/* Set our generator for all DH parameters */
r = BN_set_word(generator, DH_GENERATOR);
tor_assert(r);
tor_assert(circuit_dh_prime);
/* This is from rfc2409, section 6.2. It's a safe prime, and
supposedly it equals:
@ -2150,7 +2212,8 @@ init_dh_param(void)
/* Set the new values as the global DH parameters. */
dh_param_p = circuit_dh_prime;
dh_param_g = generator;
crypto_set_dh_generator();
tor_assert(0 == crypto_validate_dh_params(dh_param_p, dh_param_g));
if (!dh_param_p_tls) {
crypto_set_tls_dh_prime();