diff options
| -rw-r--r-- | changes/bug18221 | 3 | ||||
| -rw-r--r-- | src/common/crypto.c | 79 |
2 files changed, 74 insertions, 8 deletions
diff --git a/changes/bug18221 b/changes/bug18221 new file mode 100644 index 0000000000..afc240422a --- /dev/null +++ b/changes/bug18221 @@ -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. diff --git a/src/common/crypto.c b/src/common/crypto.c index a42c461b14..431ff306c5 100644 --- a/src/common/crypto.c +++ b/src/common/crypto.c @@ -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(); |