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[or-cvs] Backport cleaned and improved DH key checking patch
Update of /home/or/cvsroot/tor/src/common
In directory moria:/tmp/cvs-serv32256/src/common
Modified Files:
Tag: tor-0_1_0-patches
crypto.c
Log Message:
Backport cleaned and improved DH key checking patch
Index: crypto.c
===================================================================
RCS file: /home/or/cvsroot/tor/src/common/crypto.c,v
retrieving revision 1.137.2.5
retrieving revision 1.137.2.6
diff -u -d -r1.137.2.5 -r1.137.2.6
--- crypto.c 5 Aug 2005 19:01:04 -0000 1.137.2.5
+++ crypto.c 8 Aug 2005 17:30:49 -0000 1.137.2.6
@@ -107,8 +107,8 @@
EVP_PKEY *_crypto_pk_env_get_evp_pkey(crypto_pk_env_t *env, int private);
DH *_crypto_dh_env_get_dh(crypto_dh_env_t *dh);
-static int tor_check_bignum(BIGNUM *bn);
static int setup_openssl_threading(void);
+static int tor_check_dh_key(BIGNUM *bn);
/** Return the number of bytes added by padding method <b>padding</b>.
*/
@@ -1224,16 +1224,12 @@
static BIGNUM *dh_param_p = NULL;
/** Shared G parameter for our DH key exchanges. */
static BIGNUM *dh_param_g = NULL;
-#define N_XX_GX 10
-static BIGNUM *dh_gx_xx[N_XX_GX];
/** Initialize dh_param_p and dh_param_g if they are not already
* set. */
static void init_dh_param(void) {
- static int xx[] = { 0, 1, 2, -1, -2 };
BIGNUM *p, *g;
- BN_CTX *ctx;
- int r, i;
+ int r;
if (dh_param_p && dh_param_g)
return;
@@ -1275,23 +1271,6 @@
tor_assert(r);
dh_param_p = p;
dh_param_g = g;
-
- ctx = BN_CTX_new();
- for (i=0; i<5; ++i) {
- BIGNUM *x = BN_new(), *g_x = BN_new();
- char *x_s, *g_x_s;
- BN_copy(x, dh_param_p);
- if (xx[i]<0) BN_sub_word(x,-xx[i]); else BN_set_word(x,xx[i]);
- BN_mod_exp(g_x, dh_param_g, x, dh_param_p, ctx);
- x_s = BN_bn2hex(x);
- g_x_s = BN_bn2hex(g_x);
- dh_gx_xx[i*2]=x;
- dh_gx_xx[i*2+1]=g_x;
- log_fn(LOG_DEBUG, "%d,%d <- %s, %s", i*2, i*2+1, x_s, g_x_s);
- OPENSSL_free(x_s);
- OPENSSL_free(g_x_s);
- }
- BN_CTX_free(ctx);
}
/** Allocate and return a new DH object for a key exchange.
@@ -1335,10 +1314,19 @@
*/
int crypto_dh_generate_public(crypto_dh_env_t *dh)
{
+ again:
if (!DH_generate_key(dh->dh)) {
crypto_log_errors(LOG_WARN, "generating DH key");
return -1;
}
+ if (tor_check_dh_key(dh->dh->pub_key)<0) {
+ log_fn(LOG_WARN, "Weird! Our own DH key was invalid. I guess once-in-the-universe chances really do happen. Trying again.");
+ /* Free and clear the keys, so openssl will actually try again. */
+ BN_free(dh->dh->pub_key);
+ BN_free(dh->dh->priv_key);
+ dh->dh->pub_key = dh->dh->priv_key = NULL;
+ goto again;
+ }
return 0;
}
@@ -1367,31 +1355,62 @@
return 0;
}
-/** DOCDOC later; 0 on ok, -1 on bad. */
+/** Check for bad diffie-hellman public keys (g^x). Return 0 if the key is
+ * okay, or -1 if it's bad.
+ */
static int
-tor_check_bignum(BIGNUM *bn)
+tor_check_dh_key(BIGNUM *bn)
{
- int i;
+ /* There are about 2^116 ways to have a 1024-bit key with <= 16 bits set,
+ * and similarly for <= 16 bits unset. This is negligible compared to the
+ * 2^1024 entry keyspace. */
+#define MIN_DIFFERING_BITS 16
+ /* This covers another 2^25 keys, which is still negligible. */
+#define MIN_DIST_FROM_EDGE (1<<24)
+ int i, n_bits, n_set;
+ BIGNUM *x = NULL;
+ char *s;
tor_assert(bn);
+ x = BN_new();
if (!dh_param_p)
init_dh_param();
if (bn->neg) {
- log_fn(LOG_WARN, "bn<0");
+ log_fn(LOG_WARN, "Rejecting DH key < 0");
return -1;
}
- if (BN_cmp(bn,dh_param_p)>=0) {
- log_fn(LOG_WARN, "bn>=p");
+ if (BN_cmp(bn, dh_param_p)>=0){
+ log_fn(LOG_WARN, "Rejecting DH key >= p");
return -1;
}
- for (i=0; i < N_XX_GX; ++i) {
- if (!BN_cmp(bn, dh_gx_xx[i])) {
- char *which = BN_bn2hex(dh_gx_xx[i]);
- log_fn(LOG_WARN, "Tell Nick and Roger: bn #%d [%s]",i,which);
- OPENSSL_free(which);
- return -1;
- }
+ n_bits = BN_num_bits(bn);
+ n_set = 0;
+ for (i=0; i <= n_bits; ++i) {
+ if (BN_is_bit_set(bn, i))
+ ++n_set;
+ }
+ if (n_set < MIN_DIFFERING_BITS || n_set >= n_bits-MIN_DIFFERING_BITS) {
+ log_fn(LOG_WARN, "Too few/many bits in DH key (%d)", n_set);
+ goto err;
+ }
+ BN_set_word(x, MIN_DIST_FROM_EDGE);
+ if (BN_cmp(bn,x)<=0) {
+ log_fn(LOG_WARN, "DH key is too close to 0");
+ goto err;
}
+ BN_copy(x,dh_param_p);
+ BN_sub_word(x, MIN_DIST_FROM_EDGE);
+ if (BN_cmp(bn,x)>=0) {
+ log_fn(LOG_WARN, "DH key is too close to p");
+ goto err;
+ }
+ BN_free(x);
return 0;
+ err:
+ BN_free(x);
+ s = BN_bn2hex(bn);
+ log_fn(LOG_WARN, "Rejecting invalid DH key [%s]", s);
+ OPENSSL_free(s);
+ return -1;
}
#undef MIN
@@ -1421,7 +1440,7 @@
if (!(pubkey_bn = BN_bin2bn((const unsigned char*)pubkey, pubkey_len, NULL)))
goto error;
- if (tor_check_bignum(pubkey_bn)<0) {
+ if (tor_check_dh_key(pubkey_bn)<0) {
/* Check for invalid public keys. */
log_fn(LOG_WARN,"Rejected invalid g^x");
goto error;