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Tolerate curve25519 backends where the high bit of the pk isn't ignored

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Right now, all our curve25519 backends ignore the high bit of the
public key. But possibly, others could treat the high bit of the
public key as encoding out-of-bounds values, or as something to be
preserved. This could be used to distinguish clients with different
backends, at the cost of killing a circuit.

As a workaround, let's just clear the high bit of each public key
indiscriminately before we use it. Fix for bug 8121, reported by
rransom. Bugfix on 0.2.4.8-alpha.
This commit is contained in:
Nick Mathewson 2013-02-04 12:50:01 -05:00
parent 898f2d7c27
commit 266419d244
3 changed files with 28 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
changes/bug8121 Normal file
View File

@ -0,0 +1,7 @@
o Minor features:
- Clear the high bit on curve25519 public keys before passing them to
our backend, in case we ever wind up using a backend that doesn't do
so itself. If we used such a backend, and *didn't* clear the high bit,
we could wind up in a situation where users with such backends would
be distinguishable from users without. Fix for bug 8121; bugfix on
0.2.4.8-alpha.

11
src/common/crypto_curve25519.c
View File

@ -33,13 +33,20 @@ int
curve25519_impl(uint8_t *output, const uint8_t *secret,
const uint8_t *basepoint)
{
uint8_t bp[CURVE25519_PUBKEY_LEN];
int r;
memcpy(bp, basepoint, CURVE25519_PUBKEY_LEN);
/* Clear the high bit, in case our backend foolishly looks at it. */
bp[31] &= 0x7f;
#ifdef USE_CURVE25519_DONNA
return curve25519_donna(output, secret, basepoint);
r = curve25519_donna(output, secret, bp);
#elif defined(USE_CURVE25519_NACL)
return crypto_scalarmult_curve25519(output, secret, basepoint);
r = crypto_scalarmult_curve25519(output, secret, bp);
#else
#error "No implementation of curve25519 is available."
#endif
memwipe(bp, 0, sizeof(bp));
return r;
}
/* ==============================

14
src/test/test_crypto.c
View File

@ -941,6 +941,8 @@ test_crypto_curve25519_impl(void *arg)
/* adapted from curve25519_donna, which adapted it from test-curve25519
version 20050915, by D. J. Bernstein, Public domain. */
const int randomize_high_bit = (arg != NULL);
unsigned char e1k[32];
unsigned char e2k[32];
unsigned char e1e2k[32];
@ -952,12 +954,19 @@ test_crypto_curve25519_impl(void *arg)
const int loop_max=10000;
char *mem_op_hex_tmp = NULL;
(void)arg;
for (loop = 0; loop < loop_max; ++loop) {
curve25519_impl(e1k,e1,k);
curve25519_impl(e2e1k,e2,e1k);
curve25519_impl(e2k,e2,k);
if (randomize_high_bit) {
/* We require that the high bit of the public key be ignored. So if
* we're doing this variant test, we randomize the high bit of e2k, and
* make sure that the handshake still works out the same as it would
* otherwise. */
uint8_t byte;
crypto_rand((char*)&byte, 1);
e2k[31] |= (byte & 0x80);
}
curve25519_impl(e1e2k,e1,e2k);
test_memeq(e1e2k, e2e1k, 32);
if (loop == loop_max-1) {
@ -1135,6 +1144,7 @@ struct testcase_t crypto_tests[] = {
{ "hkdf_sha256", test_crypto_hkdf_sha256, 0, NULL, NULL },
#ifdef CURVE25519_ENABLED
{ "curve25519_impl", test_crypto_curve25519_impl, 0, NULL, NULL },
{ "curve25519_impl_hibit", test_crypto_curve25519_impl, 0, NULL, (void*)"y" },
{ "curve25519_wrappers", test_crypto_curve25519_wrappers, 0, NULL, NULL },
{ "curve25519_encode", test_crypto_curve25519_encode, 0, NULL, NULL },
{ "curve25519_persist", test_crypto_curve25519_persist, 0, NULL, NULL },