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Use constant time memcmp when comparing HMACs in openvpn_decrypt.

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Signed-off-by: Steffan Karger <steffan.karger@fox-it.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 11d21349a4)
This commit is contained in:
Steffan Karger 2013-03-19 13:01:50 +01:00 committed by Gert Doering
parent 1be49401a3
commit d106cf240f
2 changed files with 27 additions and 1 deletions
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8
buffer.h
View File

@ -615,6 +615,10 @@ buf_read_u32 (struct buffer *buf, bool *good)
}
}
/**
* Compare src buffer contents with match.
* *NOT* constant time. Do not use when comparing HMACs.
*/
static inline bool
buf_string_match (const struct buffer *src, const void *match, int size)
{
@ -623,6 +627,10 @@ buf_string_match (const struct buffer *src, const void *match, int size)
return memcmp (BPTR (src), match, size) == 0;
}
/**
* Compare first size bytes of src buffer contents with match.
* *NOT* constant time. Do not use when comparing HMACs.
*/
static inline bool
buf_string_match_head (const struct buffer *src, const void *match, int size)
{

20
crypto.c
View File

@ -70,6 +70,24 @@
#define CRYPT_ERROR(format) \
do { msg (D_CRYPT_ERRORS, "%s: " format, error_prefix); goto error_exit; } while (false)
/**
* As memcmp(), but constant-time.
* Returns 0 when data is equal, non-zero otherwise.
*/
static int
memcmp_constant_time (const void *a, const void *b, size_t size) {
const uint8_t * a1 = a;
const uint8_t * b1 = b;
int ret = 0;
size_t i;
for (i = 0; i < size; i++) {
ret |= *a1++ ^ *b1++;
}
return ret;
}
void
openvpn_encrypt (struct buffer *buf, struct buffer work,
const struct crypto_options *opt,
@ -254,7 +272,7 @@ openvpn_decrypt (struct buffer *buf, struct buffer work,
ASSERT (hmac_len == in_hmac_len);
/* Compare locally computed HMAC with packet HMAC */
if (memcmp (local_hmac, BPTR (buf), hmac_len))
if (memcmp_constant_time (local_hmac, BPTR (buf), hmac_len))
CRYPT_ERROR ("packet HMAC authentication failed");
ASSERT (buf_advance (buf, hmac_len));