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util: add function for constant-time memory comparison

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Add a function to check if two buffers of the same length contain the
same data, but do the comparison in a constant time with respect to the
returned value to avoid creating a timing side channel, i.e. the time
depends only on the buffer length, not on the content.

Use the gnutls_memcmp() or nettle_memeql_sec() functions if available,
otherwise use the same algorithm as nettle - bitwise ORing XORed data.
This commit is contained in:
Miroslav Lichvar
2025-04-02 15:32:05 +02:00
parent dd8738119b
commit dab98fa8da
4 changed files with 47 additions and 1 deletions
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2
configure vendored
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@@ -888,6 +888,7 @@ if [ $feat_sechash = "1" ] && [ "x$HASH_LINK" = "x" ] && [ $try_nettle = "1" ];
HASH_OBJ="hash_nettle.o"
HASH_LINK="$test_link"
MYCPPFLAGS="$MYCPPFLAGS $test_cflags"
add_def HAVE_NETTLE
add_def FEAT_SECHASH
if test_code 'CMAC in nettle' 'nettle/cmac.h' "$test_cflags" "$test_link" \
@@ -910,6 +911,7 @@ if [ $feat_sechash = "1" ] && [ "x$HASH_LINK" = "x" ] && [ $try_gnutls = "1" ];
HASH_OBJ="hash_gnutls.o"
HASH_LINK="$test_link"
MYCPPFLAGS="$MYCPPFLAGS $test_cflags"
add_def HAVE_GNUTLS
add_def FEAT_SECHASH
if test_code 'CMAC in gnutls' 'gnutls/crypto.h' "$test_cflags" "$test_link" \

16
test/unit/util.c
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@@ -32,8 +32,8 @@ handle_signal(int signal)
void
test_unit(void)
{
char buf[16], buf2[16], *s, *s2, *words[3];
struct timespec ts, ts2, ts3, ts4;
char buf[16], *s, *s2, *words[3];
NTP_int64 ntp_ts, ntp_ts2, ntp_fuzz;
NTP_int32 ntp32_ts;
struct timeval tv;
@@ -797,5 +797,19 @@ test_unit(void)
TEST_CHECK(strcmp(words[0], "a") == 0);
TEST_CHECK(strcmp(words[1], "b") == 0);
for (i = 0; i < 1000; i++) {
UTI_GetRandomBytes(buf, sizeof (buf));
memcpy(buf2, buf, sizeof (buf));
for (j = 0; j < sizeof (buf); j++)
TEST_CHECK(UTI_IsMemoryEqual(buf, buf2, j));
for (j = 0; j < 8 * sizeof (buf); j++) {
buf2[j / 8] ^= 1U << j % 8;
TEST_CHECK(!UTI_IsMemoryEqual(buf, buf2, sizeof (buf)));
buf2[j / 8] ^= 1U << j % 8;
TEST_CHECK(UTI_IsMemoryEqual(buf, buf2, sizeof (buf)));
}
}
HSH_Finalise();
}

25
util.c
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@@ -29,6 +29,12 @@
#include "sysincl.h"
#if defined(HAVE_NETTLE)
#include <nettle/memops.h>
#elif defined(HAVE_GNUTLS)
#include <gnutls/gnutls.h>
#endif
#include "logging.h"
#include "memory.h"
#include "util.h"
@@ -1648,3 +1654,22 @@ UTI_SplitString(char *string, char **words, int max_saved_words)
return i;
}
/* ================================================== */
int
UTI_IsMemoryEqual(const void *s1, const void *s2, unsigned int len)
{
#if defined(HAVE_NETTLE)
return nettle_memeql_sec(s1, s2, len);
#elif defined(HAVE_GNUTLS)
return gnutls_memcmp(s1, s2, len) == 0;
#else
unsigned int i, x;
for (i = 0, x = 0; i < len; i++)
x |= ((const unsigned char *)s1)[i] ^ ((const unsigned char *)s2)[i];
return x == 0;
#endif
}

5
util.h
View File

@@ -257,6 +257,11 @@ extern unsigned int UTI_HexToBytes(const char *hex, void *buf, unsigned int len)
number of pointers to the words. */
extern int UTI_SplitString(char *string, char **words, int max_saved_words);
/* Check if two buffers of the same length contain the same data, but do the
comparison in constant time with respect to the returned value to avoid
creating a timing side channel */
extern int UTI_IsMemoryEqual(const void *s1, const void *s2, unsigned int len);
/* Macros to get maximum and minimum of two values */
#ifdef MAX
#undef MAX