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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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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();
}