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Fix low-probability arithmetic error in ECC

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Fix the subtraction in fix_negative, which was incorrectly not looking
for a carry. This caused the result to be wrong when the least
significant limb of N was 0. Fix #4296.

The bug was introduced by d10e8fae9e
"Optimize fix_negative". Thanks to Philippe Antoine (catenacyber) for
reporting the bug which was found by his EC differential fuzzer.
Credit to OSS-Fuzz.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-04-03 20:21:43 +02:00
parent 349b37273e
commit ff6a32d79c
1 changed files with 10 additions and 2 deletions
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12
library/ecp_curves.c
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@ -1041,12 +1041,20 @@ void mbedtls_ecp_fix_negative( mbedtls_mpi *N, signed char c, size_t bits )
{
size_t i;
/* Set N := N - 2^bits */
--N->p[0];
/* Set N := 2^bits - 1 - N. We know that 0 <= N < 2^bits, so
* set the absolute value to 0xfff...fff - N. There is no carry
* since we're subtracting from all-bits-one. */
for( i = 0; i <= bits / 8 / sizeof( mbedtls_mpi_uint ); i++ )
{
N->p[i] = ~(mbedtls_mpi_uint)0 - N->p[i];
}
/* Add 1, taking care of the carry. */
i = 0;
do
++N->p[i];
while( N->p[i++] == 0 && i <= bits / 8 / sizeof( mbedtls_mpi_uint ) );
/* Invert the sign.
* Now N = N0 - 2^bits where N0 is the initial value of N. */
N->s = -1;
/* Add |c| * 2^bits to the absolute value. Since c and N are