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mbedtls_ecp_gen_privkey_mx: simplify the size calculation logic

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mbedtls_ecp_gen_privkey_mx generates a random number with a certain
top bit set. Depending on the size, it would either generate a number
with that top bit being random, then forcibly set the top bit to
1 (when high_bit is not a multiple of 8); or generate a number with
that top bit being 0, then set the top bit to 1 (when high_bit is a
multiple of 8). Change it to always generate the top bit randomly
first.

This doesn't make any difference in practice: the probability
distribution is the same either way, and no supported or plausible
curve has a size of the form 8n+1 anyway. But it slightly simplifies
reasoning about the behavior of this function.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-03-24 12:46:46 +01:00
parent 4f7767445b
commit eadf31d56a
1 changed files with 7 additions and 4 deletions
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11
library/ecp.c
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@ -3048,13 +3048,16 @@ int mbedtls_ecp_gen_privkey_mx( size_t high_bit,
void *p_rng )
{
int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
size_t n_bytes = ( high_bit + 7 ) / 8;
size_t n_random_bytes = high_bit / 8 + 1;
/* [Curve25519] page 5 */
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_bytes, f_rng, p_rng ) );
/* Generate a (high_bit+1)-bit random number by generating just enough
* random bytes, then shifting out extra bits from the top (necessary
* when (high_bit+1) is not a multiple of 8). */
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_random_bytes,
f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_random_bytes - high_bit - 1 ) );
/* Make sure the most significant bit is exactly at high_bit */
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_bytes - high_bit - 1 ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_set_bit( d, high_bit, 1 ) );
/* Make sure the last two bits are unset for Curve448, three bits for