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mbedtls_ecp_gen_privkey_sw: generalize to mbedtls_mpi_random

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Rename mbedtls_ecp_gen_privkey_sw to mbedtls_mpi_random since it has
no particular connection to elliptic curves beyond the fact that its
operation is defined by the deterministic ECDSA specification. This is
a generic function that generates a random MPI between 1 inclusive and
N exclusive.

Slightly generalize the function to accept a different lower bound,
which adds a negligible amount of complexity.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine
2021-03-29 21:53:58 +02:00
parent 6373fab865
commit 7967ec5d25
4 changed files with 119 additions and 99 deletions
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27
library/ecp.c
View File

@@ -3076,15 +3076,17 @@ cleanup:
#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
MBEDTLS_STATIC_TESTABLE
int mbedtls_ecp_gen_privkey_sw( const mbedtls_mpi *N, size_t n_bits,
mbedtls_mpi *d,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
int mbedtls_mpi_random( mbedtls_mpi *X,
mbedtls_mpi_sint min,
const mbedtls_mpi *N,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng )
{
/* SEC1 3.2.1: Generate d such that 1 <= n < N */
int ret = MBEDTLS_ERR_ECP_BAD_INPUT_DATA;
/* SEC1 3.2.1: Generate X such that 1 <= n < N */
int ret = MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
int count = 0;
unsigned cmp = 0;
size_t n_bits = mbedtls_mpi_bitlen( N );
size_t n_bytes = ( n_bits + 7 ) / 8;
/*
@@ -3097,8 +3099,8 @@ int mbedtls_ecp_gen_privkey_sw( const mbedtls_mpi *N, size_t n_bits,
*/
do
{
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( d, n_bytes, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( d, 8 * n_bytes - n_bits ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_fill_random( X, n_bytes, f_rng, p_rng ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_shift_r( X, 8 * n_bytes - n_bits ) );
/*
* Each try has at worst a probability 1/2 of failing (the msb has
@@ -3111,17 +3113,17 @@ int mbedtls_ecp_gen_privkey_sw( const mbedtls_mpi *N, size_t n_bits,
*/
if( ++count > 30 )
{
ret = MBEDTLS_ERR_ECP_RANDOM_FAILED;
ret = MBEDTLS_ERR_MPI_NOT_ACCEPTABLE;
goto cleanup;
}
ret = mbedtls_mpi_lt_mpi_ct( d, N, &cmp );
ret = mbedtls_mpi_lt_mpi_ct( X, N, &cmp );
if( ret != 0 )
{
goto cleanup;
}
}
while( mbedtls_mpi_cmp_int( d, 1 ) < 0 || cmp != 1 );
while( mbedtls_mpi_cmp_int( X, min ) < 0 || cmp != 1 );
cleanup:
return( ret );
@@ -3147,8 +3149,7 @@ int mbedtls_ecp_gen_privkey( const mbedtls_ecp_group *grp,
#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
if( mbedtls_ecp_get_type( grp ) == MBEDTLS_ECP_TYPE_SHORT_WEIERSTRASS )
return( mbedtls_ecp_gen_privkey_sw( &grp->N, grp->nbits, d,
f_rng, p_rng ) );
return( mbedtls_mpi_random( d, 1, &grp->N, f_rng, p_rng ) );
#endif /* MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED */
return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );

39
library/ecp_invasive.h
View File

@@ -77,25 +77,36 @@ int mbedtls_ecp_gen_privkey_mx( size_t n_bits,
#endif /* MBEDTLS_ECP_MONTGOMERY_ENABLED */
#if defined(MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED)
/** Generate a private key on a short Weierstrass curve.
/** Generate a random number uniformly in a range.
*
* This function generates a random number between \p min inclusive and
* \p N exclusive.
*
* The procedure complies with RFC 6979 §3.3 (deterministic ECDSA)
* when the RNG is a suitably parametrized instance of HMAC_DRBG.
* when the RNG is a suitably parametrized instance of HMAC_DRBG
* and \p min is \c 1.
*
* \p N The upper bound of the range.
* \p n_bits The size of \p N in bits. This value must be correct,
* otherwise the result is unpredictable.
* \param d A random number, uniformly generated in the range [1, N-1].
* \param f_rng The RNG function.
* \param p_rng The RNG context to be passed to \p f_rng.
* \note There are `N - min` possible outputs. The lower bound
* \p min can be reached, but the upper bound \p N cannot.
*
* \return \c 0 on success.
* \return \c MBEDTLS_ERR_ECP_xxx or MBEDTLS_ERR_MPI_xxx on failure.
* \param X The destination MPI. This must point to an initialized MPI.
* \param min The minimum value to return.
* It must be nonnegative.
* \param N The upper bound of the range, exclusive.
* In other words, this is one plus the maximum value to return.
* \p N must be strictly larger than \p min.
* \param f_rng The RNG function to use. This must not be \c NULL.
* \param p_rng The RNG parameter to be passed to \p f_rng.
*
* \return \c 0 if successful.
* \return #MBEDTLS_ERR_MPI_ALLOC_FAILED if a memory allocation failed.
* \return Another negative error code on failure.
*/
int mbedtls_ecp_gen_privkey_sw( const mbedtls_mpi *N, size_t n_bits,
mbedtls_mpi *d,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng );
int mbedtls_mpi_random( mbedtls_mpi *X,
mbedtls_mpi_sint min,
const mbedtls_mpi *N,
int (*f_rng)(void *, unsigned char *, size_t),
void *p_rng );
#endif /* MBEDTLS_ECP_SHORT_WEIERSTRASS_ENABLED */
#endif /* MBEDTLS_TEST_HOOKS && MBEDTLS_ECP_C */