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Limit keys to 65528 bits

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65528 bits is more than any reasonable key until we start supporting
post-quantum cryptography.

This limit is chosen to allow bit-sizes to be stored in 16 bits, with
65535 left to indicate an invalid value. It's a whole number of bytes,
which facilitates some calculations, in particular allowing a key of
exactly PSA_CRYPTO_MAX_STORAGE_SIZE to be created but not one bit
more.

As a resource usage limit, this is arguably too large, but that's out
of scope of the current commit.

Test that key import, generation and derivation reject overly large
sizes.
This commit is contained in:
Gilles Peskine 2019-07-30 17:26:54 +02:00
parent 7e0cff90b9
commit c744d99386
7 changed files with 158 additions and 14 deletions
Download patch file Download diff file Expand all files Collapse all files

16
library/psa_crypto.c
View file

@ -706,11 +706,14 @@ psa_status_t psa_import_key_into_slot( psa_key_slot_t *slot,
if( key_type_is_raw_bytes( slot->type ) )
{
/* Ensure that a bytes-to-bit conversion won't overflow. */
size_t bit_size = PSA_BYTES_TO_BITS( data_length );
/* Ensure that the bytes-to-bit conversion doesn't overflow. */
if( data_length > SIZE_MAX / 8 )
return( PSA_ERROR_NOT_SUPPORTED );
status = prepare_raw_data_slot( slot->type,
PSA_BYTES_TO_BITS( data_length ),
/* Ensure that the key is not overly large. */
if( bit_size > PSA_MAX_KEY_BITS )
return( PSA_ERROR_NOT_SUPPORTED );
status = prepare_raw_data_slot( slot->type, bit_size,
&slot->data.raw );
if( status != PSA_SUCCESS )
return( status );
@ -1470,6 +1473,13 @@ static psa_status_t psa_start_key_creation(
}
slot->type = attributes->core.type;
/* Refuse to create overly large keys.
* Note that this doesn't trigger on import if the attributes don't
* explicitly specify a size (so psa_get_key_bits returns 0), so
* psa_import_key() needs its own checks. */
if( psa_get_key_bits( attributes ) > PSA_MAX_KEY_BITS )
return( PSA_ERROR_NOT_SUPPORTED );
#if defined(MBEDTLS_PSA_CRYPTO_SE_C)
/* For a key in a secure element, we need to do three things:
* create the key file in internal storage, create the

11
library/psa_crypto_storage.h
View file

@ -35,9 +35,14 @@ extern "C" {
#include <stdint.h>
#include <string.h>
/* Limit the maximum key size to 30kB (just in case someone tries to
* inadvertently store an obscene amount of data) */
#define PSA_CRYPTO_MAX_STORAGE_SIZE ( 30 * 1024 )
/* Limit the maximum key size in storage. This should have no effect
* since the key size is limited in memory. */
#define PSA_CRYPTO_MAX_STORAGE_SIZE ( PSA_BITS_TO_BYTES( PSA_MAX_KEY_BITS ) )
/* Sanity check: a file size must fit in 32 bits. Allow a generous
* 64kB of metadata. */
#if PSA_CRYPTO_MAX_STORAGE_SIZE > 0xffff0000
#error PSA_CRYPTO_MAX_STORAGE_SIZE > 0xffff0000
#endif
/** The maximum permitted persistent slot number.
*