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Improve the rules on key derivation input types

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Use separate step types for a KDF secret and for the private key in a
key agreement.

Determine which key type is allowed from the step type, independently
of the KDF.

Forbid raw inputs for certain steps. They definitely should be
forbidden for asymmetric keys, which are structured. Also forbid them
for KDF secrets: the secrets are supposed to be keys, even if they're
unstructured.
This commit is contained in:
Gilles Peskine 2019-01-08 10:31:27 +01:00
parent b70a0fd1a5
commit 6cdfdb75a9
3 changed files with 95 additions and 30 deletions
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42
include/psa/crypto.h
View file

@ -2147,29 +2147,21 @@ psa_status_t psa_generator_abort(psa_crypto_generator_t *generator);
psa_status_t psa_key_derivation_setup(psa_crypto_generator_t *generator,
psa_algorithm_t alg);
/** Provide an input for key derivation.
/** Provide an input for key derivation or key agreement.
*
* Which inputs are required and in what order depends on the type of
* key derivation algorithm.
* Which inputs are required and in what order depends on the algorithm.
* Refer to the documentation of each key derivation or key agreement
* algorithm for information.
*
* - For HKDF (#PSA_ALG_HKDF), the following inputs are supported:
* - #PSA_KDF_STEP_SALT is the salt used in the "extract" step.
* It is optional; if omitted, the derivation uses an empty salt.
* - #PSA_KDF_STEP_SECRET is the secret key used in the "extract" step.
* It may be a key of type #PSA_KEY_TYPE_DERIVE with the
* usage flag #PSA_KEY_USAGE_DERIVE.
* - #PSA_KDF_STEP_INFO is the info string used in the "expand" step.
* You must pass #PSA_KDF_STEP_SALT before #PSA_KDF_STEP_SECRET.
* #PSA_KDF_STEP_INFO may be passed at any time before starting to
* generate output.
* This function passes direct inputs. Some inputs must be passed as keys
* using psa_key_derivation_input_key() instead of this function. Refer to
* the documentation of individual step types for information.
*
* \param[in,out] generator The generator object to use. It must
* have been set up with
* psa_key_derivation_setup() and must not
* have produced any output yet.
* \param step Which step the input data is for.
* See above for the permitted values
* depending on the algorithm.
* \param[in] data Input data to use.
* \param data_length Size of the \p data buffer in bytes.
*
@ -2177,6 +2169,8 @@ psa_status_t psa_key_derivation_setup(psa_crypto_generator_t *generator,
* Success.
* \retval #PSA_ERROR_INVALID_ARGUMENT
* \c step is not compatible with the generator's algorithm.
* \retval #PSA_ERROR_INVALID_ARGUMENT
* \c step does not allow direct inputs.
* \retval #PSA_ERROR_INSUFFICIENT_MEMORY
* \retval #PSA_ERROR_COMMUNICATION_FAILURE
* \retval #PSA_ERROR_HARDWARE_FAILURE
@ -2195,17 +2189,23 @@ psa_status_t psa_key_derivation_input_bytes(psa_crypto_generator_t *generator,
/** Provide an input for key derivation in the form of a key.
*
* See the descrition of psa_key_derivation_input_bytes() regarding
* what inputs are supported and in what order. An input step may only be
* a key if the descrition of psa_key_derivation_input_bytes() explicitly
* allows it.
* Which inputs are required and in what order depends on the algorithm.
* Refer to the documentation of each key derivation or key agreement
* algorithm for information.
*
* This function passes key inputs. Some inputs must be passed as keys
* of the appropriate type using this function, while others must be
* passed as direct inputs using psa_key_derivation_input_bytes(). Refer to
* the documentation of individual step types for information.
*
* \param[in,out] generator The generator object to use. It must
* have been set up with
* psa_key_derivation_setup() and must not
* have produced any output yet.
* \param step Which step the input data is for.
* \param handle Handle to the secret key.
* \param handle Handle to the key. It must have an
* appropriate type for \p step and must
* allow the usage #PSA_KEY_USAGE_DERIVE.
*
* \retval #PSA_SUCCESS
* Success.
@ -2214,6 +2214,8 @@ psa_status_t psa_key_derivation_input_bytes(psa_crypto_generator_t *generator,
* \retval #PSA_ERROR_NOT_PERMITTED
* \retval #PSA_ERROR_INVALID_ARGUMENT
* \c step is not compatible with the generator's algorithm.
* \retval #PSA_ERROR_INVALID_ARGUMENT
* \c step does not allow key inputs.
* \retval #PSA_ERROR_INSUFFICIENT_MEMORY
* \retval #PSA_ERROR_COMMUNICATION_FAILURE
* \retval #PSA_ERROR_HARDWARE_FAILURE

44
include/psa/crypto_values.h
View file

@ -1104,6 +1104,15 @@
*
* For example, `PSA_ALG_HKDF(PSA_ALG_SHA256)` is HKDF using HMAC-SHA-256.
*
* This key derivation algorithm uses the following inputs:
* - #PSA_KDF_STEP_SALT is the salt used in the "extract" step.
* It is optional; if omitted, the derivation uses an empty salt.
* - #PSA_KDF_STEP_SECRET is the secret key used in the "extract" step.
* - #PSA_KDF_STEP_INFO is the info string used in the "expand" step.
* You must pass #PSA_KDF_STEP_SALT before #PSA_KDF_STEP_SECRET.
* You may pass #PSA_KDF_STEP_INFO at any time after steup and before
* starting to generate output.
*
* \param hash_alg A hash algorithm (\c PSA_ALG_XXX value such that
* #PSA_ALG_IS_HASH(\p hash_alg) is true).
*
@ -1421,11 +1430,44 @@
* @{
*/
/** A secret input for key derivation.
*
* This must be a key of type #PSA_KEY_TYPE_DERIVE.
*/
#define PSA_KDF_STEP_SECRET ((psa_key_derivation_step_t)0x0101)
/** A label for key derivation.
*
* This must be a direct input.
*/
#define PSA_KDF_STEP_LABEL ((psa_key_derivation_step_t)0x0201)
/** A salt for key derivation.
*
* This must be a direct input.
*/
#define PSA_KDF_STEP_SALT ((psa_key_derivation_step_t)0x0202)
/** An information string for key derivation.
*
* This must be a direct input.
*/
#define PSA_KDF_STEP_INFO ((psa_key_derivation_step_t)0x0203)
#define PSA_KDF_STEP_PEER_KEY ((psa_key_derivation_step_t)0x0301)
/** The private key in a key agreement.
*
* This must be a key pair of the appropriate type for the key agreement
* algorithm.
*/
#define PSA_KDF_STEP_OUR_KEY ((psa_key_derivation_step_t)0x0301)
/** A label for key derivation.
*
* This may be a key pair of the appropriate type for the key agreement
* algorithm, or a direct input which is parsed as a public key in the
* same format as psa_import_key().
*/
#define PSA_KDF_STEP_PEER_KEY ((psa_key_derivation_step_t)0x0302)
/**@}*/

39
library/psa_crypto.c
View file

@ -4128,10 +4128,11 @@ static psa_status_t psa_hkdf_input( psa_hkdf_generator_t *hkdf,
}
#endif /* MBEDTLS_MD_C */
psa_status_t psa_key_derivation_input_bytes( psa_crypto_generator_t *generator,
psa_key_derivation_step_t step,
const uint8_t *data,
size_t data_length )
static psa_status_t psa_key_derivation_input_raw(
psa_crypto_generator_t *generator,
psa_key_derivation_step_t step,
const uint8_t *data,
size_t data_length )
{
psa_status_t status;
@ -4165,6 +4166,23 @@ psa_status_t psa_key_derivation_input_bytes( psa_crypto_generator_t *generator,
return( status );
}
psa_status_t psa_key_derivation_input_bytes( psa_crypto_generator_t *generator,
psa_key_derivation_step_t step,
const uint8_t *data,
size_t data_length )
{
switch( step )
{
case PSA_KDF_STEP_LABEL:
case PSA_KDF_STEP_SALT:
case PSA_KDF_STEP_INFO:
return( psa_key_derivation_input_raw( generator, step,
data, data_length ) );
default:
return( PSA_ERROR_INVALID_ARGUMENT );
}
}
psa_status_t psa_key_derivation_input_key( psa_crypto_generator_t *generator,
psa_key_derivation_step_t step,
psa_key_handle_t handle )
@ -4176,18 +4194,21 @@ psa_status_t psa_key_derivation_input_key( psa_crypto_generator_t *generator,
generator->alg );
if( status != PSA_SUCCESS )
return( status );
// TODO: for a key agreement algorithm, allow the corresponding key type and step
if( slot->type != PSA_KEY_TYPE_DERIVE )
return( PSA_ERROR_INVALID_ARGUMENT );
/* Don't allow a key to be used as an input that is usually public.
* This is debatable. It's ok from a cryptographic perspective to
* use secret material as an input that is usually public. However
* this is usually not intended, so be conservative at least for now. */
* the material should be dedicated to a particular input step,
* otherwise this may allow the key to be used in an unintended way
* and leak values derived from the key. So be conservative. */
if( step != PSA_KDF_STEP_SECRET )
return( PSA_ERROR_INVALID_ARGUMENT );
return( psa_key_derivation_input_bytes( generator,
step,
slot->data.raw.data,
slot->data.raw.bytes ) );
return( psa_key_derivation_input_raw( generator,
step,
slot->data.raw.data,
slot->data.raw.bytes ) );
}