attributes

# Key attributes

struct psa_key_attributes_s typedef struct psa_key_attributes_s psa_key_attributes_t psa_key_attributes_t

The type of a structure containing key attributes.

This is an opaque structure that can represent the metadata of a key object. Metadata that can be stored in attributes includes:

- The location of the key in storage, indicated by its key identifier and its lifetime.

- The key's policy, comprising usage flags and a specification of the permitted algorithm(s).

- Information about the key itself: the key type and its size.

- Additional implementation-defined attributes.

The actual key material is not considered an attribute of a key. Key attributes do not contain information that is generally considered highly confidential.

An attribute structure works like a simple data structure where each function `psa_set_key_xxx` sets a field and the corresponding function `psa_get_key_xxx` retrieves the value of the corresponding field. However, a future version of the library may report values that are equivalent to the original one, but have a different encoding. Invalid values may be mapped to different, also invalid values.

An attribute structure may contain references to auxiliary resources, for example pointers to allocated memory or indirect references to pre-calculated values. In order to free such resources, the application must call psa_reset_key_attributes(). As an exception, calling psa_reset_key_attributes() on an attribute structure is optional if the structure has only been modified by the following functions since it was initialized or last reset with psa_reset_key_attributes():

- psa_set_key_id()

- psa_set_key_lifetime()

- psa_set_key_type()

- psa_set_key_bits()

- psa_set_key_usage_flags()

- psa_set_key_algorithm()

Before calling any function on a key attribute structure, the application must initialize it by any of the following means:

- Set the structure to all-bits-zero, for example:

      psa_key_attributes_tattributes;
      memset(&attributes,0,sizeof(attributes));

- Initialize the structure to logical zero values, for example:

      psa_key_attributes_tattributes={0};

- Initialize the structure to the initializer PSA_KEY_ATTRIBUTES_INIT, for example:

      psa_key_attributes_tattributes=PSA_KEY_ATTRIBUTES_INIT;

- Assign the result of the function psa_key_attributes_init() to the structure, for example:

      psa_key_attributes_tattributes;
      attributes=psa_key_attributes_init();

A freshly initialized attribute structure contains the following values:

- lifetime: PSA_KEY_LIFETIME_VOLATILE.

- key identifier: 0 (which is not a valid key identifier).

- type: `0` (meaning that the type is unspecified).

- key size: `0` (meaning that the size is unspecified).

- usage flags: `0` (which allows no usage except exporting a public key).

- algorithm: `0` (which allows no cryptographic usage, but allows exporting).

A typical sequence to create a key is as follows:

1. Create and initialize an attribute structure.

2. If the key is persistent, call psa_set_key_id(). Also call psa_set_key_lifetime() to place the key in a non-default location.

3. Set the key policy with psa_set_key_usage_flags() and psa_set_key_algorithm().

4. Set the key type with psa_set_key_type(). Skip this step if copying an existing key with psa_copy_key().

5. When generating a random key with psa_generate_key() or deriving a key with psa_key_derivation_output_key(), set the desired key size with psa_set_key_bits().

6. Call a key creation function: psa_import_key(), psa_generate_key(), psa_key_derivation_output_key() or psa_copy_key(). This function reads the attribute structure, creates a key with these attributes, and outputs a key identifier to the newly created key.

7. The attribute structure is now no longer necessary. You may call psa_reset_key_attributes(), although this is optional with the workflow presented here because the attributes currently defined in this specification do not require any additional resources beyond the structure itself.

A typical sequence to query a key's attributes is as follows:

1. Call psa_get_key_attributes().

2. Call `psa_get_key_xxx` functions to retrieve the attribute(s) that you are interested in.

3. Call psa_reset_key_attributes() to free any resources that may be used by the attribute structure.

Once a key has been created, it is impossible to change its attributes.

psa_key_attributes_t static psa_key_attributes_t psa_key_attributes_init (void) psa_key_attributes_init void

Return an initial value for a key attributes structure.

void static void psa_set_key_id (psa_key_attributes_t *attributes, mbedtls_svc_key_id_t key) psa_set_key_id psa_key_attributes_t * attributes mbedtls_svc_key_id_t key

Declare a key as persistent and set its key identifier.

If the attribute structure currently declares the key as volatile (which is the default content of an attribute structure), this function sets the lifetime attribute to PSA_KEY_LIFETIME_PERSISTENT.

This function does not access storage, it merely stores the given value in the structure. The persistent key will be written to storage when the attribute structure is passed to a key creation function such as psa_import_key(), psa_generate_key(), psa_key_derivation_output_key() or psa_copy_key().

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

key

The persistent identifier for the key.

void static void psa_set_key_lifetime (psa_key_attributes_t *attributes, psa_key_lifetime_t lifetime) psa_set_key_lifetime psa_key_attributes_t * attributes psa_key_lifetime_t lifetime

Set the location of a persistent key.

To make a key persistent, you must give it a persistent key identifier with psa_set_key_id(). By default, a key that has a persistent identifier is stored in the default storage area identifier by PSA_KEY_LIFETIME_PERSISTENT. Call this function to choose a storage area, or to explicitly declare the key as volatile.

This function does not access storage, it merely stores the given value in the structure. The persistent key will be written to storage when the attribute structure is passed to a key creation function such as psa_import_key(), psa_generate_key(), psa_key_derivation_output_key() or psa_copy_key().

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

lifetime

The lifetime for the key. If this is PSA_KEY_LIFETIME_VOLATILE, the key will be volatile, and the key identifier attribute is reset to 0.

mbedtls_svc_key_id_t static mbedtls_svc_key_id_t psa_get_key_id (const psa_key_attributes_t *attributes) psa_get_key_id const psa_key_attributes_t * attributes

Retrieve the key identifier from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The persistent identifier stored in the attribute structure. This value is unspecified if the attribute structure declares the key as volatile.

psa_key_lifetime_t static psa_key_lifetime_t psa_get_key_lifetime (const psa_key_attributes_t *attributes) psa_get_key_lifetime const psa_key_attributes_t * attributes

Retrieve the lifetime from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The lifetime value stored in the attribute structure.

void static void psa_set_key_usage_flags (psa_key_attributes_t *attributes, psa_key_usage_t usage_flags) psa_set_key_usage_flags psa_key_attributes_t * attributes psa_key_usage_t usage_flags

Declare usage flags for a key.

Usage flags are part of a key's usage policy. They encode what kind of operations are permitted on the key. For more details, refer to the documentation of the type psa_key_usage_t.

This function overwrites any usage flags previously set in `attributes`.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

usage_flags

The usage flags to write.

psa_key_usage_t static psa_key_usage_t psa_get_key_usage_flags (const psa_key_attributes_t *attributes) psa_get_key_usage_flags const psa_key_attributes_t * attributes

Retrieve the usage flags from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The usage flags stored in the attribute structure.

void static void psa_set_key_algorithm (psa_key_attributes_t *attributes, psa_algorithm_t alg) psa_set_key_algorithm psa_key_attributes_t * attributes psa_algorithm_t alg

Declare the permitted algorithm policy for a key.

The permitted algorithm policy of a key encodes which algorithm or algorithms are permitted to be used with this key. The following algorithm policies are supported:

- 0 does not allow any cryptographic operation with the key. The key may be used for non-cryptographic actions such as exporting (if permitted by the usage flags).

- An algorithm value permits this particular algorithm.

- An algorithm wildcard built from PSA_ALG_ANY_HASH allows the specified signature scheme with any hash algorithm.

- An algorithm built from PSA_ALG_AT_LEAST_THIS_LENGTH_MAC allows any MAC algorithm from the same base class (e.g. CMAC) which generates/verifies a MAC length greater than or equal to the length encoded in the wildcard algorithm.

- An algorithm built from PSA_ALG_AEAD_WITH_AT_LEAST_THIS_LENGTH_TAG allows any AEAD algorithm from the same base class (e.g. CCM) which generates/verifies a tag length greater than or equal to the length encoded in the wildcard algorithm.

This function overwrites any algorithm policy previously set in `attributes`.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

alg

The permitted algorithm policy to write.

psa_algorithm_t static psa_algorithm_t psa_get_key_algorithm (const psa_key_attributes_t *attributes) psa_get_key_algorithm const psa_key_attributes_t * attributes

Retrieve the algorithm policy from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The algorithm stored in the attribute structure.

void static void psa_set_key_type (psa_key_attributes_t *attributes, psa_key_type_t type) psa_set_key_type psa_key_attributes_t * attributes psa_key_type_t type

Declare the type of a key.

This function overwrites any key type previously set in `attributes`.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

type

The key type to write. If this is 0, the key type in `attributes` becomes unspecified.

void static void psa_set_key_bits (psa_key_attributes_t *attributes, size_t bits) psa_set_key_bits psa_key_attributes_t * attributes size_t bits

Declare the size of a key.

This function overwrites any key size previously set in `attributes`.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate each of its arguments exactly once.

attributes

The attribute structure to write to.

bits

The key size in bits. If this is 0, the key size in `attributes` becomes unspecified. Keys of size 0 are not supported.

psa_key_type_t static psa_key_type_t psa_get_key_type (const psa_key_attributes_t *attributes) psa_get_key_type const psa_key_attributes_t * attributes

Retrieve the key type from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The key type stored in the attribute structure.

size_t static size_t psa_get_key_bits (const psa_key_attributes_t *attributes) psa_get_key_bits const psa_key_attributes_t * attributes

Retrieve the key size from key attributes.

This function may be declared as `static` (i.e. without external linkage). This function may be provided as a function-like macro, but in this case it must evaluate its argument exactly once.

attributes

The key attribute structure to query.

The key size stored in the attribute structure, in bits.

psa_status_t psa_status_t psa_get_key_attributes (mbedtls_svc_key_id_t key, psa_key_attributes_t *attributes) psa_get_key_attributes mbedtls_svc_key_id_t key psa_key_attributes_t * attributes

Retrieve the attributes of a key.

This function first resets the attribute structure as with psa_reset_key_attributes(). It then copies the attributes of the given key into the given attribute structure.

This function may allocate memory or other resources. Once you have called this function on an attribute structure, you must call psa_reset_key_attributes() to free these resources.

key

Identifier of the key to query.

attributes

On success, the attributes of the key. On failure, equivalent to a freshly-initialized structure.

PSA_SUCCESS

PSA_ERROR_INVALID_HANDLE

PSA_ERROR_INSUFFICIENT_MEMORY

PSA_ERROR_COMMUNICATION_FAILURE

PSA_ERROR_CORRUPTION_DETECTED

PSA_ERROR_STORAGE_FAILURE

PSA_ERROR_DATA_CORRUPT

PSA_ERROR_DATA_INVALID

PSA_ERROR_BAD_STATE

The library has not been previously initialized by psa_crypto_init(). It is implementation-dependent whether a failure to initialize results in this error code.

void void psa_reset_key_attributes (psa_key_attributes_t *attributes) psa_reset_key_attributes psa_key_attributes_t * attributes

Reset a key attribute structure to a freshly initialized state.

You must initialize the attribute structure as described in the documentation of the type psa_key_attributes_t before calling this function. Once the structure has been initialized, you may call this function at any time.

This function frees any auxiliary resources that the structure may contain.

attributes

The attribute structure to reset.

void static void psa_set_key_enrollment_algorithm (psa_key_attributes_t *attributes, psa_algorithm_t alg2) psa_set_key_enrollment_algorithm psa_key_attributes_t * attributes psa_algorithm_t alg2

Declare the enrollment algorithm for a key.

An operation on a key may indifferently use the algorithm set with psa_set_key_algorithm() or with this function.

attributes

The attribute structure to write to.

alg2

A second algorithm that the key may be used for, in addition to the algorithm set with psa_set_key_algorithm().

Setting an enrollment algorithm is not recommended, because using the same key with different algorithms can allow some attacks based on arithmetic relations between different computations made with the same key, or can escalate harmless side channels into exploitable ones. Use this function only if it is necessary to support a protocol for which it has been verified that the usage of the key with multiple algorithms is safe.

psa_key_policy_s::alg2
psa_key_attributes_s::core
psa_core_key_attributes_t::policy
psa_algorithm_t static psa_algorithm_t psa_get_key_enrollment_algorithm (const psa_key_attributes_t *attributes) psa_get_key_enrollment_algorithm const psa_key_attributes_t * attributes

Retrieve the enrollment algorithm policy from key attributes.

attributes

The key attribute structure to query.

The enrollment algorithm stored in the attribute structure.

psa_key_policy_s::alg2
psa_key_attributes_s::core
psa_core_key_attributes_t::policy
psa_status_t psa_status_t psa_set_key_domain_parameters (psa_key_attributes_t *attributes, psa_key_type_t type, const uint8_t *data, size_t data_length) psa_set_key_domain_parameters psa_key_attributes_t * attributes psa_key_type_t type const uint8_t * data size_t data_length

Set domain parameters for a key.

Some key types require additional domain parameters in addition to the key type identifier and the key size. Use this function instead of psa_set_key_type() when you need to specify domain parameters.

The format for the required domain parameters varies based on the key type.

- For RSA keys (PSA_KEY_TYPE_RSA_PUBLIC_KEY or PSA_KEY_TYPE_RSA_KEY_PAIR), the domain parameter data consists of the public exponent, represented as a big-endian integer with no leading zeros. This information is used when generating an RSA key pair. When importing a key, the public exponent is read from the imported key data and the exponent recorded in the attribute structure is ignored. As an exception, the public exponent 65537 is represented by an empty byte string.

- For DSA keys (PSA_KEY_TYPE_DSA_PUBLIC_KEY or PSA_KEY_TYPE_DSA_KEY_PAIR), the `Dss-Params` format as defined by RFC 3279 2.3.2.

      Dss-Params::=SEQUENCE{
      pINTEGER,
      qINTEGER,
      gINTEGER
      }

- For Diffie-Hellman key exchange keys (PSA_KEY_TYPE_DH_PUBLIC_KEY(PSA_DH_FAMILY_CUSTOM) or PSA_KEY_TYPE_DH_KEY_PAIR(PSA_DH_FAMILY_CUSTOM)), the `DomainParameters` format as defined by RFC 3279 2.3.3.

      DomainParameters::=SEQUENCE{
      pINTEGER,--oddprime,p=jq+1
      gINTEGER,--generator,g
      qINTEGER,--factorofp-1
      jINTEGEROPTIONAL,--subgroupfactor
      validationParamsValidationParamsOPTIONAL
      }
      ValidationParams::=SEQUENCE{
      seedBITSTRING,
      pgenCounterINTEGER
      }

This function may allocate memory or other resources. Once you have called this function on an attribute structure, you must call psa_reset_key_attributes() to free these resources.

This is an experimental extension to the interface. It may change in future versions of the library.

attributes

Attribute structure where the specified domain parameters will be stored. If this function fails, the content of `attributes` is not modified.

type

Key type (a `PSA_KEY_TYPE_XXX` value).

data

Buffer containing the key domain parameters. The content of this buffer is interpreted according to `type` as described above.

data_length

Size of the `data` buffer in bytes.

PSA_SUCCESS

PSA_ERROR_INVALID_ARGUMENT

PSA_ERROR_NOT_SUPPORTED

PSA_ERROR_INSUFFICIENT_MEMORY

psa_status_t psa_status_t psa_get_key_domain_parameters (const psa_key_attributes_t *attributes, uint8_t *data, size_t data_size, size_t *data_length) psa_get_key_domain_parameters const psa_key_attributes_t * attributes uint8_t * data size_t data_size size_t * data_length

Get domain parameters for a key.

Get the domain parameters for a key with this function, if any. The format of the domain parameters written to `data` is specified in the documentation for psa_set_key_domain_parameters().

This is an experimental extension to the interface. It may change in future versions of the library.

attributes

The key attribute structure to query.

data

On success, the key domain parameters.

data_size

Size of the `data` buffer in bytes. The buffer is guaranteed to be large enough if its size in bytes is at least the value given by PSA_KEY_DOMAIN_PARAMETERS_SIZE().

data_length

On success, the number of bytes that make up the key domain parameters data.

PSA_SUCCESS

PSA_ERROR_BUFFER_TOO_SMALL

PSA_KEY_ATTRIBUTES_INIT
{ PSA_CORE_KEY_ATTRIBUTES_INIT, NULL, 0 }

This macro returns a suitable initializer for a key attribute structure of type psa_key_attributes_t.

PSA_DH_FAMILY_CUSTOM
((psa_dh_family_t) 0x7e)

Custom Diffie-Hellman group.

For keys of type PSA_KEY_TYPE_DH_PUBLIC_KEY(PSA_DH_FAMILY_CUSTOM) or PSA_KEY_TYPE_DH_KEY_PAIR(PSA_DH_FAMILY_CUSTOM), the group data comes from domain parameters set by psa_set_key_domain_parameters().

PSA_KEY_DOMAIN_PARAMETERS_SIZE
key_type
key_bits
(PSA_KEY_TYPE_IS_RSA(key_type) ? sizeof(int) : \ PSA_KEY_TYPE_IS_DH(key_type) ? PSA_DH_KEY_DOMAIN_PARAMETERS_SIZE(key_bits) : \ PSA_KEY_TYPE_IS_DSA(key_type) ? PSA_DSA_KEY_DOMAIN_PARAMETERS_SIZE(key_bits) : \ 0)

Safe output buffer size for psa_get_key_domain_parameters().

This macro returns a compile-time constant if its arguments are compile-time constants.

This function may call its arguments multiple times or zero times, so you should not pass arguments that contain side effects.

This is an experimental extension to the interface. It may change in future versions of the library.

key_type

A supported key type.

key_bits

The size of the key in bits.

If the parameters are valid and supported, return a buffer size in bytes that guarantees that psa_get_key_domain_parameters() will not fail with PSA_ERROR_BUFFER_TOO_SMALL. If the parameters are a valid combination that is not supported by the implementation, this macro shall return either a sensible size or 0. If the parameters are not valid, the return value is unspecified.

PSA_DH_KEY_DOMAIN_PARAMETERS_SIZE
key_bits
(4 + (PSA_BITS_TO_BYTES(key_bits) + 5) * 3 /*without optional parts*/)
PSA_DSA_KEY_DOMAIN_PARAMETERS_SIZE
key_bits
(4 + (PSA_BITS_TO_BYTES(key_bits) + 5) * 2 /*p, g*/ + 34 /*q*/)