Key attributes

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.
• Implementations may define additional 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 can be 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, implementations may report values that are equivalent to the original one, but have a different encoding. For example, an implementation may use a more compact representation for types where many bit-patterns are invalid or not supported, and store all values that it does not support as a special marker value. In such an implementation, after setting an invalid value, the corresponding get function returns an invalid value which may not be the one that was originally stored.

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

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

     psa_key_attributes_t attributes = {0};
  

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

     psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
  

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

     psa_key_attributes_t attributes;
     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 handle 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.

Definition at line 319 of file crypto_types.h.

Register a key that is already present in a secure element.

The key must be located in a secure element designated by the lifetime field in attributes, in the slot set with psa_set_key_slot_number() in the attribute structure. This function makes the key available through the key identifier specified in attributes.

Parameters:

[in]

attributes

The attributes of the existing key.

Return values:

PSA_SUCCESS	The key was successfully registered. Note that depending on the design of the driver, this may or may not guarantee that a key actually exists in the designated slot and is compatible with the specified attributes.
PSA_ERROR_ALREADY_EXISTS	There is already a key with the identifier specified in attributes.
PSA_ERROR_NOT_SUPPORTED	The secure element driver for the specified lifetime does not support registering a key.
PSA_ERROR_INVALID_ARGUMENT	attributes specifies a lifetime which is not located in a secure element.
PSA_ERROR_INVALID_ARGUMENT	No slot number is specified in attributes, or the specified slot number is not valid.
PSA_ERROR_NOT_PERMITTED	The caller is not authorized to register the specified key slot.
PSA_ERROR_INSUFFICIENT_MEMORY
PSA_ERROR_COMMUNICATION_FAILURE
PSA_ERROR_HARDWARE_FAILURE
PSA_ERROR_CORRUPTION_DETECTED
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.

Definition at line 1889 of file psa_crypto.c.

Remove the slot number attribute from a key attribute structure.

This function undoes the action of psa_set_key_slot_number().

Parameters:

[out]

attributes

The attribute structure to write to.

Definition at line 165 of file crypto_extra.h.

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

The algorithm stored in the attribute structure.

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.

Note:

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.

Parameters:

[in]	handle	Handle to the key to query.
[in,out]	attributes	On success, the attributes of the key. On failure, equivalent to a freshly-initialized structure.

Return values:

PSA_SUCCESS
PSA_ERROR_INVALID_HANDLE
PSA_ERROR_INSUFFICIENT_MEMORY
PSA_ERROR_COMMUNICATION_FAILURE
PSA_ERROR_CORRUPTION_DETECTED
PSA_ERROR_STORAGE_FAILURE
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.

Definition at line 1186 of file psa_crypto.c.

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

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

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().

Note:

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

Parameters:

[in]	attributes	The key attribute structure to query.
[out]	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().
[out]	data_length	On success, the number of bytes that make up the key domain parameters data.

Return values:

PSA_SUCCESS
PSA_ERROR_BUFFER_TOO_SMALL

Definition at line 1129 of file psa_crypto.c.

Retrieve the enrollment algorithm policy from key attributes.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

The enrollment algorithm stored in the attribute structure.

Definition at line 94 of file crypto_extra.h.

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

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

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

The lifetime value stored in the attribute structure.

Retrieve the slot number where a key is stored.

A slot number is only defined for keys that are stored in a secure element.

This information is only useful if the secure element is not entirely managed through the PSA Cryptography API. It is up to the secure element driver to decide how PSA slot numbers map to any other interface that the secure element may have.

Parameters:

[in]	attributes	The key attribute structure to query.
[out]	slot_number	On success, the slot number containing the key.

Return values:

PSA_SUCCESS	The key is located in a secure element, and *slot_number indicates the slot number that contains it.
PSA_ERROR_NOT_PERMITTED	The caller is not permitted to query the slot number. Mbed Crypto currently does not return this error.
PSA_ERROR_INVALID_ARGUMENT	The key is not located in a secure element.

Definition at line 1234 of file psa_crypto.c.

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

The key type stored in the attribute structure.

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.

Parameters:

[in]

attributes

The key attribute structure to query.

Returns:

The usage flags stored in the attribute structure.

Return an initial value for a key attributes structure.

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.

Parameters:

[in,out]

attributes

The attribute structure to reset.

Definition at line 1093 of file psa_crypto.c.

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.

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.

Parameters:

[out]	attributes	The attribute structure to write to.
	alg	The permitted algorithm policy to write.

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.

Parameters:

[out]	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.

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-Parms` format as defined by RFC 3279 2.3.2. ``` Dss-Parms ::= SEQUENCE { p INTEGER, q INTEGER, g INTEGER } ```
• For Diffie-Hellman key exchange keys (PSA_KEY_TYPE_DH_PUBLIC_KEY(PSA_DH_GROUP_CUSTOM) or PSA_KEY_TYPE_DH_KEY_PAIR(PSA_DH_GROUP_CUSTOM)), the `DomainParameters` format as defined by RFC 3279 2.3.3. ``` DomainParameters ::= SEQUENCE { p INTEGER, -- odd prime, p=jq +1 g INTEGER, -- generator, g q INTEGER, -- factor of p-1 j INTEGER OPTIONAL, -- subgroup factor validationParms ValidationParms OPTIONAL } ValidationParms ::= SEQUENCE { seed BIT STRING, pgenCounter INTEGER } ```

Note:

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.

Parameters:

[in,out]	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).
[in]	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.

Return values:

PSA_SUCCESS
PSA_ERROR_INVALID_ARGUMENT
PSA_ERROR_NOT_SUPPORTED
PSA_ERROR_INSUFFICIENT_MEMORY

Definition at line 1099 of file psa_crypto.c.

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.

Parameters:

[out]	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().

Warning:

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.

Definition at line 81 of file crypto_extra.h.

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.

Parameters:

[out]	attributes	The attribute structure to write to.
	id	The persistent identifier for the key.

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.

Parameters:

[out]	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.

Choose the slot number where a key is stored.

This function declares a slot number in the specified attribute structure.

A slot number is only meaningful for keys that are stored in a secure element. It is up to the secure element driver to decide how PSA slot numbers map to any other interface that the secure element may have.

Note:

Setting a slot number in key attributes for a key creation can cause the following errors when creating the key:

• PSA_ERROR_NOT_SUPPORTED if the selected secure element does not support choosing a specific slot number.
• PSA_ERROR_NOT_PERMITTED if the caller is not permitted to choose slot numbers in general or to choose this specific slot.
• PSA_ERROR_INVALID_ARGUMENT if the chosen slot number is not valid in general or not valid for this specific key.
• PSA_ERROR_ALREADY_EXISTS if there is already a key in the selected slot.

Parameters:

[out]	attributes	The attribute structure to write to.
	slot_number	The slot number to set.

Definition at line 151 of file crypto_extra.h.

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.

Parameters:

[out]	attributes	The attribute structure to write to.
	type	The key type to write. If this is 0, the key type in attributes becomes unspecified.

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.

Parameters:

[out]	attributes	The attribute structure to write to.
	usage_flags	The usage flags to write.