psa_key_derivation_set_capacity() function

The key derivation operation object to modify.

The new capacity of the operation. It must be less or equal to the operation's current capacity.

Set up a key derivation operation. A key derivation algorithm takes some inputs and uses them to generate a byte stream in a deterministic way. This byte stream can be used to produce keys and other cryptographic material. To derive a key: -# Start with an initialized object of type #psa_key_derivation_operation_t. -# Call psa_key_derivation_setup() to select the algorithm. -# Provide the inputs for the key derivation by calling psa_key_derivation_input_bytes() or psa_key_derivation_input_key() as appropriate. Which inputs are needed, in what order, and whether they may be keys and if so of what type depends on the algorithm. -# Optionally set the operation's maximum capacity with psa_key_derivation_set_capacity(). You may do this before, in the middle of or after providing inputs. For some algorithms, this step is mandatory because the output depends on the maximum capacity. -# To derive a key, call psa_key_derivation_output_key(). To derive a byte string for a different purpose, call psa_key_derivation_output_bytes(). Successive calls to these functions use successive output bytes calculated by the key derivation algorithm. -# Clean up the key derivation operation object with psa_key_derivation_abort(). If this function returns an error, the key derivation operation object is not changed. If an error occurs at any step after a call to psa_key_derivation_setup(), the operation will need to be reset by a call to psa_key_derivation_abort(). Implementations must reject an attempt to derive a key of size 0.

Retrieve the current capacity of a key derivation operation. The capacity of a key derivation is the maximum number of bytes that it can return. When you get *N* bytes of output from a key derivation operation, this reduces its capacity by *N*.

Provide an input for key derivation or key agreement. 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 direct inputs, which is usually correct for non-secret inputs. To pass a secret input, which should be in a key object, call psa_key_derivation_input_key() instead of this function. Refer to the documentation of individual step types (`PSA_KEY_DERIVATION_INPUT_xxx` values of type ::psa_key_derivation_step_t) for more information. If this function returns an error status, the operation enters an error state and must be aborted by calling psa_key_derivation_abort().

Provide an input for key derivation in the form of a key. 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 obtains input from a key object, which is usually correct for secret inputs or for non-secret personalization strings kept in the key store. To pass a non-secret parameter which is not in the key store, call psa_key_derivation_input_bytes() instead of this function. Refer to the documentation of individual step types (`PSA_KEY_DERIVATION_INPUT_xxx` values of type ::psa_key_derivation_step_t) for more information. If this function returns an error status, the operation enters an error state and must be aborted by calling psa_key_derivation_abort().

Perform a key agreement and use the shared secret as input to a key derivation. A key agreement algorithm takes two inputs: a private key \p private_key a public key \p peer_key. The result of this function is passed as input to a key derivation. The output of this key derivation can be extracted by reading from the resulting operation to produce keys and other cryptographic material. If this function returns an error status, the operation enters an error state and must be aborted by calling psa_key_derivation_abort().

Read some data from a key derivation operation. This function calculates output bytes from a key derivation algorithm and return those bytes. If you view the key derivation's output as a stream of bytes, this function destructively reads the requested number of bytes from the stream. The operation's capacity decreases by the number of bytes read. If this function returns an error status other than #PSA_ERROR_INSUFFICIENT_DATA, the operation enters an error state and must be aborted by calling psa_key_derivation_abort().

Derive a key from an ongoing key derivation operation. This function calculates output bytes from a key derivation algorithm and uses those bytes to generate a key deterministically. The key's location, usage policy, type and size are taken from \p attributes. If you view the key derivation's output as a stream of bytes, this function destructively reads as many bytes as required from the stream. The operation's capacity decreases by the number of bytes read. If this function returns an error status other than #PSA_ERROR_INSUFFICIENT_DATA, the operation enters an error state and must be aborted by calling psa_key_derivation_abort(). How much output is produced and consumed from the operation, and how the key is derived, depends on the key type and on the key size (denoted \c bits below): - For key types for which the key is an arbitrary sequence of bytes of a given size, this function is functionally equivalent to calling #psa_key_derivation_output_bytes and passing the resulting output to #psa_import_key. However, this function has a security benefit: if the implementation provides an isolation boundary then the key material is not exposed outside the isolation boundary. As a consequence, for these key types, this function always consumes exactly (\c bits / 8) bytes from the operation. The following key types defined in this specification follow this scheme: - #PSA_KEY_TYPE_AES; - #PSA_KEY_TYPE_ARC4; - #PSA_KEY_TYPE_ARIA; - #PSA_KEY_TYPE_CAMELLIA; - #PSA_KEY_TYPE_DERIVE; - #PSA_KEY_TYPE_HMAC. - For ECC keys on a Montgomery elliptic curve (#PSA_KEY_TYPE_ECC_KEY_PAIR(\c curve) where \c curve designates a Montgomery curve), this function always draws a byte string whose length is determined by the curve, and sets the mandatory bits accordingly. That is: - Curve25519 (#PSA_ECC_FAMILY_MONTGOMERY, 255 bits): draw a 32-byte string and process it as specified in RFC 7748 §5. - Curve448 (#PSA_ECC_FAMILY_MONTGOMERY, 448 bits): draw a 56-byte string and process it as specified in RFC 7748 §5. - For key types for which the key is represented by a single sequence of \c bits bits with constraints as to which bit sequences are acceptable, this function draws a byte string of length (\c bits / 8) bytes rounded up to the nearest whole number of bytes. If the resulting byte string is acceptable, it becomes the key, otherwise the drawn bytes are discarded. This process is repeated until an acceptable byte string is drawn. The byte string drawn from the operation is interpreted as specified for the output produced by psa_export_key(). The following key types defined in this specification follow this scheme: - #PSA_KEY_TYPE_DES. Force-set the parity bits, but discard forbidden weak keys. For 2-key and 3-key triple-DES, the three keys are generated successively (for example, for 3-key triple-DES, if the first 8 bytes specify a weak key and the next 8 bytes do not, discard the first 8 bytes, use the next 8 bytes as the first key, and continue reading output from the operation to derive the other two keys). - Finite-field Diffie-Hellman keys (#PSA_KEY_TYPE_DH_KEY_PAIR(\c group) where \c group designates any Diffie-Hellman group) and ECC keys on a Weierstrass elliptic curve (#PSA_KEY_TYPE_ECC_KEY_PAIR(\c curve) where \c curve designates a Weierstrass curve). For these key types, interpret the byte string as integer in big-endian order. Discard it if it is not in the range [0, *N* - 2] where *N* is the boundary of the private key domain (the prime *p* for Diffie-Hellman, the subprime *q* for DSA, or the order of the curve's base point for ECC). Add 1 to the resulting integer and use this as the private key *x*. This method allows compliance to NIST standards, specifically the methods titled "key-pair generation by testing candidates" in NIST SP 800-56A §5.6.1.1.4 for Diffie-Hellman, in FIPS 186-4 §B.1.2 for DSA, and in NIST SP 800-56A §5.6.1.2.2 or FIPS 186-4 §B.4.2 for elliptic curve keys. - For other key types, including #PSA_KEY_TYPE_RSA_KEY_PAIR, the way in which the operation output is consumed is implementation-defined. In all cases, the data that is read is discarded from the operation. The operation's capacity is decreased by the number of bytes read. For algorithms that take an input step #PSA_KEY_DERIVATION_INPUT_SECRET, the input to that step must be provided with psa_key_derivation_input_key(). Future versions of this specification may include additional restrictions on the derived key based on the attributes and strength of the secret key.

Abort a key derivation operation. Aborting an operation frees all associated resources except for the \c operation structure itself. Once aborted, the operation object can be reused for another operation by calling psa_key_derivation_setup() again. This function may be called at any time after the operation object has been initialized as described in #psa_key_derivation_operation_t. In particular, it is valid to call psa_key_derivation_abort() twice, or to call psa_key_derivation_abort() on an operation that has not been set up.