mbed TLS v2.23.0: Key import and export

mbed TLS v2.23.0

Functions
Key import and export

## Functions
psa_status_t | psa_import_key (const psa_key_attributes_t *attributes, const uint8_t *data, size_t data_length, psa_key_handle_t *handle)
| Import a key in binary format. More...
psa_status_t | psa_export_key (psa_key_handle_t handle, uint8_t *data, size_t data_size, size_t *data_length)
| Export a key in binary format. More...
psa_status_t | psa_export_public_key (psa_key_handle_t handle, uint8_t *data, size_t data_size, size_t *data_length)
| Export a public key or the public part of a key pair in binary format. More...

## Detailed Description

## Function Documentation

## ◆ psa_export_key()

psa_status_t psa_export_key | ( | psa_key_handle_t | handle,
|  | uint8_t * | data,
|  | size_t | data_size,
|  | size_t * | data_length
| ) |  |

Export a key in binary format.

The output of this function can be passed to psa_import_key() to create an equivalent object.

If the implementation of psa_import_key() supports other formats beyond the format specified here, the output from psa_export_key() must use the representation specified here, not the original representation.

For standard key types, the output format is as follows:

- For symmetric keys (including MAC keys), the format is the raw bytes of the key.
- For DES, the key data consists of 8 bytes. The parity bits must be correct.
- For Triple-DES, the format is the concatenation of the two or three DES keys.
- For RSA key pairs (PSA_KEY_TYPE_RSA_KEY_PAIR), the format is the non-encrypted DER encoding of the representation defined by PKCS#1 (RFC 8017) as `RSAPrivateKey`, version 0.
  RSAPrivateKey ::= SEQUENCE {
  version INTEGER, -- must be 0
  modulus INTEGER, -- n
  publicExponent INTEGER, -- e
  privateExponent INTEGER, -- d
  prime1 INTEGER, -- p
  prime2 INTEGER, -- q
  exponent1 INTEGER, -- d mod (p-1)
  exponent2 INTEGER, -- d mod (q-1)
  coefficient INTEGER, -- (inverse of q) mod p
  }
- For elliptic curve key pairs (key types for which PSA_KEY_TYPE_IS_ECC_KEY_PAIR is true), the format is a representation of the private value as a `ceiling(m/8)`-byte string where `m` is the bit size associated with the curve, i.e. the bit size of the order of the curve's coordinate field. This byte string is in little-endian order for Montgomery curves (curve types `PSA_ECC_CURVE_CURVEXXX`), and in big-endian order for Weierstrass curves (curve types `PSA_ECC_CURVE_SECTXXX`, `PSA_ECC_CURVE_SECPXXX` and `PSA_ECC_CURVE_BRAINPOOL_PXXX`). This is the content of the `privateKey` field of the `ECPrivateKey` format defined by RFC 5915.
- For Diffie-Hellman key exchange key pairs (key types for which PSA_KEY_TYPE_IS_DH_KEY_PAIR is true), the format is the representation of the private key `x` as a big-endian byte string. The length of the byte string is the private key size in bytes (leading zeroes are not stripped).
- For public keys (key types for which PSA_KEY_TYPE_IS_PUBLIC_KEY is true), the format is the same as for psa_export_public_key().

The policy on the key must have the usage flag PSA_KEY_USAGE_EXPORT set.

Parameters

  | handle | Handle to the key to export.
  [out] | data | Buffer where the key data is to be written.
  | data_size | Size of the `data` buffer in bytes.
  [out] | data_length | On success, the number of bytes that make up the key data.

Return values

  PSA_SUCCESS |
  PSA_ERROR_INVALID_HANDLE |
  PSA_ERROR_NOT_PERMITTED | The key does not have the PSA_KEY_USAGE_EXPORT flag.
  PSA_ERROR_NOT_SUPPORTED |
  PSA_ERROR_BUFFER_TOO_SMALL | The size of the `data` buffer is too small. You can determine a sufficient buffer size by calling PSA_KEY_EXPORT_MAX_SIZE(`type`, `bits`) where `type` is the key type and `bits` is the key size in bits.
  PSA_ERROR_COMMUNICATION_FAILURE |
  PSA_ERROR_HARDWARE_FAILURE |
  PSA_ERROR_CORRUPTION_DETECTED |
  PSA_ERROR_STORAGE_FAILURE |
  PSA_ERROR_INSUFFICIENT_MEMORY |
  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.

## ◆ psa_export_public_key()

psa_status_t psa_export_public_key | ( | psa_key_handle_t | handle,
|  | uint8_t * | data,
|  | size_t | data_size,
|  | size_t * | data_length
| ) |  |

Export a public key or the public part of a key pair in binary format.

The output of this function can be passed to psa_import_key() to create an object that is equivalent to the public key.

This specification supports a single format for each key type. Implementations may support other formats as long as the standard format is supported. Implementations that support other formats should ensure that the formats are clearly unambiguous so as to minimize the risk that an invalid input is accidentally interpreted according to a different format.

For standard key types, the output format is as follows:

- For RSA public keys (PSA_KEY_TYPE_RSA_PUBLIC_KEY), the DER encoding of the representation defined by RFC 3279 §2.3.1 as `RSAPublicKey`.
  RSAPublicKey ::= SEQUENCE {
  modulus INTEGER, -- n
  publicExponent INTEGER } -- e
- For elliptic curve public keys (key types for which PSA_KEY_TYPE_IS_ECC_PUBLIC_KEY is true), the format is the uncompressed representation defined by SEC1 §2.3.3 as the content of an ECPoint. Let `m` be the bit size associated with the curve, i.e. the bit size of `q` for a curve over `F_q`. The representation consists of:
  - The byte 0x04;
  - `x_P` as a `ceiling(m/8)`-byte string, big-endian;
  - `y_P` as a `ceiling(m/8)`-byte string, big-endian.
- For Diffie-Hellman key exchange public keys (key types for which PSA_KEY_TYPE_IS_DH_PUBLIC_KEY is true), the format is the representation of the public key `y = g^x mod p` as a big-endian byte string. The length of the byte string is the length of the base prime `p` in bytes.

Exporting a public key object or the public part of a key pair is always permitted, regardless of the key's usage flags.

Parameters

  | handle | Handle to the key to export.
  [out] | data | Buffer where the key data is to be written.
  | data_size | Size of the `data` buffer in bytes.
  [out] | data_length | On success, the number of bytes that make up the key data.

Return values

  PSA_SUCCESS |
  PSA_ERROR_INVALID_HANDLE |
  PSA_ERROR_INVALID_ARGUMENT | The key is neither a public key nor a key pair.
  PSA_ERROR_NOT_SUPPORTED |
  PSA_ERROR_BUFFER_TOO_SMALL | The size of the `data` buffer is too small. You can determine a sufficient buffer size by calling PSA_KEY_EXPORT_MAX_SIZE(PSA_KEY_TYPE_PUBLIC_KEY_OF_KEY_PAIR(`type`), `bits`) where `type` is the key type and `bits` is the key size in bits.
  PSA_ERROR_COMMUNICATION_FAILURE |
  PSA_ERROR_HARDWARE_FAILURE |
  PSA_ERROR_CORRUPTION_DETECTED |
  PSA_ERROR_STORAGE_FAILURE |
  PSA_ERROR_INSUFFICIENT_MEMORY |
  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.

## ◆ psa_import_key()

psa_status_t psa_import_key | ( | const psa_key_attributes_t * | attributes,
|  | const uint8_t * | data,
|  | size_t | data_length,
|  | psa_key_handle_t * | handle
| ) |  |

Import a key in binary format.

This function supports any output from psa_export_key(). Refer to the documentation of psa_export_public_key() for the format of public keys and to the documentation of psa_export_key() for the format for other key types.

The key data determines the key size. The attributes may optionally specify a key size; in this case it must match the size determined from the key data. A key size of 0 in `attributes` indicates that the key size is solely determined by the key data.

Implementations must reject an attempt to import a key of size 0.

This specification supports a single format for each key type. Implementations may support other formats as long as the standard format is supported. Implementations that support other formats should ensure that the formats are clearly unambiguous so as to minimize the risk that an invalid input is accidentally interpreted according to a different format.

Parameters

  [in] | attributes | The attributes for the new key. The key size is always determined from the `data` buffer. If the key size in `attributes` is nonzero, it must be equal to the size from `data`.
  [out] | handle | On success, a handle to the newly created key. `0` on failure.
  [in] | data | Buffer containing the key data. The content of this buffer is interpreted according to the type declared in `attributes`. All implementations must support at least the format described in the documentation of psa_export_key() or psa_export_public_key() for the chosen type. Implementations may allow other formats, but should be conservative: implementations should err on the side of rejecting content if it may be erroneous (e.g. wrong type or truncated data).
  | data_length | Size of the `data` buffer in bytes.

Return values

  PSA_SUCCESS | Success. If the key is persistent, the key material and the key's metadata have been saved to persistent storage.
  PSA_ERROR_ALREADY_EXISTS | This is an attempt to create a persistent key, and there is already a persistent key with the given identifier.
  PSA_ERROR_NOT_SUPPORTED | The key type or key size is not supported, either by the implementation in general or in this particular persistent location.
  PSA_ERROR_INVALID_ARGUMENT | The key attributes, as a whole, are invalid.
  PSA_ERROR_INVALID_ARGUMENT | The key data is not correctly formatted.
  PSA_ERROR_INVALID_ARGUMENT | The size in `attributes` is nonzero and does not match the size of the key data.
  PSA_ERROR_INSUFFICIENT_MEMORY |
  PSA_ERROR_INSUFFICIENT_STORAGE |
  PSA_ERROR_COMMUNICATION_FAILURE |
  PSA_ERROR_STORAGE_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.