# PSKC encryption

Some of the information in PSKC files (e.g. key material) can be encrypted with either pre-shared keys, passphrase-based keys or asymmetric keys (asymmetric keys are currently unimplemented).

Embedded PSKC encryption is handled inside the `Encryption` class that defines encryption key or means of deriving keys. It is accessed from the `encryption` attribute of a `PSKC` instance:

    >>> from binascii import a2b_hex
    >>> from pskc import PSKC
    >>> pskc = PSKC('somefile.pskcxml')
    >>> pskc.encryption.key = a2b_hex('12345678901234567890123456789012')

or:

    >>> pskc.encryption.derive_key('qwerty')

Once the encryption key has been set up, any encrypted key values from the PSKC file are available transparently.

If no key or an incorrect key has been configured, upon accessing encrypted information (e.g. the `secret` attribute of a `Key` instance) a `DecryptionError` exception will be raised.

When writing out a PSKC file, encryption can be configured with the `setup_preshared_key()` or `setup_pbkdf2()` function:

    >>> from pskc import PSKC
    >>> pskc = PSKC()
    >>> pskc.encryption.setup_preshared_key(algorithm='AES256-CBC')

or:

    >>> pskc.encryption.setup_pbkdf2(password='verysecure')

## The Encryption class

class `pskc.encryption.``Encryption`
  `id`

    Optional identifier of the encryption key.

  `algorithm`

    A URI of the encryption algorithm used. See the section Supported encryption algorithms below for a list of algorithms URIs.

    Assigned values to this attribute will be converted to the canonical URI for the algorithm if it is known. For instance, the value `3DES-CBC` will automatically be converted into `http://www.w3.org/2001/04/xmlenc#tripledes-cbc`.

  `is_encrypted`

    An indicator of whether the PSKC file requires an additional pre-shared key or passphrase to decrypt the contents of the file. Will be `True` if a key or passphrase is needed, `False` otherwise.

  `key_names`

    List of names provided for the encryption key.

  `key_name`

    Since usually only one name is defined for a key but the schema allows for multiple names, this is a shortcut for accessing the first value of `key_names`. It will return `None` if no name is available.

  `key`

    The binary value of the encryption key. In the case of pre-shared keys this value should be set before trying to access encrypted information in the PSKC file.

    When using key derivation the secret key is available in this attribute after calling `derive_key()`.

  `derive_key`(password)

    Derive a key from the supplied password and information in the PSKC file (generally algorithm, salt, etc.).

    This function may raise a `KeyDerivationError` exception if key derivation fails for some reason.

  `fields`

    A list of `Key` instance field names that will be encrypted when the PSKC file is written. List values can contain `secret`, `counter`, `time_offset`, `time_interval` and `time_drift`.

  `setup_preshared_key`(...)

    Configure pre-shared key encryption when writing the file.

    Parameters:

    - key (bytes) – the encryption key to use
    - id (str) – encryption key identifier
    - algorithm (str) – encryption algorithm
    - key_length (int) – encryption key length in bytes
    - key_name (str) – a name for the key
    - key_names (list) – a number of names for the key
    - fields (list) – a list of fields to encrypt

    This is a utility function to easily set up encryption. Encryption can also be set up by manually by setting the `Encryption` properties.

    This method will generate a key if required and set the passed values. By default AES128-CBC encryption will be configured and unless a key is specified one of the correct length will be generated. If the algorithm does not provide integrity checks (e.g. CBC-mode algorithms) integrity checking in the PSKC file will be set up using `setup()`.

    By default only the `secret` property will be encrypted when writing the file.

  `setup_pbkdf2`(...)

    Configure password-based PSKC encryption when writing the file.

    Parameters:

    - password (str) – the password to use (required)
    - id (str) – encryption key identifier
    - algorithm (str) – encryption algorithm
    - key_length (int) – encryption key length in bytes
    - key_name (str) – a name for the key
    - key_names (list) – a number of names for the key
    - fields (list) – a list of fields to encrypt
    - salt (bytes) – PBKDF2 salt
    - salt_length (int) – used when generating random salt
    - iterations (int) – number of PBKDF2 iterations
    - prf (function) – PBKDF2 pseudorandom function

    Defaults for the above parameters are similar to those for `setup_preshared_key()` but the password parameter is required.

    By default 12000 iterations will be used and a random salt with the length of the to-be-generated encryption key will be used.

## Supported encryption algorithms

The following encryption algorithms are currently supported by python-pskc.

URI | Description
--- | ---
`http://www.w3.org/2001/04/xmlenc#aes128-cbc` `http://www.w3.org/2001/04/xmlenc#aes192-cbc` `http://www.w3.org/2001/04/xmlenc#aes256-cbc` | AES encryption in CBC mode with various key lengths
`http://www.w3.org/2001/04/xmlenc#kw-aes128` `http://www.w3.org/2001/04/xmlenc#kw-aes192` `http://www.w3.org/2001/04/xmlenc#kw-aes256` | AES key wrap with various key lengths
`http://www.w3.org/2001/04/xmlenc#tripledes-cbc` | Triple DES (3DES) encryption in CBC mode
`http://www.w3.org/2001/04/xmlenc#kw-tripledes` | Triple DES (3DES) key wrap