Basic usage

The pskc module implements a simple and efficient API for parsing and creating PSKC files. The PSKC class is used to access the file as a whole which provides access to a list of Device and Key instances which contain most of the useful information of the PSKC file.

Reading a PSKC file

Importing data from a PSKC file can be done by instantiating the PSKC class with a file name argument:

>>> from pskc import PSKC
>>> pskc = PSKC('somefile.pskcxml')
>>> pskc.version
'1.0'

The keys attribute contains a list of keys in the PSKC file. Key instances have a number of attributes that provide information on the transmitted keys:

>>> key = pskc.keys[0]
>>> key.id
'some-id'
>>> key.algorithm
'urn:ietf:params:xml:ns:keyprov:pskc:hotp'
>>> key.secret
'SOME_SECRET_VALUE'

Attribute values will be None if it the value is not present in the PSKC file.

The secret, counter, time_offset, time_interval or time_drift attributes may be stored in encrypted form in the PSKC file. Decryption of these properties is done when they are accessed. If decryption is unsuccessful a DecryptionError exception is raised. See PSKC encryption for more information.

Writing a PSKC file

Creating a PSKC file can be done by creating a PSKC instance, adding keys with add_key() and writing the result:

>>> from pskc import PSKC
>>> pskc = PSKC()
>>> key = pskc.add_key(
...     id='456', secret='987654321', manufacturer='Manufacturer',
...     algorithm = 'urn:ietf:params:xml:ns:keyprov:pskc:hotp')
>>> pskc.write('output.pskcxml')

By default an unencrypted PSKC file will be created but an encryption can be configured using the setup_preshared_key() or setup_pbkdf2() function.

The PSKC class

class pskc.PSKC([filename])

The PSKC class is used as a wrapper to access information from a PSKC file.

The filename argument can be either the name of a file or a file-like object. The whole file is parsed in one go. If parsing the PSKC file fails, a ParseError exception is raised. If no argument is provided, an instance without any keys is created.

Instances of this class provide the following attributes and functions:

version

The PSKC format version used. Only version 1.0 is currently specified in RFC 6030 and supported.

id

A unique identifier for the container.

devices

A list of Device instances that represent the key containers within the PSKC file.

keys

A list of Key instances that represent the keys within the PSKC file.

encryption

Encryption instance that handles PSKC file encryption. See PSKC encryption for more information.

mac

MAC instance for handling integrity checking. See Integrity checking for more information.

add_device([**kwargs])

Add a new key package to the PSKC instance. The keyword arguments may refer to any attributes of the Device class with which the new device is initialised.

add_key([**kwargs])

Add a new key to the PSKC instance. The keyword arguments may refer to any attributes of the Key or Device class with which the new key is initialised.

write(filename)

Write the PSKC object to the provided file. The filename argument can be either the name of a file or a file-like object.

The Key class

class pskc.key.Key

Instances of this class provide the following attributes and functions:

id

A unique identifier for the key. If there are multiple interactions with the same key in multiple instances of PSKC files the id is supposed to remain the same.

algorithm

A URI that identifies the PSKC algorithm profile. The algorithm profile associates specific semantics to the key. Some known profiles are:

URI Purpose
urn:ietf:params:xml:ns:keyprov:pskc:pin Symmetric static credential comparison
urn:ietf:params:xml:ns:keyprov:pskc:hotp OATH event-based OTP
urn:ietf:params:xml:ns:keyprov:pskc#totp urn:ietf:params:xml:ns:keyprov:pskc:totp OATH time-based OTP
urn:ietf:params:xml:ns:keyprov:pskc#OCRA-1 OATH challenge-response algorithm
secret

The binary value of the transported secret key. If the key information is encrypted in the PSKC file it is transparently decrypted if possible. Accessing the value may raise DecryptionError if decryption fails.

counter

The event counter (integer) for event-based OTP algorithms. Will also be transparently decrypted and may also raise DecryptionError.

time_offset

The time offset (integer) for time-based OTP algorithms. If time intervals are used it carries the number of time intervals passed from an algorithm-dependent start point. Will also be transparently decrypted and may also raise DecryptionError.

time_interval

The time interval in seconds (integer) for time-based OTP algorithms (usually 30 or 60). Will also be transparently decrypted and may also raise DecryptionError.

time_drift

For time-based OTP algorithms this contains the device clock drift in number of intervals (integer). Will also be transparently decrypted and may also raise DecryptionError.

issuer

The name of the party that issued the key. This may be different from the manufacturer of the device.

key_profile

A reference to a pre-shared key profile agreed upon between the sending and receiving parties. The profile information itself is not transmitted within the container. See RFC 6030.

key_reference

A reference to an external key that is not contained within the PSKC file (e.g., a PKCS #11 key label). If this attribute is present, the secret attribute will generally be missing.

friendly_name

A human-readable name for the secret key.

key_userid

The distinguished name of the user associated with the key. Also see device_userid.

algorithm_suite

Additional algorithm-specific characteristics. For example, in an HMAC-based algorithm it could specify the hash algorithm used (SHA1 or SHA256).

challenge_encoding

Encoding of the challenge accepted by the device for challenge-response authentication. One of:

  • DECIMAL: only numerical digits
  • HEXADECIMAL: hexadecimal
  • ALPHANUMERIC: all letters and numbers (case sensitive)
  • BASE64: base-64 encoded
  • BINARY: binary data
challenge_min_length

The minimum size of the challenge accepted by the device.

challenge_max_length

The maximum size of the challenge accepted by the device.

challenge_check

Boolean that indicates whether the device will check an embedded Luhn check digit contained in the challenge.

response_encoding

Format of the response that is generated by the device. If must be one of the values as described under challenge_encoding.

response_length

The length of the response generated by the device.

response_check

Boolean that indicates whether the device will append a Luhn check digit to the response.

policy

Policy instance that provides key and PIN policy information. See Key usage policy.

check()

Check if any MACs in the key data embedded in the PSKC file are valid. This will return None if there is no MAC to be checked. It will return True if all the MACs match. If any MAC fails a DecryptionError exception is raised.

Apart from the above, all properties of the Device class are also transparently available in Key instances.

The Device class

class pskc.device.Device

Instances of this class provide the following attributes and functions:

keys

A list of Key instances that represent the keys that are linked to this device. Most PSKC files only allow one key per device which is why all Device attributes are available in Key.

add_key([**kwargs])

Add a new key to the device. The keyword arguments may refer to any attributes of the Key or Device class with which the new key is initialised.

manufacturer

The name of the manufacturer of the device to which the key is provisioned. RFC 6030 prescribes that the value is of the form oath.prefix for OATH Manufacturer Prefixes or iana.organisation for IANA Private Enterprise Numbers however, it is generally just a string. The value may be different from the issuer of the key on the device.

serial

The serial number of the device to which the key is provisioned. Together with manufacturer (and possibly issue_no) this should uniquely identify the device.

model

A manufacturer-specific description of the model of the device.

issue_no

The issue number in case there are devices with the same serial number so that they can be distinguished by different issue numbers.

device_binding

Reference to a device identifier (e.g. IMEI) that allows a provisioning server to ensure that the key is going to be loaded into a specific device.

start_date

datetime.datetime value that indicates that the device should only be used after this date.

expiry_date

datetime.datetime value that indicates that the device should only be used before this date. Systems should not rely upon the device to enforce key usage date restrictions, as some devices do not have an internal clock.

device_userid

The distinguished name of the user associated with the device. Also see key_userid.

crypto_module

Implementation specific unique identifier of the cryptographic module on the device to which the keys have been (or will be) provisioned.