#!/usr/bin/env python

"""
<Program Name>
  formats.py

<Author>
  Geremy Condra
  Vladimir Diaz <vladimir.v.diaz@gmail.com>

<Started>
  Refactored April 30, 2012. -vladimir.v.diaz

<Copyright>
  See LICENSE for licensing information.

<Purpose>
  A central location for all format-related checking of TUF objects.
  Note: 'formats.py' depends heavily on 'schema.py', so the 'schema.py'
  module should be read and understood before tackling this module.

  'formats.py' can be broken down into three sections.  (1) Schemas and object
  matching.  (2) Classes that represent Role Metadata and help produce
  correctly formatted files.  (3) Functions that help produce or verify TUF
  objects.

  The first section deals with schemas and object matching based on format.
  There are two ways of checking the format of objects.  The first method
  raises a 'securesystemslib.exceptions.FormatError' exception if the match
  fails and the other returns a Boolean result.

  securesystemslib.formats.<SCHEMA>.check_match(object)
  securesystemslib.formats.<SCHEMA>.matches(object)

  Example:

  rsa_key = {'keytype': 'rsa'
             'keyid': 34892fc465ac76bc3232fab
             'keyval': {'public': 'public_key',
                        'private': 'private_key'}

  securesystemslib.formats.RSAKEY_SCHEMA.check_match(rsa_key)
  securesystemslib.formats.RSAKEY_SCHEMA.matches(rsa_key)

  In this example, if a dict key or dict value is missing or incorrect,
  the match fails.  There are numerous variations of object checking
  provided by 'formats.py' and 'schema.py'.

  The second section deals with the role metadata classes.  There are
  multiple top-level roles, each with differing metadata formats.
  Example:

  root_object = securesystemslib.formats.RootFile.from_metadata(root_metadata_file)
  targets_metadata = securesystemslib.formats.TargetsFile.make_metadata(...)

  The input and output of these classes are checked against their respective
  schema to ensure correctly formatted metadata.

  The last section contains miscellaneous functions related to the format of
  TUF objects.
  Example:

  signable_object = make_signable(unsigned_object)
"""

# Help with Python 3 compatibility, where the print statement is a function, an
# implicit relative import is invalid, and the '/' operator performs true
# division.  Example:  print 'hello world' raises a 'SyntaxError' exception.
from __future__ import print_function
from __future__ import absolute_import
from __future__ import division
from __future__ import unicode_literals

import binascii
import calendar
import re
import string
import datetime
import time
import six

import securesystemslib.schema as SCHEMA
import securesystemslib.exceptions

# Note that in the schema definitions below, the 'SCHEMA.Object' types allow
# additional keys which are not defined. Thus, any additions to them will be
# easily backwards compatible with clients that are already deployed.

# A datetime in 'YYYY-MM-DDTHH:MM:SSZ' ISO 8601 format.  The "Z" zone designator
# for the zero UTC offset is always used (i.e., a numerical offset is not
# supported.)  Example: '2015-10-21T13:20:00Z'.  Note:  This is a simple format
# check, and an ISO8601 string should be fully verified when it is parsed.
ISO8601_DATETIME_SCHEMA = SCHEMA.RegularExpression(r'\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z')

# A Unix/POSIX time format.  An integer representing the number of seconds
# since the epoch (January 1, 1970.)  Metadata uses this format for the
# 'expires' field.  Set 'hi' to the upper timestamp limit (year 2038), the max
# value of an int.
UNIX_TIMESTAMP_SCHEMA = SCHEMA.Integer(lo=0, hi=2147483647)

# A hexadecimal value in '23432df87ab..' format.
HASH_SCHEMA = SCHEMA.RegularExpression(r'[a-fA-F0-9]+')

# A dict in {'sha256': '23432df87ab..', 'sha512': '34324abc34df..', ...} format.
HASHDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = SCHEMA.AnyString(),
  value_schema = HASH_SCHEMA)

# A hexadecimal value in '23432df87ab..' format.
HEX_SCHEMA = SCHEMA.RegularExpression(r'[a-fA-F0-9]+')

# A key identifier (e.g., a hexadecimal value identifying an RSA key).
KEYID_SCHEMA = HASH_SCHEMA

# A list of KEYID_SCHEMA.
KEYIDS_SCHEMA = SCHEMA.ListOf(KEYID_SCHEMA)

# The signing scheme used by a key to generate a signature (e.g.,
# 'rsassa-pss-sha256' is one of the signing schemes for key type 'rsa').
SCHEME_SCHEMA = SCHEMA.AnyString()

# A relative file path (e.g., 'metadata/root/').
RELPATH_SCHEMA = SCHEMA.AnyString()
RELPATHS_SCHEMA = SCHEMA.ListOf(RELPATH_SCHEMA)

# An absolute path.
PATH_SCHEMA = SCHEMA.AnyString()
PATHS_SCHEMA = SCHEMA.ListOf(PATH_SCHEMA)

# Uniform Resource Locator identifier (e.g., 'https://www.updateframework.com/').
URL_SCHEMA = SCHEMA.AnyString()

# A dictionary holding version information.
VERSION_SCHEMA = SCHEMA.Object(
  object_name = 'VERSION_SCHEMA',
  major = SCHEMA.Integer(lo=0),
  minor = SCHEMA.Integer(lo=0),
  fix = SCHEMA.Integer(lo=0))

# An integer representing the numbered version of a metadata file.
# Must be 1, or greater.
METADATAVERSION_SCHEMA = SCHEMA.Integer(lo=0)

# An integer representing length.  Must be 0, or greater.
LENGTH_SCHEMA = SCHEMA.Integer(lo=0)

# An integer representing logger levels, such as logging.CRITICAL (=50).
# Must be between 0 and 50.
LOGLEVEL_SCHEMA = SCHEMA.Integer(lo=0, hi=50)

# A string representing a named object.
NAME_SCHEMA = SCHEMA.AnyString()
NAMES_SCHEMA = SCHEMA.ListOf(NAME_SCHEMA)

# A byte string representing data.
DATA_SCHEMA = SCHEMA.AnyBytes()

# A text string.  For instance, a string entered by the user.
TEXT_SCHEMA = SCHEMA.AnyString()

# Supported hash algorithms.
HASHALGORITHMS_SCHEMA = SCHEMA.ListOf(SCHEMA.OneOf(
  [SCHEMA.String('md5'), SCHEMA.String('sha1'),
   SCHEMA.String('sha224'), SCHEMA.String('sha256'),
   SCHEMA.String('sha384'), SCHEMA.String('sha512')]))

# The contents of an encrypted TUF key.  Encrypted TUF keys are saved to files
# in this format.
ENCRYPTEDKEY_SCHEMA = SCHEMA.AnyString()

# A value that is either True or False, on or off, etc.
BOOLEAN_SCHEMA = SCHEMA.Boolean()

# A role's threshold value (i.e., the minimum number
# of signatures required to sign a metadata file).
# Must be 1 and greater.
THRESHOLD_SCHEMA = SCHEMA.Integer(lo=1)

# A string representing a role's name.
ROLENAME_SCHEMA = SCHEMA.AnyString()

# The minimum number of bits for an RSA key.  Must be 2048 bits, or greater
# (recommended by TUF).  Recommended RSA key sizes:
# http://www.emc.com/emc-plus/rsa-labs/historical/twirl-and-rsa-key-size.htm#table1
RSAKEYBITS_SCHEMA = SCHEMA.Integer(lo=2048)

# The supported ECDSA signature schemes (ecdsa-sha2-nistp256 is supported by
# default).
ECDSA_SCHEME_SCHEMA = SCHEMA.OneOf([SCHEMA.String('ecdsa-sha2-nistp256')])

# The number of hashed bins, or the number of delegated roles.  See
# delegate_hashed_bins() in 'repository_tool.py' for an example.  Note:
# Tools may require further restrictions on the number of bins, such
# as requiring them to be a power of 2.
NUMBINS_SCHEMA = SCHEMA.Integer(lo=1)

# A pyca-cryptography signature.
PYCACRYPTOSIGNATURE_SCHEMA = SCHEMA.AnyBytes()

# An RSA key in PEM format.
PEMRSA_SCHEMA = SCHEMA.AnyString()

# An ECDSA key in PEM format.
PEMECDSA_SCHEMA = SCHEMA.AnyString()

# A string representing a password.
PASSWORD_SCHEMA = SCHEMA.AnyString()

# A list of passwords.
PASSWORDS_SCHEMA = SCHEMA.ListOf(PASSWORD_SCHEMA)

# The actual values of a key, as opposed to meta data such as a key type and
# key identifier ('rsa', 233df889cb).  For RSA keys, the key value is a pair of
# public and private keys in PEM Format stored as strings.
KEYVAL_SCHEMA = SCHEMA.Object(
  object_name = 'KEYVAL_SCHEMA',
  public = SCHEMA.AnyString(),
  private = SCHEMA.Optional(SCHEMA.AnyString()))

# Public keys CAN have a private portion (for backwards compatibility) which
# MUST be an empty string
PUBLIC_KEYVAL_SCHEMA = SCHEMA.Object(
  object_name = 'KEYVAL_SCHEMA',
  public = SCHEMA.AnyString(),
  private = SCHEMA.Optional(SCHEMA.String("")))

# Supported TUF key types.
KEYTYPE_SCHEMA = SCHEMA.OneOf(
  [SCHEMA.String('rsa'), SCHEMA.String('ed25519'),
   SCHEMA.String('ecdsa-sha2-nistp256')])

# A generic TUF key.  All TUF keys should be saved to metadata files in this
# format.
KEY_SCHEMA = SCHEMA.Object(
  object_name = 'KEY_SCHEMA',
  keytype = SCHEMA.AnyString(),
  scheme = SCHEME_SCHEMA,
  keyval = KEYVAL_SCHEMA,
  expires = SCHEMA.Optional(ISO8601_DATETIME_SCHEMA))

# Like KEY_SCHEMA, but requires keyval's private portion to be unset or empty,
# and optionally includes the supported keyid hash algorithms used to generate
# the key's keyid.
PUBLIC_KEY_SCHEMA = SCHEMA.Object(
  object_name = 'PUBLIC_KEY_SCHEMA',
  keytype = SCHEMA.AnyString(),
  keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
  keyval = PUBLIC_KEYVAL_SCHEMA,
  expires = SCHEMA.Optional(ISO8601_DATETIME_SCHEMA))

# A TUF key object.  This schema simplifies validation of keys that may be one
# of the supported key types.  Supported key types: 'rsa', 'ed25519'.
ANYKEY_SCHEMA = SCHEMA.Object(
  object_name = 'ANYKEY_SCHEMA',
  keytype = KEYTYPE_SCHEMA,
  scheme = SCHEME_SCHEMA,
  keyid = KEYID_SCHEMA,
  keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
  keyval = KEYVAL_SCHEMA,
  expires = SCHEMA.Optional(ISO8601_DATETIME_SCHEMA))

# A list of TUF key objects.
ANYKEYLIST_SCHEMA = SCHEMA.ListOf(ANYKEY_SCHEMA)

# RSA signature schemes.
RSA_SCHEME_SCHEMA = SCHEMA.OneOf([SCHEMA.String('rsassa-pss-sha256')])

# An RSA TUF key.
RSAKEY_SCHEMA = SCHEMA.Object(
  object_name = 'RSAKEY_SCHEMA',
  keytype = SCHEMA.String('rsa'),
  scheme = RSA_SCHEME_SCHEMA,
  keyid = KEYID_SCHEMA,
  keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
  keyval = KEYVAL_SCHEMA)

# An ECDSA TUF key.
ECDSAKEY_SCHEMA = SCHEMA.Object(
  object_name = 'ECDSAKEY_SCHEMA',
  keytype = SCHEMA.String('ecdsa-sha2-nistp256'),
  scheme = ECDSA_SCHEME_SCHEMA,
  keyid = KEYID_SCHEMA,
  keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
  keyval = KEYVAL_SCHEMA)

# An ED25519 raw public key, which must be 32 bytes.
ED25519PUBLIC_SCHEMA = SCHEMA.LengthBytes(32)

# An ED25519 raw seed key, which must be 32 bytes.
ED25519SEED_SCHEMA = SCHEMA.LengthBytes(32)

# An ED25519 raw signature, which must be 64 bytes.
ED25519SIGNATURE_SCHEMA = SCHEMA.LengthBytes(64)

# An ECDSA signature.
ECDSASIGNATURE_SCHEMA = SCHEMA.AnyBytes()

# Required installation libraries expected by the repository tools and other
# cryptography modules.
REQUIRED_LIBRARIES_SCHEMA = SCHEMA.ListOf(SCHEMA.OneOf(
  [SCHEMA.String('general'), SCHEMA.String('ed25519'), SCHEMA.String('rsa'),
   SCHEMA.String('ecdsa-sha2-nistp256')]))

# Ed25519 signature schemes.  The vanilla Ed25519 signature scheme is currently
# supported.
ED25519_SIG_SCHEMA = SCHEMA.OneOf([SCHEMA.String('ed25519')])

# An ed25519 TUF key.
ED25519KEY_SCHEMA = SCHEMA.Object(
  object_name = 'ED25519KEY_SCHEMA',
  keytype = SCHEMA.String('ed25519'),
  scheme = ED25519_SIG_SCHEMA,
  keyid = KEYID_SCHEMA,
  keyid_hash_algorithms = SCHEMA.Optional(HASHALGORITHMS_SCHEMA),
  keyval = KEYVAL_SCHEMA)

# Information about target files, like file length and file hash(es).  This
# schema allows the storage of multiple hashes for the same file (e.g., sha256
# and sha512 may be computed for the same file and stored).
FILEINFO_SCHEMA = SCHEMA.Object(
  object_name = 'FILEINFO_SCHEMA',
  length = LENGTH_SCHEMA,
  hashes = HASHDICT_SCHEMA,
  version = SCHEMA.Optional(METADATAVERSION_SCHEMA),
  custom = SCHEMA.Optional(SCHEMA.Object()))

# Version information specified in "snapshot.json" for each role available on
# the TUF repository.  The 'FILEINFO_SCHEMA' object was previously listed in
# the snapshot role, but was switched to this object format to reduce the
# amount of metadata that needs to be downloaded.  Listing version numbers in
# "snapshot.json" also prevents rollback attacks for roles that clients have
# not downloaded.
VERSIONINFO_SCHEMA = SCHEMA.Object(
  object_name = 'VERSIONINFO_SCHEMA',
  version = METADATAVERSION_SCHEMA)

# A dict holding the version information for a particular metadata role.  The
# dict keys hold the relative file paths, and the dict values the corresponding
# version numbers.
VERSIONDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = RELPATH_SCHEMA,
  value_schema = VERSIONINFO_SCHEMA)

# A dict holding the information for a particular target / file.  The dict keys
# hold the relative file paths, and the dict values the corresponding file
# information.
FILEDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = RELPATH_SCHEMA,
  value_schema = FILEINFO_SCHEMA)

# A single signature of an object.  Indicates the signature, and the KEYID of
# the signing key.  I debated making the signature schema not contain the key
# ID and instead have the signatures of a file be a dictionary with the key
# being the keyid and the value being the signature schema without the keyid.
# That would be under the argument that a key should only be able to sign a
# file once.
SIGNATURE_SCHEMA = SCHEMA.Object(
  object_name = 'SIGNATURE_SCHEMA',
  keyid = KEYID_SCHEMA,
  sig = HEX_SCHEMA)

# List of SIGNATURE_SCHEMA.
SIGNATURES_SCHEMA = SCHEMA.ListOf(SIGNATURE_SCHEMA)

# A schema holding the result of checking the signatures of a particular
# 'SIGNABLE_SCHEMA' role.
# For example, how many of the signatures for the 'Target' role are
# valid?  This SCHEMA holds this information.  See 'sig.py' for
# more information.
SIGNATURESTATUS_SCHEMA = SCHEMA.Object(
  object_name = 'SIGNATURESTATUS_SCHEMA',
  threshold = SCHEMA.Integer(),
  good_sigs = KEYIDS_SCHEMA,
  bad_sigs = KEYIDS_SCHEMA,
  unknown_sigs = KEYIDS_SCHEMA,
  untrusted_sigs = KEYIDS_SCHEMA)

# A signable object.  Holds the signing role and its associated signatures.
SIGNABLE_SCHEMA = SCHEMA.Object(
  object_name = 'SIGNABLE_SCHEMA',
  signed = SCHEMA.Any(),
  signatures = SCHEMA.ListOf(SIGNATURE_SCHEMA))

# A dict where the dict keys hold a keyid and the dict values a key object.
KEYDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = KEYID_SCHEMA,
  value_schema = KEY_SCHEMA)

# The format used by the key database to store keys.  The dict keys hold a key
# identifier and the dict values any object.  The key database should store
# key objects in the values (e.g., 'RSAKEY_SCHEMA', 'DSAKEY_SCHEMA').
KEYDB_SCHEMA = SCHEMA.DictOf(
  key_schema = KEYID_SCHEMA,
  value_schema = SCHEMA.Any())

# A path hash prefix is a hexadecimal string.
PATH_HASH_PREFIX_SCHEMA = HEX_SCHEMA

# A list of path hash prefixes.
PATH_HASH_PREFIXES_SCHEMA = SCHEMA.ListOf(PATH_HASH_PREFIX_SCHEMA)

# Role object in {'keyids': [keydids..], 'name': 'ABC', 'threshold': 1,
# 'paths':[filepaths..]} format.
ROLE_SCHEMA = SCHEMA.Object(
  object_name = 'ROLE_SCHEMA',
  name = SCHEMA.Optional(ROLENAME_SCHEMA),
  keyids = KEYIDS_SCHEMA,
  threshold = THRESHOLD_SCHEMA,
  backtrack = SCHEMA.Optional(BOOLEAN_SCHEMA),
  paths = SCHEMA.Optional(RELPATHS_SCHEMA),
  path_hash_prefixes = SCHEMA.Optional(PATH_HASH_PREFIXES_SCHEMA))

# A dict of roles where the dict keys are role names and the dict values holding
# the role data/information.
ROLEDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = ROLENAME_SCHEMA,
  value_schema = ROLE_SCHEMA)

# Like ROLEDICT_SCHEMA, except that ROLE_SCHEMA instances are stored in order.
ROLELIST_SCHEMA = SCHEMA.ListOf(ROLE_SCHEMA)

# The delegated roles of a Targets role (a parent).
DELEGATIONS_SCHEMA = SCHEMA.Object(
  keys = KEYDICT_SCHEMA,
  roles = ROLELIST_SCHEMA)

# The fileinfo format of targets specified in the repository and
# developer tools.  The second element of this list holds custom data about the
# target, such as file permissions, author(s), last modified, etc.
CUSTOM_SCHEMA = SCHEMA.Object()

PATH_FILEINFO_SCHEMA = SCHEMA.DictOf(
  key_schema = RELPATH_SCHEMA,
  value_schema = CUSTOM_SCHEMA)

# TUF roledb
ROLEDB_SCHEMA = SCHEMA.Object(
  object_name = 'ROLEDB_SCHEMA',
  keyids = SCHEMA.Optional(KEYIDS_SCHEMA),
  signing_keyids = SCHEMA.Optional(KEYIDS_SCHEMA),
  previous_keyids = SCHEMA.Optional(KEYIDS_SCHEMA),
  threshold = SCHEMA.Optional(THRESHOLD_SCHEMA),
  previous_threshold = SCHEMA.Optional(THRESHOLD_SCHEMA),
  version = SCHEMA.Optional(METADATAVERSION_SCHEMA),
  expires = SCHEMA.Optional(ISO8601_DATETIME_SCHEMA),
  signatures = SCHEMA.Optional(SIGNATURES_SCHEMA),
  paths = SCHEMA.Optional(SCHEMA.OneOf([RELPATHS_SCHEMA, PATH_FILEINFO_SCHEMA])),
  path_hash_prefixes = SCHEMA.Optional(PATH_HASH_PREFIXES_SCHEMA),
  delegations = SCHEMA.Optional(DELEGATIONS_SCHEMA),
  partial_loaded = SCHEMA.Optional(BOOLEAN_SCHEMA))

# Root role: indicates root keys and top-level roles.
ROOT_SCHEMA = SCHEMA.Object(
  object_name = 'ROOT_SCHEMA',
  _type = SCHEMA.String('root'),
  version = METADATAVERSION_SCHEMA,
  consistent_snapshot = BOOLEAN_SCHEMA,
  expires = ISO8601_DATETIME_SCHEMA,
  keys = KEYDICT_SCHEMA,
  roles = ROLEDICT_SCHEMA)

# Targets role: Indicates targets and delegates target paths to other roles.
TARGETS_SCHEMA = SCHEMA.Object(
  object_name = 'TARGETS_SCHEMA',
  _type = SCHEMA.String('targets'),
  version = METADATAVERSION_SCHEMA,
  expires = ISO8601_DATETIME_SCHEMA,
  targets = FILEDICT_SCHEMA,
  delegations = SCHEMA.Optional(DELEGATIONS_SCHEMA))

# Snapshot role: indicates the latest versions of all metadata (except
# timestamp).
SNAPSHOT_SCHEMA = SCHEMA.Object(
  object_name = 'SNAPSHOT_SCHEMA',
  _type = SCHEMA.String('snapshot'),
  version = METADATAVERSION_SCHEMA,
  expires = ISO8601_DATETIME_SCHEMA,
  meta = VERSIONDICT_SCHEMA)

# Timestamp role: indicates the latest version of the snapshot file.
TIMESTAMP_SCHEMA = SCHEMA.Object(
  object_name = 'TIMESTAMP_SCHEMA',
  _type = SCHEMA.String('timestamp'),
  version = METADATAVERSION_SCHEMA,
  expires = ISO8601_DATETIME_SCHEMA,
  meta = FILEDICT_SCHEMA)

# project.cfg file: stores information about the project in a json dictionary
PROJECT_CFG_SCHEMA = SCHEMA.Object(
    object_name = 'PROJECT_CFG_SCHEMA',
    project_name = SCHEMA.AnyString(),
    layout_type = SCHEMA.OneOf([SCHEMA.String('repo-like'), SCHEMA.String('flat')]),
    targets_location = PATH_SCHEMA,
    metadata_location = PATH_SCHEMA,
    prefix = PATH_SCHEMA,
    public_keys = KEYDICT_SCHEMA,
    threshold = SCHEMA.Integer(lo = 0, hi = 2)
    )

# A schema containing information a repository mirror may require,
# such as a url, the path of the directory metadata files, etc.
MIRROR_SCHEMA = SCHEMA.Object(
  object_name = 'MIRROR_SCHEMA',
  url_prefix = URL_SCHEMA,
  metadata_path = RELPATH_SCHEMA,
  targets_path = RELPATH_SCHEMA,
  confined_target_dirs = RELPATHS_SCHEMA,
  custom = SCHEMA.Optional(SCHEMA.Object()))

# A dictionary of mirrors where the dict keys hold the mirror's name and
# and the dict values the mirror's data (i.e., 'MIRROR_SCHEMA').
# The repository class of 'updater.py' accepts dictionaries
# of this type provided by the TUF client.
MIRRORDICT_SCHEMA = SCHEMA.DictOf(
  key_schema = SCHEMA.AnyString(),
  value_schema = MIRROR_SCHEMA)

# A Mirrorlist: indicates all the live mirrors, and what documents they
# serve.
MIRRORLIST_SCHEMA = SCHEMA.Object(
  object_name = 'MIRRORLIST_SCHEMA',
  _type = SCHEMA.String('mirrors'),
  version = METADATAVERSION_SCHEMA,
  expires = ISO8601_DATETIME_SCHEMA,
  mirrors = SCHEMA.ListOf(MIRROR_SCHEMA))

# Any of the role schemas (e.g., TIMESTAMP_SCHEMA, SNAPSHOT_SCHEMA, etc.)
ANYROLE_SCHEMA = SCHEMA.OneOf([ROOT_SCHEMA, TARGETS_SCHEMA, SNAPSHOT_SCHEMA,
                               TIMESTAMP_SCHEMA, MIRROR_SCHEMA])



def datetime_to_unix_timestamp(datetime_object):
  """
  <Purpose>
    Convert 'datetime_object' (in datetime.datetime()) format) to a Unix/POSIX
    timestamp.  For example, Python's time.time() returns a Unix timestamp, and
    includes the number of microseconds.  'datetime_object' is converted to UTC.

    >>> datetime_object = datetime.datetime(1985, 10, 26, 1, 22)
    >>> timestamp = datetime_to_unix_timestamp(datetime_object)
    >>> timestamp
    499137720

  <Arguments>
    datetime_object:
      The datetime.datetime() object to convert to a Unix timestamp.

  <Exceptions>
    securesystemslib.exceptions.FormatError, if 'datetime_object' is not a
    datetime.datetime() object.

  <Side Effects>
    None.

  <Returns>
    A unix (posix) timestamp (e.g., 499137660).
  """

  # Is 'datetime_object' a datetime.datetime() object?
  # Raise 'securesystemslib.exceptions.FormatError' if not.
  if not isinstance(datetime_object, datetime.datetime):
    message = repr(datetime_object) + ' is not a datetime.datetime() object.'
    raise securesystemslib.exceptions.FormatError(message)

  unix_timestamp = calendar.timegm(datetime_object.timetuple())

  return unix_timestamp




def unix_timestamp_to_datetime(unix_timestamp):
  """
  <Purpose>
    Convert 'unix_timestamp' (i.e., POSIX time, in UNIX_TIMESTAMP_SCHEMA format)
    to a datetime.datetime() object.  'unix_timestamp' is the number of seconds
    since the epoch (January 1, 1970.)

    >>> datetime_object = unix_timestamp_to_datetime(1445455680)
    >>> datetime_object
    datetime.datetime(2015, 10, 21, 19, 28)

  <Arguments>
    unix_timestamp:
      An integer representing the time (e.g., 1445455680).  Conformant to
      'securesystemslib.formats.UNIX_TIMESTAMP_SCHEMA'.

  <Exceptions>
    securesystemslib.exceptions.FormatError, if 'unix_timestamp' is improperly
    formatted.

  <Side Effects>
    None.

  <Returns>
    A datetime.datetime() object corresponding to 'unix_timestamp'.
  """

  # Is 'unix_timestamp' properly formatted?
  # Raise 'securesystemslib.exceptions.FormatError' if there is a mismatch.
  securesystemslib.formats.UNIX_TIMESTAMP_SCHEMA.check_match(unix_timestamp)

  # Convert 'unix_timestamp' to a 'time.struct_time',  in UTC.  The Daylight
  # Savings Time (DST) flag is set to zero.  datetime.fromtimestamp() is not
  # used because it returns a local datetime.
  struct_time = time.gmtime(unix_timestamp)

  # Extract the (year, month, day, hour, minutes, seconds) arguments for the
  # datetime object to be returned.
  datetime_object = datetime.datetime(*struct_time[:6])

  return datetime_object




def format_base64(data):
  """
  <Purpose>
    Return the base64 encoding of 'data' with whitespace and '=' signs omitted.

  <Arguments>
    data:
      Binary or buffer of data to convert.

  <Exceptions>
    securesystemslib.exceptions.FormatError, if the base64 encoding fails or the
    argument is invalid.

  <Side Effects>
    None.

  <Returns>
    A base64-encoded string.
  """

  try:
    return binascii.b2a_base64(data).decode('utf-8').rstrip('=\n ')

  except (TypeError, binascii.Error) as e:
    raise securesystemslib.exceptions.FormatError('Invalid base64'
      ' encoding: ' + str(e))




def parse_base64(base64_string):
  """
  <Purpose>
    Parse a base64 encoding with whitespace and '=' signs omitted.

  <Arguments>
    base64_string:
      A string holding a base64 value.

  <Exceptions>
    securesystemslib.exceptions.FormatError, if 'base64_string' cannot be parsed
    due to an invalid base64 encoding.

  <Side Effects>
    None.

  <Returns>
    A byte string representing the parsed based64 encoding of
    'base64_string'.
  """

  if not isinstance(base64_string, six.string_types):
    message = 'Invalid argument: '+repr(base64_string)
    raise securesystemslib.exceptions.FormatError(message)

  extra = len(base64_string) % 4
  if extra:
    padding = '=' * (4 - extra)
    base64_string = base64_string + padding

  try:
    return binascii.a2b_base64(base64_string.encode('utf-8'))

  except (TypeError, binascii.Error) as e:
    raise securesystemslib.exceptions.FormatError('Invalid base64'
      ' encoding: ' + str(e))




def _canonical_string_encoder(string):
  """
  <Purpose>
    Encode 'string' to canonical string format.

  <Arguments>
    string:
      The string to encode.

  <Exceptions>
    None.

  <Side Effects>
    None.

  <Returns>
    A string with the canonical-encoded 'string' embedded.
  """

  string = '"%s"' % re.sub(r'(["\\])', r'\\\1', string)

  return string


def _encode_canonical(object, output_function):
  # Helper for encode_canonical.  Older versions of json.encoder don't
  # even let us replace the separators.

  if isinstance(object, six.string_types):
    output_function(_canonical_string_encoder(object))
  elif object is True:
    output_function("true")
  elif object is False:
    output_function("false")
  elif object is None:
    output_function("null")
  elif isinstance(object, six.integer_types):
    output_function(str(object))
  elif isinstance(object, (tuple, list)):
    output_function("[")
    if len(object):
      for item in object[:-1]:
        _encode_canonical(item, output_function)
        output_function(",")
      _encode_canonical(object[-1], output_function)
    output_function("]")
  elif isinstance(object, dict):
    output_function("{")
    if len(object):
      items = sorted(six.iteritems(object))
      for key, value in items[:-1]:
        output_function(_canonical_string_encoder(key))
        output_function(":")
        _encode_canonical(value, output_function)
        output_function(",")
      key, value = items[-1]
      output_function(_canonical_string_encoder(key))
      output_function(":")
      _encode_canonical(value, output_function)
    output_function("}")
  else:
    raise securesystemslib.exceptions.FormatError('I cannot encode '+repr(object))


def encode_canonical(object, output_function=None):
  """
  <Purpose>
    Encode 'object' in canonical JSON form, as specified at
    http://wiki.laptop.org/go/Canonical_JSON .  It's a restricted
    dialect of JSON in which keys are always lexically sorted,
    there is no whitespace, floats aren't allowed, and only quote
    and backslash get escaped.  The result is encoded in UTF-8,
    and the resulting bits are passed to output_function (if provided),
    or joined into a string and returned.

    Note: This function should be called prior to computing the hash or
    signature of a JSON object in TUF.  For example, generating a signature
    of a signing role object such as 'ROOT_SCHEMA' is required to ensure
    repeatable hashes are generated across different json module versions
    and platforms.  Code elsewhere is free to dump JSON objects in any format
    they wish (e.g., utilizing indentation and single quotes around object
    keys).  These objects are only required to be in "canonical JSON" format
    when their hashes or signatures are needed.

    >>> encode_canonical("")
    '""'
    >>> encode_canonical([1, 2, 3])
    '[1,2,3]'
    >>> encode_canonical([])
    '[]'
    >>> encode_canonical({"A": [99]})
    '{"A":[99]}'
    >>> encode_canonical({"x" : 3, "y" : 2})
    '{"x":3,"y":2}'

  <Arguments>
    object:
      The object to be encoded.

    output_function:
      The result will be passed as arguments to 'output_function'
      (e.g., output_function('result')).

  <Exceptions>
    securesystemslib.exceptions.FormatError, if 'object' cannot be encoded or
    'output_function' is not callable.

  <Side Effects>
    The results are fed to 'output_function()' if 'output_function' is set.

  <Returns>
    A string representing the 'object' encoded in canonical JSON form.
  """

  result = None
  # If 'output_function' is unset, treat it as
  # appending to a list.
  if output_function is None:
    result = []
    output_function = result.append

  try:
    _encode_canonical(object, output_function)

  except (TypeError, securesystemslib.exceptions.FormatError) as e:
    message = 'Could not encode ' + repr(object) + ': ' + str(e)
    raise securesystemslib.exceptions.FormatError(message)

  # Return the encoded 'object' as a string.
  # Note: Implies 'output_function' is None,
  # otherwise results are sent to 'output_function'.
  if result is not None:
    return ''.join(result)
