#!/usr/bin/env python3

# Copyright © 2020 Xavier G. <xavier.yamltab@kindwolf.org>
# This work is free. You can redistribute it and/or modify it under the
# terms of the Do What The Fuck You Want To Public License, Version 2,
# as published by Sam Hocevar. See the COPYING file for more details.

from io import BufferedReader, BytesIO
import os
import sys
import json
import yaml
import struct
import argparse
from datetime import datetime
from binascii import hexlify, unhexlify

# Documents used as reference to implement the keytab format:
# [1] https://web.mit.edu/kerberos/krb5-1.12/doc/formats/keytab_file_format.html
# [2] https://github.com/krb5/krb5/blob/master/src/lib/krb5/keytab/kt_file.c#L892
# [3] https://github.com/krb5/krb5/blob/master/src/include/krb5/krb5.hin#L230

DATA_LAYOUT_RAW = 0
DATA_LAYOUT_FULL = 1
DATA_LAYOUT_SIMPLE = 2
DATA_LAYOUTS = {'raw': DATA_LAYOUT_RAW, 'full': DATA_LAYOUT_FULL, 'simple': DATA_LAYOUT_SIMPLE}

DATE_TIME_FORMAT = '%Y-%m-%dT%H:%M:%SZ'

KEYTAB_FIRST_BYTE = 0x05
# Default prefix for struct's format strings, defining big-endian byte order:
BIG_ENDIAN='>'
LITTLE_ENDIAN='<'
NATIVE_ENDIANNESS='='
DEFAULT_PREFIX=BIG_ENDIAN
DEFAULT_ENCODING='ascii'
VERBOSITY=1
CALCSIZE={}

# The following table is based on [3]:
NAME_TYPES = {
	'KRB5_NT_UNKNOWN': 0,
	'KRB5_NT_PRINCIPAL': 1,
	'KRB5_NT_SRV_INST': 2,
	'KRB5_NT_SRV_HST': 3,
	'KRB5_NT_SRV_XHST': 4,
	'KRB5_NT_UID': 5,
	'KRB5_NT_X500_PRINCIPAL': 6,
	'KRB5_NT_SMTP_NAME': 7,
	'KRB5_NT_ENTERPRISE_PRINCIPAL': 10,
	'KRB5_NT_WELLKNOWN': 11,
	'KRB5_NT_MS_PRINCIPAL': -128,
	'KRB5_NT_MS_PRINCIPAL_AND_ID': -129,
	'KRB5_NT_ENT_PRINCIPAL_AND_ID': -130,
}

ENC_TYPES = {
	'NULL': 0,
	'DES_CBC_CRC': 1,
	'DES_CBC_MD4': 2,
	'DES_CBC_MD5': 3,
	'DES_CBC_RAW': 4,
	'DES3_CBC_SHA': 5,
	'DES3_CBC_RAW': 6,
	'DES_HMAC_SHA1': 8,
	'DSA_SHA1_CMS': 9,
	'MD5_RSA_CMS': 10,
	'SHA1_RSA_CMS': 11,
	'RC2_CBC_ENV': 12,
	'RSA_ENV': 13,
	'RSA_ES_OAEP_ENV': 14,
	'DES3_CBC_ENV': 15,
	'DES3_CBC_SHA1': 16,
	'AES128_CTS_HMAC_SHA1_96': 17,
	'AES256_CTS_HMAC_SHA1_96': 18,
	'AES128_CTS_HMAC_SHA256_128': 19,
	'AES256_CTS_HMAC_SHA384_192': 20,
	'ARCFOUR_HMAC': 23,
	'ARCFOUR_HMAC_EXP': 24,
	'CAMELLIA128_CTS_CMAC': 25,
	'CAMELLIA256_CTS_CMAC': 26,
	'UNKNOWN': 511,
}

class KeytabParsingError(Exception):
	MESSAGE = 'Parsing eror: expected {size} bytes to unpack {format} but read {length} bytes instead: {data}'
	def __init__(self, data, size, frmt):
		self.data = data
		self.size = size
		self.format = frmt
	def __str__(self):
		return __class__.MESSAGE.format(**self.__dict__, length=len(self.data))

class KeytabComposingError(Exception):
	pass

def lookup(lookup_value, dictionary, default):
	for name, value in dictionary.items():
		if value == lookup_value:
			return name
	return default

def int_to_name_type(lookup_value):
	return lookup(lookup_value, NAME_TYPES, 'KRB5_NT_UNKNOWN')

def int_to_enc_type(lookup_value):
	return lookup(lookup_value, ENC_TYPES, 'ENCTYPE_UNKNOWN')

def principal_to_spn(principal):
	if principal.get('name_type_raw') != NAME_TYPES['KRB5_NT_PRINCIPAL']:
		return None
	components = principal['components']
	count = len(components)
	if count < 1 or count > 3:
		return None
	for component in components:
		if not component:
			return None
	spn = components[0]
	if count >= 2:
		spn += '/' + components[1]
	if count == 3:
		spn += ':' + components[2]
	spn += '@' + principal['realm']
	return spn

def verbose(level, msg, *args, **kwargs):
	if level <= VERBOSITY:
		message = msg.format(*args, **kwargs)
		sys.stderr.write(message + '\n')

def unpack(buf, prefix, format):
	"""
	Wrapper around read(), struct.unpack() and struct.calcsize().
	"""
	actual_format = prefix + format
	size = CALCSIZE.get(actual_format)
	if size is None:
		size = CALCSIZE[actual_format] = struct.calcsize(actual_format)
	data = buf.read(size)
	if len(data) < size:
		raise KeytabParsingError(data, size, actual_format)
	results = struct.unpack(actual_format, data)
	return results[0] if len(results) == 1 else results

def parse_data(buf, prefix=DEFAULT_PREFIX):
	length = unpack(buf, prefix, 'H')
	return buf.read(length)

def parse_str(buf, prefix=DEFAULT_PREFIX, encoding=DEFAULT_ENCODING):
	return parse_data(buf, prefix).decode(encoding)

def parse_principal(buf, prefix=DEFAULT_PREFIX, version=2):
	principal = {}
	# [1] states "count of components (32 bits)" but [2] says int16:
	component_count = unpack(buf, prefix, 'H')
	# [1] states "[includes realm in version 1]"
	if version == 1:
		component_count -= 1
	principal['realm'] = parse_str(buf, prefix)
	components = []
	for i in range(component_count):
		components.append(parse_str(buf, prefix))
	principal['components'] = components
	# [1] states "[omitted in version 1]"
	if version != 1:
		# [3] states int32:
		principal['name_type_raw'] = unpack(buf, prefix, 'i')
	return principal

def parse_entry(buf, prefix=DEFAULT_PREFIX, version=2):
	entry = {}
	entry['principal'] = parse_principal(buf, prefix, version)
	entry['timestamp'], entry['kvno'], entry['enctype_raw'], entry['key_length'] = unpack(buf, prefix, 'IBHH')
	entry['key'] = buf.read(entry['key_length'])
	return entry

def parse_record(buf, prefix=DEFAULT_PREFIX, version=2):
	record = {'type': 'record'}
	record['entry'] = parse_entry(buf, prefix, version)
	record['tail'] = buf.read()
	return record

def parse_keytab(buf, args):
	second_byte = buf.read(2)[1]
	verbose(2, 'keytab v{}', second_byte)
	if second_byte == 1:
		# Version 1 uses native byte order:
		prefix = NATIVE_ENDIANNESS
		if args.v1_endianness == 'big':
			prefix = BIG_ENDIAN
		elif args.v1_endianness == 'little':
			prefix = LITTLE_ENDIAN
	elif second_byte == 2:
		# Version 2 always uses big-endian byte order:
		prefix = BIG_ENDIAN
	else:
		verbose(1, 'Unknown keytab version: v{}', second_byte)
		sys.exit(1)

	keytab = {
		'version': second_byte,
		'records': [],
	}
	while True:
		try:
			record_length = unpack(buf, prefix, 'i')
		except KeytabParsingError as kpe:
			if len(kpe.data):
				verbose(1, 'Premature end of file? Got {} as record length.', kpe.data)
			break
		if not record_length:
			break
		verbose(3, 'Record #{} of length {}', len(keytab['records']) + 1, record_length)
		record = buf.read(abs(record_length))
		if record_length > 0:
			record = parse_record(BufferedReader(BytesIO(record)), prefix, second_byte)
		else:
			record = {'type': 'hole', 'data': record}
		record['length'] = record_length
		keytab['records'].append(record)
	return keytab

def enrich_keytab(keytab):
	"""
	Enrich records with extra information suitable for human readers.
	"""
	for record in keytab['records']:
		if 'entry' not in record:
			continue
		entry = record['entry']
		entry['date'] = datetime.utcfromtimestamp(entry['timestamp']).strftime(DATE_TIME_FORMAT)
		if 'name_type_raw' in entry['principal']:
			entry['principal']['name_type'] = int_to_name_type(entry['principal']['name_type_raw'])
		spn = principal_to_spn(entry['principal'])
		if spn:
			entry['spn'] = spn
		entry['enctype'] = int_to_enc_type(entry['enctype_raw'])
		if 'tail' in record:
			# [1] states: Some implementations of Kerberos recognize a 32-bit key version at the end of
			# an entry, if the record length is at least 4 bytes longer than the entry and the value of
			# those 32 bits is not 0. If present, this key version supersedes the 8-bit key version.
			if len(record['tail']) >= 4:
				tail_kvno = struct.unpack('>I', record['tail'][0:4])[0]
				if tail_kvno:
					entry['tail_kvno'] = tail_kvno
					# tail_kvno overrides kvno if non-zero:
					if entry['tail_kvno']:
						entry['actual_kvno'] = entry['tail_kvno']
			if 'actual_kvno' not in entry:
				entry['actual_kvno'] = entry['kvno']
	return keytab

def simplify_keytab(keytab):
	"""
	Simplify the keytab to make it suitable for edition.
	"""
	simplified = {'version': keytab['version'], 'entries': []}
	for record in keytab['records']:
		if 'entry' not in record:
			continue
		entry = record['entry']
		simple_entry = {}
		if 'spn' in entry:
			simple_entry['spn'] = entry['spn']
		simple_entry['principal'] = {}
		for key in ('name_type', 'components', 'realm'):
			if key in entry['principal']:
				simple_entry['principal'][key] = entry['principal'][key]
		simple_entry['kvno'] = entry.get('actual_kvno', entry['kvno'])
		for key in ('date', 'enctype', 'key'):
			if key in entry:
				simple_entry[key] = entry[key]
		simplified['entries'].append(simple_entry)
	return simplified

def prepare_serialization(obj):
	"""
	Prepare keytab for serialization.
	"""
	if type(obj) is dict:
		for key, value in obj.items():
			obj[key] = prepare_serialization(value)
	elif type(obj) is list:
		for index, value in enumerate(obj):
			obj[index] = prepare_serialization(value)
	elif type(obj) is bytes:
		obj = hexlify(obj).decode(DEFAULT_ENCODING)
	return obj

def keytab_data(buf, args):
	keytab = parse_keytab(buf, args)
	layout = DATA_LAYOUTS.get(args.data_layout, DATA_LAYOUT_FULL)
	if layout >= DATA_LAYOUT_FULL:
		keytab = enrich_keytab(keytab)
	if layout >= DATA_LAYOUT_SIMPLE:
		keytab = simplify_keytab(keytab)
	return keytab

def keytab_to_yaml(buf, args):
	keytab = keytab_data(buf, args)
	final_keytab = prepare_serialization(keytab)
	if args.output_format == 'yaml':
		yaml.dump(final_keytab, sys.stdout, width=160, sort_keys=False)
	else:
		json.dump(final_keytab, sys.stdout, indent=4)

def pack_data(data):
	return struct.pack('>H', len(data)) + data

def pack_str(string, encoding=DEFAULT_ENCODING):
	return pack_data(string.encode(encoding))

def raw_principal_to_binary(principal, index):
	for key in ('realm', 'components'):
		if key not in principal:
			raise KeytabComposingError('Mandatory key "%s" not found in principal #%d' % (key, index))
	name_type_raw = principal.get('name_type_raw', NAME_TYPES['KRB5_NT_PRINCIPAL'])
	try:
		_ = int(name_type_raw)
	except:
		raise KeytabComposingError('invalid name_type_raw value in principal #%d' % index)
	data = struct.pack('>H', len(principal['components']))
	data += pack_str(principal['realm'])
	for component in principal['components']:
		data += pack_str(component)
	data += struct.pack('>i', name_type_raw)
	return data

def raw_entry_to_binary(entry, index):
	for key in ('principal', 'timestamp', 'kvno', 'enctype_raw', 'key'):
		if key not in entry:
			raise KeytabComposingError('Mandatory key "%s" not found in entry #%d' % (key, index))
	for key in ('timestamp', 'kvno', 'enctype_raw'):
		try:
			assert(int(entry[key]) >= 0)
		except:
			raise KeytabComposingError('invalid %s data in entry #%d' % (key, index))
	data = raw_principal_to_binary(entry['principal'], index)
	try:
		key_data = unhexlify(entry['key'])
	except:
		raise KeytabComposingError('invalid key data in entry #%d' % index)
	data += struct.pack('>IBHH', entry['timestamp'], entry['kvno'], entry['enctype_raw'], len(key_data))
	data += key_data
	return data

def raw_record_to_binary(record, index):
	data = b''
	if 'entry' not in record:
		raise KeytabComposingError('missing entry in record #%d' % index)
	data += raw_entry_to_binary(record['entry'], index)
	if 'tail' in record:
		try:
			data += unhexlify(record['tail'])
		except:
			raise KeytabComposingError('invalid tail data in record #%d' % index)
	return struct.pack('>i', len(data)) + data

def raw_hole_to_binary(hole, index):
	data = b''
	if 'length' not in record:
		raise KeytabComposingError('missing length in record #%d' % index)
	try:
		length = abs(int(record['length']))
		if not length:
			raise KeytabComposingError('illegal zero-length hole in record #%d' % index)
		data += struct.pack('>i', -length)
	except:
		raise KeytabComposingError('invalid length in record #%d' % index)
	if 'data' in record:
		try:
			hole_data = unhexlify(record['data'])
		except:
			raise KeytabComposingError('invalid data in record #%d' % index)
		if len(hole_data) != length:
			raise KeytabComposingError('length and data do not match in record #%d' % index)
		data += hole_data
	else:
		data += b'\x00' * length
	return data

def raw_keytab_to_binary(indata):
	data = bytes([KEYTAB_FIRST_BYTE, 2])
	for index, record in enumerate(indata.get('records', [])):
		record_type = record.get('type', 'record')
		if record_type == 'hole':
			data += raw_hole_to_binary(record, index)
		elif record_type == 'record':
			data += raw_record_to_binary(record, index)
		else:
			raise KeytabComposingError('Unknown record type in record #%d' % index)
	return data

def yaml_to_keytab(buf, args):
	data = yaml.load(buf, Loader=yaml.SafeLoader)
	result = raw_keytab_to_binary(data)
	sys.stdout.buffer.write(result)

def parse_args():
	parser = argparse.ArgumentParser(description='Keytab <-> YAML/JSON convertor.')
	parser.add_argument('--verbose', '-v', dest='verbose', action='count', help='increase verbosity level', default=VERBOSITY)
	parser.add_argument('--data-layout', '-l',  dest='data_layout', choices=DATA_LAYOUTS.keys(), default='simple', help='data layout (keytab to YAML/JSON only)')
	parser.add_argument('--output-format', '-f', dest='output_format', choices=['json', 'yaml'], default='yaml', help='output format (keytab to YAML/JSON only)')
	parser.add_argument('--v1-endianness', '-e', dest='v1_endianness', choices=['native', 'little', 'big'], default='native', help='Enforce endianness (keytab v1 to YAML/JSON only)')
	parser.add_argument('input', nargs='?', type=argparse.FileType('rb'), default=sys.stdin.buffer, help='input file; defaults to standard input')
	args = parser.parse_args()
	return args

def main():
	args = parse_args()
	global VERBOSITY
	VERBOSITY=args.verbose
	buf = args.input
	first_byte = buf.peek(1)[0]
	if first_byte == KEYTAB_FIRST_BYTE:
		keytab_to_yaml(buf, args)
	else:
		yaml_to_keytab(buf, args)

if __name__ == '__main__':
	main()