tlv8 0.1.0

Python module to handle type-length-value (TLV) encoded data 8-bit type, 8-bit length, and N-byte value as described within the Apple HomeKit Accessory Protocol Specification Non-Commercial Version Release R2.

Type-Length-Value8 (TLV8) for python

Type-Length-Value (TLV) are used to encode arbitrary data. In this case the type and length are represented by 1 byte each. Hence the name TLV8.

A TLV8 entry consists of the following parts:

• the type: this 8 bit field denotes the type of information that is represented by the data.
• the length: this 8 bit field denotes the length of the data (this does not include the 2 bytes for type and length. For data longer than 255 bytes, there is a defined procedure available.
• the value: these length bytes represent the value of this TLV. The different types of data is represented differently:
  • bytes: this is raw binary data and will be used as is, no further interpretation takes place
  • tlv8: this is a specialized case of bytes values. Using this instead of pure bytes enables nesting of data and creating a hierarchy.
  • integer: integers are stored in little-endian byte order and are encoded with the minimal number of bytes possible (1, 2, 4 or 8)
  • float: floats are stored as little-endian ieee754 numbers
  • string: strings are always UTF-8 encoded and do not contain the terminating NULL byte

TLV8 entries whose content is longer than 255 bytes are split up into fragments. The type is repeated is repeated in each fragment, only the last fragment may contain less than 255 bytes. Fragments of one TLV8 entry must be continuous.

Multiple TLV8 entries can be combined to create larger structures. Entries of different types can placed one after another. Entries of the same type must be separated by a TLV8 entry of a different type (and probably zero length).

TLV8 entries of unknown or unwanted type are to be silently ignored.

Examples

simple TLV8s

Encoding of some atomic examples:

• an empty TLV of type 42: [42, None] will be encoded as b'\x2a\x00'.
• a TLV of type 2 with 2 bytes 0x12, 0x34: [2, b'\x12\x34'] will be encoded as b'\x02\x02\x12\x34'
• a TLV of type 3 that contains the TLV from above: [3, [2, b'\x12\x34']] will be encoded as b'\x03\x04\x02\x02\x12\x34'
• a TLV of type 4 that contains 1024: [4, 1024] will be encoded as b'\x04\0x02\x00\x04'
• a TLV of type 5 that contains 3.141: [4, 3.141] will be encoded as b'\x04\x04\x0a\xd7\x23\x41'
• a TLV of type 23 with string Hello 🌍: [23, 'Hello 🌍'] will be encoded as b'\x17\x0a\x48\x65\x6c\x6c\x6f\x20\xf0\x9f\x8c\x8d'

fragmented TLV8s

Encoding of a fragmented TLV8 entry:

• an TLV of type 6 that contains 256 bytes from 0 to 255: [6, b'\x00\x01...\xfe\xff'] will be encoded as b'\x06\xff\x00...\xfe\x06\x01\xff'

combined TLV8s

Encoding of two TLV8 Entries that follow each other in the input list:

• the combination of 2 TLV8 entries ([1, 123] and [2, 'Hello']) will be encoded as b'\x01\x01\x7b\x02\x05\x48\x65\x6c\x6c\x6f'

sequences of TLV8s of same type:

• a sequence of 3 TLV8 entries of type 1 ([1, 1], [1, 2] and [1, 1]) will be encoded as b'\x01\x01\x01\xff\x00\x01\x01\x02\xff\x00\x01\x01\x03'

Using in code

There are two main use cases of this module.

Create a bytes representation

Here we want to have a comfortable way to create a data structure in python and to encode this structure into a bytes value.

encode a simple list

For example, create a representation containing the following structure:

• Type: 1, Value: 23
• Type: 2, Value: 2345

This can be code like that:

import tlv8

structure = [
    tlv8.Entry(1, 23),
    tlv8.Entry(2, 2345)
]
bytes_data = tlv8.encode(structure)
print(bytes_data)

And this will result in: b'\x01\x01\x17\x02\x02)\t'

Nesting structures

Representing a line ([x: 10, y: 20] - [x: 30, y: 40]) between to points could be represented like:

• Type: 1, Value:
  • Type: 3, Value: 10
  • Type: 4, Value: 20
• Type: 2, Value:
  • Type: 3, Value: 30
  • Type: 4, Value: 40

import tlv8

structure = [
    tlv8.Entry(1, [
        tlv8.Entry(3, 10),
        tlv8.Entry(4, 10),
    ]),
    tlv8.Entry(2, [
        tlv8.Entry(3, 30),
        tlv8.Entry(4, 40),
    ])
]
bytes_data = tlv8.encode(structure)
print(bytes_data)

And this will result in: b'\x01\x06\x03\x01\n\x04\x01\n\x02\x06\x03\x01\x1e\x04\x01('

Decode a bytes representation

Decoding TLV8 entries from bytes data will return all bytes from all first level entries. This includes possible separator entries between entries of the same type.

Decoding can be assisted by hinting with an expected structure. To represent the structure in python dict objects are used and nested. The keys of the dict objects are the type ids of the TLV8 entries. If the id of an entry is not contained in the structure, it will be ignored.

decode the simple list

import tlv8

in_data = b'\x01\x01\x17\x02\x02)\t'
expected_structure = {
    1: tlv8.DataType.INTEGER,
    2: tlv8.DataType.INTEGER
}
result = tlv8.decode(in_data, expected_structure)

print(tlv8.format_string(result))

This will result in:

[
  <1, 23>,
  <2, 2345>,
]

decode nested data

</code></pre>
<p>This will result in:</p>
<pre lang="text"><code>[
  <1, [
    <3, 10>,
    <4, 10>,
  ]>,
  <2, [
    <3, 30>,
    <4, 40>,
  ]>,
]

Coding

The module offers the following primary functions and classes.

function format_string

This function formats a list of TLV8 Entry objects as str. The hierarchy of the entries will be represented by increasing the indentation of the output.

The parameters are:

• entries: a python list of tlv8.Entries objects
• indent: the level of indentation to be used, this defaults to 0 and is increased on recursive calls for nested entries.

The function returns a str instance and raises ValueError instances if the input is not a list of tlv8.Entry objects.

Example:

import tlv8

data = [
    tlv8.Entry(1, 3.141),
    tlv8.Entry(2, [
        tlv8.Entry(3, 'hello'),
        tlv8.Entry(4, 'world'),
    ]),
    tlv8.Entry(1, 2)
]
print(tlv8.format_string(data))

This will become:

[
  <1, 3.141>,
  <2, [
    <3, hello>,
    <4, world>,
  ]>,
  <1, 2>,
]

function encode

Function to encode a list of tlv8.Entry objects into a sequence of bytes following the rules for creating TLVs. The separator_type_id is used for the separating entries between two entries of the same type.

The parameters are:

• entries: a list of tlv8.Entry objects
• separator_type_id: the 8-bit type id of the separator to be used. The default is (as defined in table 5-6, page 51 of HomeKit Accessory Protocol Specification Non-Commercial Version Release R2) 0xff.

The function returns an instance of bytes. This is empty if nothing was encoded. The function raises ValueError if the input parameter is not a list of tlv8.Entry objects or a data value is not encodable.

Example:

import tlv8

data = [
    tlv8.Entry(1, 3.141),
    tlv8.Entry(2, [
        tlv8.Entry(3, 'hello'),
        tlv8.Entry(4, 'world')
    ]),
    tlv8.Entry(1, 2)
]
print(tlv8.encode(data))

This will result in:

b'\x01\x04%\x06I@\x02\x0e\x03\x05hello\x04\x05world\x01\x01\x02'

function decode

class DataType

This enumeration is used to represent the data type of a tlv8.Entry.

Enumeration Entry	TLV8 type	Python type
BYTES	bytes	bytes
TLV8	tlv8	custom class tlv8.Entry for encoding and dict for the expected structure during decoding
INTEGER	integer	int
FLOAT	float	float
STRING	string	str
AUTODETECT	n/a	this is used declare that a data type is not preset but will be determined by the python type of the data

class Entry

This class represents a single entry in a TLV8 data set. The class overrides the methods __eq__, __str__ and __repr__ to fit the needs of the application.

constructor

The constructor takes the following parameters:

• type_id: the type id of the entry. Must be between 0 and 255 (8-bit type id).
• data: the data to be stored in this entry.
• data_type: the data type of the entry. Defaults to DataType.AUTODETECT.
• length: if set, this overrides the automatic length detection. This used for integer, when there is special need to set higher byte count than the value would need.

The constructor raises a ValueError if the type_id is not within the 8-bit range.

encode() -> bytes

This function is called to encode the data stored in this Entry. The data type of the data will be used to decide how to encode the data. It uses the tlv8.encode() function to encode nested lists of tlv8.Entry objects.

format_string() -> str

This function formats the data stored in this entry as readable string. It is mostly called by tlv8.format_string().