# -*- coding: utf-8 -*-
"""
hpack/hpack
~~~~~~~~~~~
Implements the HPACK header compression algorithm as detailed by the IETF.
"""
import logging
from .table import HeaderTable, table_entry_size
from .exceptions import (
HPACKDecodingError, OversizedHeaderListError, InvalidTableSizeError
)
from .huffman import HuffmanEncoder
from .huffman_constants import (
REQUEST_CODES, REQUEST_CODES_LENGTH
)
from .huffman_table import decode_huffman
from .struct import HeaderTuple, NeverIndexedHeaderTuple
log = logging.getLogger(__name__)
INDEX_NONE = b'\x00'
INDEX_NEVER = b'\x10'
INDEX_INCREMENTAL = b'\x40'
# Precompute 2^i for 1-8 for use in prefix calcs.
# Zero index is not used but there to save a subtraction
# as prefix numbers are not zero indexed.
_PREFIX_BIT_MAX_NUMBERS = [(2 ** i) - 1 for i in range(9)]
try: # pragma: no cover
basestring = basestring
except NameError: # pragma: no cover
basestring = (str, bytes)
# We default the maximum header list we're willing to accept to 64kB. That's a
# lot of headers, but if applications want to raise it they can do.
DEFAULT_MAX_HEADER_LIST_SIZE = 2 ** 16
def _unicode_if_needed(header, raw):
"""
Provides a header as a unicode string if raw is False, otherwise returns
it as a bytestring.
"""
name = bytes(header[0])
value = bytes(header[1])
if not raw:
name = name.decode('utf-8')
value = value.decode('utf-8')
return header.__class__(name, value)
def encode_integer(integer, prefix_bits):
"""
This encodes an integer according to the wacky integer encoding rules
defined in the HPACK spec.
"""
log.debug("Encoding %d with %d bits", integer, prefix_bits)
if integer < 0:
raise ValueError(
"Can only encode positive integers, got %s" % integer
)
if prefix_bits < 1 or prefix_bits > 8:
raise ValueError(
"Prefix bits must be between 1 and 8, got %s" % prefix_bits
)
max_number = _PREFIX_BIT_MAX_NUMBERS[prefix_bits]
if integer < max_number:
return bytearray([integer]) # Seriously?
else:
elements = [max_number]
integer -= max_number
while integer >= 128:
elements.append((integer & 127) + 128)
integer >>= 7
elements.append(integer)
return bytearray(elements)
def decode_integer(data, prefix_bits):
"""
This decodes an integer according to the wacky integer encoding rules
defined in the HPACK spec. Returns a tuple of the decoded integer and the
number of bytes that were consumed from ``data`` in order to get that
integer.
"""
if prefix_bits < 1 or prefix_bits > 8:
raise ValueError(
"Prefix bits must be between 1 and 8, got %s" % prefix_bits
)
max_number = _PREFIX_BIT_MAX_NUMBERS[prefix_bits]
index = 1
shift = 0
mask = (0xFF >> (8 - prefix_bits))
try:
number = data[0] & mask
if number == max_number:
while True:
next_byte = data[index]
index += 1
if next_byte >= 128:
number += (next_byte - 128) << shift
else:
number += next_byte << shift
break
shift += 7
except IndexError:
raise HPACKDecodingError(
"Unable to decode HPACK integer representation from %r" % data
)
log.debug("Decoded %d, consumed %d bytes", number, index)
return number, index
def _dict_to_iterable(header_dict):
"""
This converts a dictionary to an iterable of two-tuples. This is a
HPACK-specific function because it pulls "special-headers" out first and
then emits them.
"""
assert isinstance(header_dict, dict)
keys = sorted(
header_dict.keys(),
key=lambda k: not _to_bytes(k).startswith(b':')
)
for key in keys:
yield key, header_dict[key]
def _to_bytes(string):
"""
Convert string to bytes.
"""
if not isinstance(string, basestring): # pragma: no cover
string = str(string)
return string if isinstance(string, bytes) else string.encode('utf-8')
[docs]class Encoder:
"""
An HPACK encoder object. This object takes HTTP headers and emits encoded
HTTP/2 header blocks.
"""
def __init__(self):
self.header_table = HeaderTable()
self.huffman_coder = HuffmanEncoder(
REQUEST_CODES, REQUEST_CODES_LENGTH
)
self.table_size_changes = []
@property
def header_table_size(self):
"""
Controls the size of the HPACK header table.
"""
return self.header_table.maxsize
@header_table_size.setter
def header_table_size(self, value):
self.header_table.maxsize = value
if self.header_table.resized:
self.table_size_changes.append(value)
[docs] def encode(self, headers, huffman=True):
"""
Takes a set of headers and encodes them into a HPACK-encoded header
block.
:param headers: The headers to encode. Must be either an iterable of
tuples, an iterable of :class:`HeaderTuple
<hpack.HeaderTuple>`, or a ``dict``.
If an iterable of tuples, the tuples may be either
two-tuples or three-tuples. If they are two-tuples, the
tuples must be of the format ``(name, value)``. If they
are three-tuples, they must be of the format
``(name, value, sensitive)``, where ``sensitive`` is a
boolean value indicating whether the header should be
added to header tables anywhere. If not present,
``sensitive`` defaults to ``False``.
If an iterable of :class:`HeaderTuple
<hpack.HeaderTuple>`, the tuples must always be
two-tuples. Instead of using ``sensitive`` as a third
tuple entry, use :class:`NeverIndexedHeaderTuple
<hpack.NeverIndexedHeaderTuple>` to request that
the field never be indexed.
.. warning:: HTTP/2 requires that all special headers
(headers whose names begin with ``:`` characters)
appear at the *start* of the header block. While
this method will ensure that happens for ``dict``
subclasses, callers using any other iterable of
tuples **must** ensure they place their special
headers at the start of the iterable.
For efficiency reasons users should prefer to use
iterables of two-tuples: fixing the ordering of
dictionary headers is an expensive operation that
should be avoided if possible.
:param huffman: (optional) Whether to Huffman-encode any header sent as
a literal value. Except for use when debugging, it is
recommended that this be left enabled.
:returns: A bytestring containing the HPACK-encoded header block.
"""
# Transforming the headers into a header block is a procedure that can
# be modeled as a chain or pipe. First, the headers are encoded. This
# encoding can be done a number of ways. If the header name-value pair
# are already in the header table we can represent them using the
# indexed representation: the same is true if they are in the static
# table. Otherwise, a literal representation will be used.
header_block = []
# Turn the headers into a list of tuples if possible. This is the
# natural way to interact with them in HPACK. Because dictionaries are
# un-ordered, we need to make sure we grab the "special" headers first.
if isinstance(headers, dict):
headers = _dict_to_iterable(headers)
# Before we begin, if the header table size has been changed we need
# to signal all changes since last emission appropriately.
if self.header_table.resized:
header_block.append(self._encode_table_size_change())
self.header_table.resized = False
# Add each header to the header block
for header in headers:
sensitive = False
if isinstance(header, HeaderTuple):
sensitive = not header.indexable
elif len(header) > 2:
sensitive = header[2]
header = (_to_bytes(header[0]), _to_bytes(header[1]))
header_block.append(self.add(header, sensitive, huffman))
header_block = b''.join(header_block)
log.debug("Encoded header block to %s", header_block)
return header_block
def add(self, to_add, sensitive, huffman=False):
"""
This function takes a header key-value tuple and serializes it.
"""
log.debug(
"Adding %s to the header table, sensitive:%s, huffman:%s",
to_add,
sensitive,
huffman
)
name, value = to_add
# Set our indexing mode
indexbit = INDEX_INCREMENTAL if not sensitive else INDEX_NEVER
# Search for a matching header in the header table.
match = self.header_table.search(name, value)
if match is None:
# Not in the header table. Encode using the literal syntax,
# and add it to the header table.
encoded = self._encode_literal(name, value, indexbit, huffman)
if not sensitive:
self.header_table.add(name, value)
return encoded
# The header is in the table, break out the values. If we matched
# perfectly, we can use the indexed representation: otherwise we
# can use the indexed literal.
index, name, perfect = match
if perfect:
# Indexed representation.
encoded = self._encode_indexed(index)
else:
# Indexed literal. We are going to add header to the
# header table unconditionally. It is a future todo to
# filter out headers which are known to be ineffective for
# indexing since they just take space in the table and
# pushed out other valuable headers.
encoded = self._encode_indexed_literal(
index, value, indexbit, huffman
)
if not sensitive:
self.header_table.add(name, value)
return encoded
def _encode_indexed(self, index):
"""
Encodes a header using the indexed representation.
"""
field = encode_integer(index, 7)
field[0] |= 0x80 # we set the top bit
return bytes(field)
def _encode_literal(self, name, value, indexbit, huffman=False):
"""
Encodes a header with a literal name and literal value. If ``indexing``
is True, the header will be added to the header table: otherwise it
will not.
"""
if huffman:
name = self.huffman_coder.encode(name)
value = self.huffman_coder.encode(value)
name_len = encode_integer(len(name), 7)
value_len = encode_integer(len(value), 7)
if huffman:
name_len[0] |= 0x80
value_len[0] |= 0x80
return b''.join(
[indexbit, bytes(name_len), name, bytes(value_len), value]
)
def _encode_indexed_literal(self, index, value, indexbit, huffman=False):
"""
Encodes a header with an indexed name and a literal value and performs
incremental indexing.
"""
if indexbit != INDEX_INCREMENTAL:
prefix = encode_integer(index, 4)
else:
prefix = encode_integer(index, 6)
prefix[0] |= ord(indexbit)
if huffman:
value = self.huffman_coder.encode(value)
value_len = encode_integer(len(value), 7)
if huffman:
value_len[0] |= 0x80
return b''.join([bytes(prefix), bytes(value_len), value])
def _encode_table_size_change(self):
"""
Produces the encoded form of all header table size change context
updates.
"""
block = b''
for size_bytes in self.table_size_changes:
size_bytes = encode_integer(size_bytes, 5)
size_bytes[0] |= 0x20
block += bytes(size_bytes)
self.table_size_changes = []
return block
[docs]class Decoder:
"""
An HPACK decoder object.
.. versionchanged:: 2.3.0
Added ``max_header_list_size`` argument.
:param max_header_list_size: The maximum decompressed size we will allow
for any single header block. This is a protection against DoS attacks
that attempt to force the application to expand a relatively small
amount of data into a really large header list, allowing enormous
amounts of memory to be allocated.
If this amount of data is exceeded, a `OversizedHeaderListError
<hpack.OversizedHeaderListError>` exception will be raised. At this
point the connection should be shut down, as the HPACK state will no
longer be usable.
Defaults to 64kB.
:type max_header_list_size: ``int``
"""
def __init__(self, max_header_list_size=DEFAULT_MAX_HEADER_LIST_SIZE):
self.header_table = HeaderTable()
#: The maximum decompressed size we will allow for any single header
#: block. This is a protection against DoS attacks that attempt to
#: force the application to expand a relatively small amount of data
#: into a really large header list, allowing enormous amounts of memory
#: to be allocated.
#:
#: If this amount of data is exceeded, a `OversizedHeaderListError
#: <hpack.OversizedHeaderListError>` exception will be raised. At this
#: point the connection should be shut down, as the HPACK state will no
#: longer be usable.
#:
#: Defaults to 64kB.
#:
#: .. versionadded:: 2.3.0
self.max_header_list_size = max_header_list_size
#: Maximum allowed header table size.
#:
#: A HTTP/2 implementation should set this to the most recent value of
#: SETTINGS_HEADER_TABLE_SIZE that it sent *and has received an ACK
#: for*. Once this setting is set, the actual header table size will be
#: checked at the end of each decoding run and whenever it is changed,
#: to confirm that it fits in this size.
self.max_allowed_table_size = self.header_table.maxsize
@property
def header_table_size(self):
"""
Controls the size of the HPACK header table.
"""
return self.header_table.maxsize
@header_table_size.setter
def header_table_size(self, value):
self.header_table.maxsize = value
[docs] def decode(self, data, raw=False):
"""
Takes an HPACK-encoded header block and decodes it into a header set.
:param data: A bytestring representing a complete HPACK-encoded header
block.
:param raw: (optional) Whether to return the headers as tuples of raw
byte strings or to decode them as UTF-8 before returning
them. The default value is False, which returns tuples of
Unicode strings
:returns: A list of two-tuples of ``(name, value)`` representing the
HPACK-encoded headers, in the order they were decoded.
:raises HPACKDecodingError: If an error is encountered while decoding
the header block.
"""
log.debug("Decoding %s", data)
data_mem = memoryview(data)
headers = []
data_len = len(data)
inflated_size = 0
current_index = 0
while current_index < data_len:
# Work out what kind of header we're decoding.
# If the high bit is 1, it's an indexed field.
current = data[current_index]
indexed = True if current & 0x80 else False
# Otherwise, if the second-highest bit is 1 it's a field that does
# alter the header table.
literal_index = True if current & 0x40 else False
# Otherwise, if the third-highest bit is 1 it's an encoding context
# update.
encoding_update = True if current & 0x20 else False
if indexed:
header, consumed = self._decode_indexed(
data_mem[current_index:]
)
elif literal_index:
# It's a literal header that does affect the header table.
header, consumed = self._decode_literal_index(
data_mem[current_index:]
)
elif encoding_update:
# It's an update to the encoding context. These are forbidden
# in a header block after any actual header.
if headers:
raise HPACKDecodingError(
"Table size update not at the start of the block"
)
consumed = self._update_encoding_context(
data_mem[current_index:]
)
header = None
else:
# It's a literal header that does not affect the header table.
header, consumed = self._decode_literal_no_index(
data_mem[current_index:]
)
if header:
headers.append(header)
inflated_size += table_entry_size(*header)
if inflated_size > self.max_header_list_size:
raise OversizedHeaderListError(
"A header list larger than %d has been received" %
self.max_header_list_size
)
current_index += consumed
# Confirm that the table size is lower than the maximum. We do this
# here to ensure that we catch when the max has been *shrunk* and the
# remote peer hasn't actually done that.
self._assert_valid_table_size()
try:
return [_unicode_if_needed(h, raw) for h in headers]
except UnicodeDecodeError:
raise HPACKDecodingError("Unable to decode headers as UTF-8.")
def _assert_valid_table_size(self):
"""
Check that the table size set by the encoder is lower than the maximum
we expect to have.
"""
if self.header_table_size > self.max_allowed_table_size:
raise InvalidTableSizeError(
"Encoder did not shrink table size to within the max"
)
def _update_encoding_context(self, data):
"""
Handles a byte that updates the encoding context.
"""
# We've been asked to resize the header table.
new_size, consumed = decode_integer(data, 5)
if new_size > self.max_allowed_table_size:
raise InvalidTableSizeError(
"Encoder exceeded max allowable table size"
)
self.header_table_size = new_size
return consumed
def _decode_indexed(self, data):
"""
Decodes a header represented using the indexed representation.
"""
index, consumed = decode_integer(data, 7)
header = HeaderTuple(*self.header_table.get_by_index(index))
log.debug("Decoded %s, consumed %d", header, consumed)
return header, consumed
def _decode_literal_no_index(self, data):
return self._decode_literal(data, False)
def _decode_literal_index(self, data):
return self._decode_literal(data, True)
def _decode_literal(self, data, should_index):
"""
Decodes a header represented with a literal.
"""
total_consumed = 0
# When should_index is true, if the low six bits of the first byte are
# nonzero, the header name is indexed.
# When should_index is false, if the low four bits of the first byte
# are nonzero the header name is indexed.
if should_index:
indexed_name = data[0] & 0x3F
name_len = 6
not_indexable = False
else:
high_byte = data[0]
indexed_name = high_byte & 0x0F
name_len = 4
not_indexable = high_byte & 0x10
if indexed_name:
# Indexed header name.
index, consumed = decode_integer(data, name_len)
name = self.header_table.get_by_index(index)[0]
total_consumed = consumed
length = 0
else:
# Literal header name. The first byte was consumed, so we need to
# move forward.
data = data[1:]
length, consumed = decode_integer(data, 7)
name = data[consumed:consumed + length]
if len(name) != length:
raise HPACKDecodingError("Truncated header block")
if data[0] & 0x80:
name = decode_huffman(name)
total_consumed = consumed + length + 1 # Since we moved forward 1.
data = data[consumed + length:]
# The header value is definitely length-based.
length, consumed = decode_integer(data, 7)
value = data[consumed:consumed + length]
if len(value) != length:
raise HPACKDecodingError("Truncated header block")
if data[0] & 0x80:
value = decode_huffman(value)
# Updated the total consumed length.
total_consumed += length + consumed
# If we have been told never to index the header field, encode that in
# the tuple we use.
if not_indexable:
header = NeverIndexedHeaderTuple(name, value)
else:
header = HeaderTuple(name, value)
# If we've been asked to index this, add it to the header table.
if should_index:
self.header_table.add(name, value)
log.debug(
"Decoded %s, total consumed %d bytes, indexed %s",
header,
total_consumed,
should_index
)
return header, total_consumed