320 lines
10 KiB
Python
320 lines
10 KiB
Python
"""Decoders for GPG v2 data structures."""
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import base64
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import functools
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import hashlib
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import io
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import logging
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import struct
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import subprocess
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import ecdsa
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import ed25519
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from . import proto
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from .. import util
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log = logging.getLogger(__name__)
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def parse_subpackets(s):
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"""See https://tools.ietf.org/html/rfc4880#section-5.2.3.1 for details."""
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subpackets = []
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total_size = s.readfmt('>H')
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data = s.read(total_size)
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s = util.Reader(io.BytesIO(data))
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while True:
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try:
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subpacket_len = s.readfmt('B')
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except EOFError:
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break
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subpackets.append(s.read(subpacket_len))
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return subpackets
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def parse_mpi(s):
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"""See https://tools.ietf.org/html/rfc4880#section-3.2 for details."""
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bits = s.readfmt('>H')
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blob = bytearray(s.read(int((bits + 7) // 8)))
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return sum(v << (8 * i) for i, v in enumerate(reversed(blob)))
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def _parse_nist256p1_verifier(mpi):
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prefix, x, y = util.split_bits(mpi, 4, 256, 256)
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assert prefix == 4
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point = ecdsa.ellipticcurve.Point(curve=ecdsa.NIST256p.curve,
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x=x, y=y)
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vk = ecdsa.VerifyingKey.from_public_point(
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point=point, curve=ecdsa.curves.NIST256p,
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hashfunc=hashlib.sha256)
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def _nist256p1_verify(signature, digest):
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result = vk.verify_digest(signature=signature,
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digest=digest,
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sigdecode=lambda rs, order: rs)
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log.debug('nist256p1 ECDSA signature is OK (%s)', result)
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return _nist256p1_verify, vk
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def _parse_ed25519_verifier(mpi):
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prefix, value = util.split_bits(mpi, 8, 256)
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assert prefix == 0x40
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vk = ed25519.VerifyingKey(util.num2bytes(value, size=32))
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def _ed25519_verify(signature, digest):
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sig = b''.join(util.num2bytes(val, size=32)
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for val in signature)
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result = vk.verify(sig, digest)
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log.debug('ed25519 ECDSA signature is OK (%s)', result)
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return _ed25519_verify, vk
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def _create_rsa_verifier(n, e):
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def verifier(signature, digest):
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s, = signature
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size = n.bit_length()
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result = pow(s, e, n) % (2 ** 256)
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digest = util.bytes2num(digest)
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if result == digest:
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log.debug('RSA-%d signature is OK', size)
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return True
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else:
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raise ValueError('invalid RSA signature')
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return verifier
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SUPPORTED_CURVES = {
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b'\x2A\x86\x48\xCE\x3D\x03\x01\x07': _parse_nist256p1_verifier,
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b'\x2B\x06\x01\x04\x01\xDA\x47\x0F\x01': _parse_ed25519_verifier,
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}
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ECDSA_ALGO_IDS = (19, 22) # (nist256, ed25519)
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def _parse_literal(stream):
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"""See https://tools.ietf.org/html/rfc4880#section-5.9 for details."""
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p = {'type': 'literal'}
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p['format'] = stream.readfmt('c')
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filename_len = stream.readfmt('B')
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p['filename'] = stream.read(filename_len)
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p['date'] = stream.readfmt('>L')
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p['content'] = stream.read()
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p['_to_hash'] = p['content']
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return p
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def _parse_embedded_signatures(subpackets):
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for packet in subpackets:
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data = bytearray(packet)
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if data[0] == 32:
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# https://tools.ietf.org/html/rfc4880#section-5.2.3.26
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stream = io.BytesIO(data[1:])
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yield _parse_signature(util.Reader(stream))
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def _parse_signature(stream):
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"""See https://tools.ietf.org/html/rfc4880#section-5.2 for details."""
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p = {'type': 'signature'}
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to_hash = io.BytesIO()
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with stream.capture(to_hash):
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p['version'] = stream.readfmt('B')
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p['sig_type'] = stream.readfmt('B')
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p['pubkey_alg'] = stream.readfmt('B')
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p['hash_alg'] = stream.readfmt('B')
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p['hashed_subpackets'] = parse_subpackets(stream)
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# https://tools.ietf.org/html/rfc4880#section-5.2.4
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tail_to_hash = b'\x04\xff' + struct.pack('>L', to_hash.tell())
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p['_to_hash'] = to_hash.getvalue() + tail_to_hash
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p['unhashed_subpackets'] = parse_subpackets(stream)
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embedded = list(_parse_embedded_signatures(p['unhashed_subpackets']))
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if embedded:
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log.debug('embedded sigs: %s', embedded)
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p['embedded'] = embedded
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p['_is_custom'] = (proto.CUSTOM_SUBPACKET in p['unhashed_subpackets'])
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p['hash_prefix'] = stream.readfmt('2s')
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if p['pubkey_alg'] in ECDSA_ALGO_IDS:
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p['sig'] = (parse_mpi(stream), parse_mpi(stream))
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else: # RSA
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p['sig'] = (parse_mpi(stream),)
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assert not stream.read()
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return p
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def _parse_pubkey(stream, packet_type='pubkey'):
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"""See https://tools.ietf.org/html/rfc4880#section-5.5 for details."""
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p = {'type': packet_type}
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packet = io.BytesIO()
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with stream.capture(packet):
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p['version'] = stream.readfmt('B')
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p['created'] = stream.readfmt('>L')
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p['algo'] = stream.readfmt('B')
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if p['algo'] in ECDSA_ALGO_IDS:
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# https://tools.ietf.org/html/rfc6637#section-11
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oid_size = stream.readfmt('B')
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oid = stream.read(oid_size)
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assert oid in SUPPORTED_CURVES, util.hexlify(oid)
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parser = SUPPORTED_CURVES[oid]
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mpi = parse_mpi(stream)
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log.debug('mpi: %x (%d bits)', mpi, mpi.bit_length())
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p['verifier'], p['verifying_key'] = parser(mpi)
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else: # RSA
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n = parse_mpi(stream)
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e = parse_mpi(stream)
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log.debug('n: %x (%d bits)', n, n.bit_length())
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log.debug('e: %x (%d bits)', e, e.bit_length())
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p['verifier'] = _create_rsa_verifier(n, e)
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assert not stream.read()
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# https://tools.ietf.org/html/rfc4880#section-12.2
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packet_data = packet.getvalue()
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data_to_hash = (b'\x99' + struct.pack('>H', len(packet_data)) +
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packet_data)
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p['key_id'] = hashlib.sha1(data_to_hash).digest()[-8:]
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p['_to_hash'] = data_to_hash
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log.debug('key ID: %s', util.hexlify(p['key_id']))
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return p
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_parse_subkey = functools.partial(_parse_pubkey, packet_type='subkey')
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def _parse_user_id(stream):
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"""See https://tools.ietf.org/html/rfc4880#section-5.11 for details."""
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value = stream.read()
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to_hash = b'\xb4' + util.prefix_len('>L', value)
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return {'type': 'user_id', 'value': value, '_to_hash': to_hash}
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PACKET_TYPES = {
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2: _parse_signature,
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6: _parse_pubkey,
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11: _parse_literal,
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13: _parse_user_id,
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14: _parse_subkey,
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}
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def parse_packets(stream):
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"""
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Support iterative parsing of available GPG packets.
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See https://tools.ietf.org/html/rfc4880#section-4.2 for details.
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"""
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while True:
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try:
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value = stream.readfmt('B')
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except EOFError:
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return
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log.debug('prefix byte: %s', bin(value))
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assert util.bit(value, 7) == 1
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assert util.bit(value, 6) == 0 # new format not supported yet
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tag = util.low_bits(value, 6)
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length_type = util.low_bits(tag, 2)
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tag = tag >> 2
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fmt = {0: '>B', 1: '>H', 2: '>L'}[length_type]
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packet_size = stream.readfmt(fmt)
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log.debug('packet length: %d', packet_size)
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packet_data = stream.read(packet_size)
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packet_type = PACKET_TYPES.get(tag)
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if packet_type:
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p = packet_type(util.Reader(io.BytesIO(packet_data)))
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else:
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raise ValueError('Unknown packet type: {}'.format(tag))
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p['tag'] = tag
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log.debug('packet "%s": %s', p['type'], p)
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yield p
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def digest_packets(packets):
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"""Compute digest on specified packets, according to '_to_hash' field."""
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data_to_hash = io.BytesIO()
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for p in packets:
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data_to_hash.write(p['_to_hash'])
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return hashlib.sha256(data_to_hash.getvalue()).digest()
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def load_public_key(stream, use_custom=False):
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"""Parse and validate GPG public key from an input stream."""
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packets = list(parse_packets(util.Reader(stream)))
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pubkey, userid, signature = packets[:3]
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packets = packets[3:]
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digest = digest_packets([pubkey, userid, signature])
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assert signature['hash_prefix'] == digest[:2]
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log.debug('loaded public key "%s"', userid['value'])
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verify_digest(pubkey=pubkey, digest=digest,
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signature=signature['sig'], label='GPG public key')
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packet = pubkey
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while use_custom:
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if packet['type'] in ('pubkey', 'subkey') and signature['_is_custom']:
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log.debug('found custom %s', packet['type'])
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break
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packet, signature = packets[:2]
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packets = packets[2:]
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packet['user_id'] = userid['value']
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return packet
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def load_signature(stream, original_data):
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"""Load signature from stream, and compute GPG digest for verification."""
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signature, = list(parse_packets(util.Reader(stream)))
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digest = digest_packets([{'_to_hash': original_data}, signature])
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assert signature['hash_prefix'] == digest[:2]
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return signature, digest
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def load_from_gpg(user_id, use_custom=False):
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"""Load existing GPG public key for `user_id` from local keyring."""
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args = ['gpg2', '--export'] + ([user_id] if user_id else [])
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pubkey_bytes = subprocess.check_output(args=args)
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if pubkey_bytes:
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return load_public_key(io.BytesIO(pubkey_bytes), use_custom=use_custom)
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else:
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log.error('could not find public key %r in local GPG keyring', user_id)
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raise KeyError(user_id)
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def verify_digest(pubkey, digest, signature, label):
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"""Verify a digest signature from a specified public key."""
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verifier = pubkey['verifier']
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try:
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verifier(signature, digest)
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log.debug('%s is OK', label)
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except ecdsa.keys.BadSignatureError:
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log.error('Bad %s!', label)
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raise
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def verify(pubkey, signature, original_data):
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"""Verify correctness of public key and signature."""
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stream = io.BytesIO(signature)
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# remove GPG armor
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lines = stream.readlines()[3:-1]
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data = base64.b64decode(''.join(lines))
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payload, checksum = data[:-3], data[-3:]
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assert util.crc24(payload) == checksum
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stream = io.BytesIO(payload)
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signature, digest = load_signature(stream, original_data)
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verify_digest(pubkey=pubkey, digest=digest,
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signature=signature['sig'], label='GPG signature')
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