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# Copyright (c) 2021 Jeff Irion and contributors
#
# This file is part of the adb-shell package. It was originally written by
# @joeleong, and it was obtained from: https://github.com/google/python-adb/pull/144
"""This file implements encoding and decoding logic for Android's custom RSA
public key binary format. Public keys are stored as a sequence of
little-endian 32 bit words. Note that Android only supports little-endian
processors, so we don't do any byte order conversions when parsing the binary
struct.
Structure from:
https://github.com/aosp-mirror/platform_system_core/blob/c55fab4a59cfa461857c6a61d8a0f1ae4591900c/libcrypto_utils/android_pubkey.c
.. code-block:: c
typedef struct RSAPublicKey {
// Modulus length. This must be ANDROID_PUBKEY_MODULUS_SIZE_WORDS
uint32_t modulus_size_words;
// Precomputed montgomery parameter: -1 / n[0] mod 2^32
uint32_t n0inv;
// RSA modulus as a little-endian array
uint8_t modulus[ANDROID_PUBKEY_MODULUS_SIZE];
// Montgomery parameter R^2 as a little-endian array of little-endian words
uint8_t rr[ANDROID_PUBKEY_MODULUS_SIZE];
// RSA modulus: 3 or 65537
uint32_t exponent;
} RSAPublicKey;
.. rubric:: Contents
* :func:`_to_bytes`
* :func:`decode_pubkey`
* :func:`decode_pubkey_file`
* :func:`encode_pubkey`
* :func:`get_user_info`
* :func:`keygen`
* :func:`write_public_keyfile`
"""
import os
import base64
import logging
import socket
import struct
import sys
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
_LOGGER = logging.getLogger(__name__)
if sys.version_info[0] == 2: # pragma: no cover
FileNotFoundError = IOError # pylint: disable=redefined-builtin
#: Size of an RSA modulus such as an encrypted block or a signature.
ANDROID_PUBKEY_MODULUS_SIZE = 2048 // 8
#: Python representation of "struct RSAPublicKey"
ANDROID_RSAPUBLICKEY_STRUCT = (
'<' # Little-endian
'L' # uint32_t modulus_size_words;
'L' # uint32_t n0inv;
'{modulus_size}s' # uint8_t modulus[ANDROID_PUBKEY_MODULUS_SIZE];
'{modulus_size}s' # uint8_t rr[ANDROID_PUBKEY_MODULUS_SIZE];
'L' # uint32_t exponent;
).format(modulus_size=ANDROID_PUBKEY_MODULUS_SIZE)
#: Size of the RSA modulus in words.
ANDROID_PUBKEY_MODULUS_SIZE_WORDS = ANDROID_PUBKEY_MODULUS_SIZE // 4
def _to_bytes(n, length, endianess='big'):
"""Partial python2 compatibility with int.to_bytes
https://stackoverflow.com/a/20793663
Parameters
----------
n : TODO
TODO
length : TODO
TODO
endianess : str, TODO
TODO
Returns
-------
TODO
TODO
"""
if not hasattr(n, 'to_bytes'):
h = '{:x}'.format(n)
s = ('0' * (len(h) % 2) + h).zfill(length * 2).decode('hex')
return s if endianess == 'big' else s[::-1]
return n.to_bytes(length, endianess)
def decode_pubkey(public_key):
"""Decode a public RSA key stored in Android's custom binary format.
Parameters
----------
public_key : TODO
TODO
"""
binary_key_data = base64.b64decode(public_key)
modulus_size_words, n0inv, modulus_bytes, rr_bytes, exponent = struct.unpack(ANDROID_RSAPUBLICKEY_STRUCT, binary_key_data)
assert modulus_size_words == ANDROID_PUBKEY_MODULUS_SIZE_WORDS
modulus = reversed(modulus_bytes)
rr = reversed(rr_bytes)
_LOGGER.debug('modulus_size_words: %s', hex(modulus_size_words))
_LOGGER.debug('n0inv: %s', hex(n0inv))
_LOGGER.debug('modulus: %s', ':'.join((hex(m) for m in modulus)))
_LOGGER.debug('rr: %s', ':'.join((hex(r) for r in rr)))
_LOGGER.debug('exponent: %s', hex(exponent))
def decode_pubkey_file(public_key_path):
"""TODO
Parameters
----------
public_key_path : str
TODO
"""
with open(public_key_path, 'rb') as fd:
decode_pubkey(fd.read())
def encode_pubkey(private_key_path):
"""Encodes a public RSA key into Android's custom binary format.
Parameters
----------
private_key_path : str
TODO
Returns
-------
TODO
TODO
"""
with open(private_key_path, 'rb') as key_file:
key = serialization.load_pem_private_key(key_file.read(), password=None, backend=default_backend()).private_numbers().public_numbers
# Compute and store n0inv = -1 / N[0] mod 2^32.
# BN_set_bit(r32, 32)
r32 = 1 << 32
# BN_mod(n0inv, key->n, r32, ctx)
n0inv = key.n % r32
# BN_mod_inverse(n0inv, n0inv, r32, ctx)
n0inv = rsa._modinv(n0inv, r32) # pylint: disable=protected-access
# BN_sub(n0inv, r32, n0inv)
n0inv = r32 - n0inv
# Compute and store rr = (2^(rsa_size)) ^ 2 mod N.
# BN_set_bit(rr, ANDROID_PUBKEY_MODULUS_SIZE * 8)
rr = 1 << (ANDROID_PUBKEY_MODULUS_SIZE * 8)
# BN_mod_sqr(rr, rr, key->n, ctx)
rr = (rr ** 2) % key.n
return struct.pack(
ANDROID_RSAPUBLICKEY_STRUCT,
ANDROID_PUBKEY_MODULUS_SIZE_WORDS,
n0inv,
_to_bytes(key.n, ANDROID_PUBKEY_MODULUS_SIZE, 'little'),
_to_bytes(rr, ANDROID_PUBKEY_MODULUS_SIZE, 'little'),
key.e
)
def get_user_info():
"""TODO
Returns
-------
str
``' <username>@<hostname>``
"""
try:
username = os.getlogin()
except (FileNotFoundError, OSError):
username = 'unknown'
if not username:
username = 'unknown'
hostname = socket.gethostname()
if not hostname:
hostname = 'unknown'
return ' ' + username + '@' + hostname
def write_public_keyfile(private_key_path, public_key_path):
"""Write a public keyfile to ``public_key_path`` in Android's custom
RSA public key format given a path to a private keyfile.
Parameters
----------
private_key_path : TODO
TODO
public_key_path : TODO
TODO
"""
public_key = encode_pubkey(private_key_path)
assert len(public_key) == struct.calcsize(ANDROID_RSAPUBLICKEY_STRUCT)
with open(public_key_path, 'wb') as public_key_file:
public_key_file.write(base64.b64encode(public_key))
public_key_file.write(get_user_info().encode())
def keygen(filepath):
"""Generate an ADB public/private key pair.
* The private key is stored in ``filepath``.
* The public key is stored in ``filepath + '.pub'``
(Existing files will be overwritten.)
Parameters
----------
filepath : str
File path to write the private/public keypair
"""
private_key = rsa.generate_private_key(public_exponent=65537, key_size=2048, backend=default_backend())
with open(filepath, 'wb') as private_key_file:
private_key_file.write(private_key.private_bytes(encoding=serialization.Encoding.PEM, format=serialization.PrivateFormat.PKCS8, encryption_algorithm=serialization.NoEncryption()))
write_public_keyfile(filepath, filepath + '.pub')
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