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# Copyright (c) 2019-2024 by Ron Frederick <ronf@timeheart.net> and others.
#
# This program and the accompanying materials are made available under
# the terms of the Eclipse Public License v2.0 which accompanies this
# distribution and is available at:
#
# http://www.eclipse.org/legal/epl-2.0/
#
# This program may also be made available under the following secondary
# licenses when the conditions for such availability set forth in the
# Eclipse Public License v2.0 are satisfied:
#
# GNU General Public License, Version 2.0, or any later versions of
# that license
#
# SPDX-License-Identifier: EPL-2.0 OR GPL-2.0-or-later
#
# Contributors:
# Ron Frederick - initial implementation, API, and documentation
"""U2F ECDSA public key encryption handler"""
from hashlib import sha256
from typing import Optional, Tuple, cast
from .asn1 import der_encode, der_decode
from .crypto import ECDSAPublicKey
from .packet import Byte, MPInt, String, UInt32, SSHPacket
from .public_key import KeyExportError, SSHKey, SSHOpenSSHCertificateV01
from .public_key import register_public_key_alg, register_certificate_alg
from .public_key import register_sk_alg
from .sk import SSH_SK_ECDSA, SSH_SK_USER_PRESENCE_REQD
from .sk import sk_enroll, sk_sign, sk_webauthn_prefix, sk_use_webauthn
_PrivateKeyArgs = Tuple[bytes, bytes, str, int, bytes, bytes]
_PublicKeyArgs = Tuple[bytes, bytes, str]
class _SKECDSAKey(SSHKey):
"""Handler for U2F ECDSA public key encryption"""
_key: ECDSAPublicKey
use_executor = True
def __init__(self, curve_id: bytes, public_value: bytes, application: str,
flags: int = 0, key_handle: Optional[bytes] = None,
reserved: bytes = b''):
super().__init__(ECDSAPublicKey.construct(curve_id, public_value))
self.algorithm = b'sk-ecdsa-sha2-' + curve_id + b'@openssh.com'
self.sig_algorithms = (self.algorithm, b'webauthn-' + self.algorithm)
self.all_sig_algorithms = set(self.sig_algorithms)
self.use_webauthn = sk_use_webauthn
self._application = application
self._app_hash = sha256(application.encode('utf-8')).digest()
self._flags = flags
self._key_handle = key_handle
self._reserved = reserved
def __eq__(self, other: object) -> bool:
# This isn't protected access - both objects are _SKECDSAKey instances
# pylint: disable=protected-access
return (isinstance(other, type(self)) and
self._key.curve_id == other._key.curve_id and
self._key.public_value == other._key.public_value and
self._application == other._application and
self._flags == other._flags and
self._key_handle == other._key_handle and
self._reserved == other._reserved)
def __hash__(self) -> int:
return hash((self._key.curve_id, self._key.public_value,
self._application, self._flags, self._key_handle,
self._reserved))
@classmethod
def generate(cls, algorithm: bytes, *, # type: ignore
application: str = 'ssh:', user: str = 'AsyncSSH',
pin: Optional[str] = None, resident: bool = False,
touch_required: bool = True) -> '_SKECDSAKey':
"""Generate a new SK ECDSA private key"""
# pylint: disable=arguments-differ
flags = SSH_SK_USER_PRESENCE_REQD if touch_required else 0
public_value, key_handle = sk_enroll(SSH_SK_ECDSA, application,
user, pin, resident)
# Strip prefix and suffix of algorithm to get curve_id
return cls(algorithm[14:-12], public_value, application,
flags, key_handle, b'')
@classmethod
def make_private(cls, key_params: object) -> SSHKey:
"""Construct a U2F ECDSA private key"""
curve_id, public_value, application, flags, key_handle, reserved = \
cast(_PrivateKeyArgs, key_params)
return cls(curve_id, public_value, application,
flags, key_handle, reserved)
@classmethod
def make_public(cls, key_params: object) -> SSHKey:
"""Construct a U2F ECDSA public key"""
curve_id, public_value, application = cast(_PublicKeyArgs, key_params)
return cls(curve_id, public_value, application)
@classmethod
def decode_ssh_private(cls, packet: SSHPacket) -> _PrivateKeyArgs:
"""Decode an SSH format SK ECDSA private key"""
curve_id = packet.get_string()
public_value = packet.get_string()
application = packet.get_string().decode('utf-8')
flags = packet.get_byte()
key_handle = packet.get_string()
reserved = packet.get_string()
return curve_id, public_value, application, flags, key_handle, reserved
@classmethod
def decode_ssh_public(cls, packet: SSHPacket) -> _PublicKeyArgs:
"""Decode an SSH format SK ECDSA public key"""
curve_id = packet.get_string()
public_value = packet.get_string()
application = packet.get_string().decode('utf-8')
return curve_id, public_value, application
def encode_ssh_private(self) -> bytes:
"""Encode an SSH format SK ECDSA private key"""
if self._key_handle is None:
raise KeyExportError('Key is not private')
return b''.join((String(self._key.curve_id),
String(self._key.public_value),
String(self._application), Byte(self._flags),
String(self._key_handle), String(self._reserved)))
def encode_ssh_public(self) -> bytes:
"""Encode an SSH format SK ECDSA public key"""
return b''.join((String(self._key.curve_id),
String(self._key.public_value),
String(self._application)))
def encode_agent_cert_private(self) -> bytes:
"""Encode U2F ECDSA certificate private key data for agent"""
if self._key_handle is None:
raise KeyExportError('Key is not private')
return b''.join((String(self._application), Byte(self._flags),
String(self._key_handle), String(self._reserved)))
def sign_ssh(self, data: bytes, sig_algorithm: bytes) -> bytes:
"""Compute an SSH-encoded signature of the specified data"""
if self._key_handle is None:
raise ValueError('Key handle needed for signing')
is_webauthn = sig_algorithm.startswith(b'webauthn')
flags, counter, sig, client_data = sk_sign(data, self._application,
self._key_handle,
self._flags, is_webauthn)
r, s = cast(Tuple[int, int], der_decode(sig))
sig = String(MPInt(r) + MPInt(s)) + Byte(flags) + UInt32(counter)
if is_webauthn:
sig += String(self._application) + String(client_data) + String('')
return sig
def verify_ssh(self, data: bytes, sig_algorithm: bytes,
packet: SSHPacket) -> bool:
"""Verify an SSH-encoded signature of the specified data"""
is_webauthn = sig_algorithm.startswith(b'webauthn')
sig = packet.get_string()
flags = packet.get_byte()
counter = packet.get_uint32()
if is_webauthn:
_ = packet.get_string() # origin
client_data = packet.get_string()
_ = packet.get_string() # extensions
prefix = sk_webauthn_prefix(data, self._application)
if not client_data.startswith(prefix):
return False
data = client_data
packet.check_end()
if self._touch_required and not flags & SSH_SK_USER_PRESENCE_REQD:
return False
packet = SSHPacket(sig)
r = packet.get_mpint()
s = packet.get_mpint()
packet.check_end()
sig = der_encode((r, s))
return self._key.verify(self._app_hash + Byte(flags) +
UInt32(counter) + sha256(data).digest(),
sig, 'sha256')
_algorithm = b'sk-ecdsa-sha2-nistp256@openssh.com'
_cert_algorithm = b'sk-ecdsa-sha2-nistp256-cert-v01@openssh.com'
register_sk_alg(SSH_SK_ECDSA, _SKECDSAKey, b'nistp256')
register_public_key_alg(_algorithm, _SKECDSAKey, True,
(_algorithm, b'webauthn-' + _algorithm))
register_certificate_alg(1, _algorithm, _cert_algorithm,
_SKECDSAKey, SSHOpenSSHCertificateV01, True)
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