src/derivepassphrase/ssh_agent.py (e8f3ec8) - derivepassphrase.git

ssh_agent.py

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# SPDX-FileCopyrightText: 2024 Marco Ricci <software@the13thletter.info>
#
# SPDX-License-Identifier: MIT

"""A bare-bones SSH agent client supporting signing and key listing."""

from __future__ import annotations

import collections
import errno
import os
import socket
from typing import TYPE_CHECKING

from typing_extensions import Self

from derivepassphrase import _types

if TYPE_CHECKING:
    from collections.abc import Sequence
    from types import TracebackType

__all__ = ('SSHAgentClient',)
__author__ = 'Marco Ricci <software@the13thletter.info>'

# In SSH bytestrings, the "length" of the byte string is stored as
# a 4-byte/32-bit unsigned integer at the beginning.
HEAD_LEN = 4

_socket = socket

class TrailingDataError(RuntimeError):
    """The result contained trailing data."""

def __init__(self) -> None:
        super().__init__('Overlong response from SSH agent')

class SSHAgentFailedError(RuntimeError):
    """The SSH agent failed to complete the requested operation."""

def __str__(self) -> str:
        match self.args:
            case (_types.SSH_AGENT.FAILURE.value, b''):  # pragma: no branch
                return 'The SSH agent failed to complete the request'
            case (_, _msg) if _msg:  # pragma: no cover
                code = self.args[0]
                msg = self.args[1].decode('utf-8', 'surrogateescape')
                return f'[Code {code:d}] {msg:s}'
            case _:  # pragma: no cover
                return repr(self)

def __repr__(self) -> str:  # pragma: no cover
        return f'{self.__class__.__name__}{self.args!r}'

class SSHAgentClient:
    """A bare-bones SSH agent client supporting signing and key listing.

The main use case is requesting the agent sign some data, after
    checking that the necessary key is already loaded.

The main fleshed out methods are `list_keys` and `sign`, which
    implement the `REQUEST_IDENTITIES` and `SIGN_REQUEST` requests.  If
    you *really* wanted to, there is enough infrastructure in place to
    issue other requests as defined in the protocol---it's merely the
    wrapper functions and the protocol numbers table that are missing.

"""

_connection: socket.socket

def __init__(
        self, /, *, socket: socket.socket | None = None, timeout: int = 125
    ) -> None:
        """Initialize the client.

Args:
            socket:
                An optional socket, connected to the SSH agent.  If not
                given, we query the `SSH_AUTH_SOCK` environment
                variable to auto-discover the correct socket address.
            timeout:
                A connection timeout for the SSH agent.  Only used if
                the socket is not yet connected.  The default value
                gives ample time for agent connections forwarded via
                SSH on high-latency networks (e.g. Tor).

Raises:
            KeyError:
                The `SSH_AUTH_SOCK` environment was not found.
            OSError:
                There was an error setting up a socket connection to the
                agent.

"""
        if socket is not None:
            self._connection = socket
        else:
            self._connection = _socket.socket(family=_socket.AF_UNIX)
        try:
            # Test whether the socket is connected.
            self._connection.getpeername()
        except OSError as e:
            # This condition is hard to test purposefully, so exclude
            # from coverage.
            if e.errno != errno.ENOTCONN:  # pragma: no cover
                raise
            if 'SSH_AUTH_SOCK' not in os.environ:
                msg = 'SSH_AUTH_SOCK environment variable'
                raise KeyError(msg) from None
            ssh_auth_sock = os.environ['SSH_AUTH_SOCK']
            self._connection.settimeout(timeout)
            self._connection.connect(ssh_auth_sock)

def __enter__(self) -> Self:
        """Close socket connection upon context manager completion.

Returns:
            Self.

"""
        self._connection.__enter__()
        return self

def __exit__(
        self,
        exc_type: type[BaseException] | None,
        exc_val: BaseException | None,
        exc_tb: TracebackType | None,
    ) -> bool:
        """Close socket connection upon context manager completion.

Args:
            exc_type: An optional exception type.
            exc_val: An optional exception value.
            exc_tb: An optional exception traceback.

Returns:
            True if the exception was handled, false if it should
            propagate.

"""
        return bool(
            self._connection.__exit__(exc_type, exc_val, exc_tb)  # type: ignore[func-returns-value]
        )

@staticmethod
    def uint32(num: int, /) -> bytes:
        r"""Format the number as a `uint32`, as per the agent protocol.

Args:
            num: A number.

Returns:
            The number in SSH agent wire protocol format, i.e. as
            a 32-bit big endian number.

Raises:
            OverflowError:
                As per [`int.to_bytes`][].

Examples:
            >>> SSHAgentClient.uint32(16777216)
            b'\x01\x00\x00\x00'

"""
        return int.to_bytes(num, 4, 'big', signed=False)

@classmethod
    def string(cls, payload: bytes | bytearray, /) -> bytes | bytearray:
        r"""Format the payload as an SSH string, as per the agent protocol.

Args:
            payload: A byte string.

Returns:
            The payload, framed in the SSH agent wire protocol format.

Examples:
            >>> bytes(SSHAgentClient.string(b'ssh-rsa'))
            b'\x00\x00\x00\x07ssh-rsa'

"""
        try:
            ret = bytearray()
            ret.extend(cls.uint32(len(payload)))
            ret.extend(payload)
        except Exception as e:
            msg = 'invalid payload type'
            raise TypeError(msg) from e  # noqa: DOC501
        return ret

@classmethod
    def unstring(cls, bytestring: bytes | bytearray, /) -> bytes | bytearray:
        r"""Unpack an SSH string.

Args:
            bytestring: A framed byte string.

Returns:
            The unframed byte string, i.e., the payload.

Raises:
            ValueError:
                The byte string is not an SSH string.

Examples:
            >>> bytes(SSHAgentClient.unstring(b'\x00\x00\x00\x07ssh-rsa'))
            b'ssh-rsa'
            >>> bytes(
            ...     SSHAgentClient.unstring(SSHAgentClient.string(b'ssh-ed25519'))
            ... )
            b'ssh-ed25519'

"""  # noqa: E501
        n = len(bytestring)
        msg = 'malformed SSH byte string'
        if n < HEAD_LEN or n != HEAD_LEN + int.from_bytes(
            bytestring[:HEAD_LEN], 'big', signed=False
        ):
            raise ValueError(msg)
        return bytestring[HEAD_LEN:]

@classmethod
    def unstring_prefix(
        cls, bytestring: bytes | bytearray, /
    ) -> tuple[bytes | bytearray, bytes | bytearray]:
        r"""Unpack an SSH string at the beginning of the byte string.

Args:
            bytestring:
                A (general) byte string, beginning with a framed/SSH
                byte string.

Returns:
            A 2-tuple `(a, b)`, where `a` is the unframed byte
            string/payload at the beginning of input byte string, and
            `b` is the remainder of the input byte string.

Raises:
            ValueError:
                The byte string does not begin with an SSH string.

Examples:
            >>> a, b = SSHAgentClient.unstring_prefix(
            ...     b'\x00\x00\x00\x07ssh-rsa____trailing data'
            ... )
            >>> (bytes(a), bytes(b))
            (b'ssh-rsa', b'____trailing data')
            >>> a, b = SSHAgentClient.unstring_prefix(
            ...     SSHAgentClient.string(b'ssh-ed25519')
            ... )
            >>> (bytes(a), bytes(b))
            (b'ssh-ed25519', b'')

"""
        n = len(bytestring)
        msg = 'malformed SSH byte string'
        if n < HEAD_LEN:
            raise ValueError(msg)
        m = int.from_bytes(bytestring[:HEAD_LEN], 'big', signed=False)
        if m + HEAD_LEN > n:
            raise ValueError(msg)
        return (
            bytestring[HEAD_LEN : m + HEAD_LEN],
            bytestring[m + HEAD_LEN :],
        )

def request(
        self, code: int, payload: bytes | bytearray, /
    ) -> tuple[int, bytes | bytearray]:
        """Issue a generic request to the SSH agent.

Args:
            code:
                The request code.  See the SSH agent protocol for
                protocol numbers to use here (and which protocol numbers
                to expect in a response).
            payload:
                A byte string containing the payload, or "contents", of
                the request.  Request-specific.  `request` will add any
                necessary wire framing around the request code and the
                payload.

Returns:
            A 2-tuple consisting of the response code and the payload,
            with all wire framing removed.

Raises:
            EOFError:
                The response from the SSH agent is truncated or missing.

"""
        request_message = bytearray([code])
        request_message.extend(payload)
        self._connection.sendall(self.string(request_message))
        chunk = self._connection.recv(HEAD_LEN)
        if len(chunk) < HEAD_LEN:
            msg = 'cannot read response length'
            raise EOFError(msg)
        response_length = int.from_bytes(chunk, 'big', signed=False)
        response = self._connection.recv(response_length)
        if len(response) < response_length:
            msg = 'truncated response from SSH agent'
            raise EOFError(msg)
        return response[0], response[1:]

def list_keys(self) -> Sequence[_types.KeyCommentPair]:
        """Request a list of keys known to the SSH agent.

Returns:
            A read-only sequence of key/comment pairs.

"""
        response_code, response = self.request(
            _types.SSH_AGENTC.REQUEST_IDENTITIES.value, b''
        )
        if response_code != _types.SSH_AGENT.IDENTITIES_ANSWER.value:
            raise SSHAgentFailedError(response_code, response)
        response_stream = collections.deque(response)

def shift(num: int) -> bytes:
            buf = collections.deque(b'')
            for _ in range(num):
                try:
                    val = response_stream.popleft()
                except IndexError:
                    response_stream.extendleft(reversed(buf))
                    msg = 'truncated response from SSH agent'
                    raise EOFError(msg) from None
                buf.append(val)
            return bytes(buf)

key_count = int.from_bytes(shift(4), 'big')
        keys: collections.deque[_types.KeyCommentPair]
        keys = collections.deque()
        for _ in range(key_count):
            key_size = int.from_bytes(shift(4), 'big')
            key = shift(key_size)
            comment_size = int.from_bytes(shift(4), 'big')
            comment = shift(comment_size)
            # Both `key` and `comment` are not wrapped as SSH strings.
            keys.append(_types.KeyCommentPair(key, comment))
        if response_stream:
            raise TrailingDataError
        return keys

def sign(
        self,
        /,
        key: bytes | bytearray,
        payload: bytes | bytearray,
        *,
        flags: int = 0,
        check_if_key_loaded: bool = False,
    ) -> bytes | bytearray:
        """Request the SSH agent sign the payload with the key.

Args:
            key:
                The public SSH key to sign the payload with, in the same
                format as returned by, e.g., the `list_keys` method.
                The corresponding private key must have previously been
                loaded into the agent to successfully issue a signature.
            payload:
                A byte string of data to sign.
            flags:
                Optional flags for the signing request.  Currently
                passed on as-is to the agent.  In real-world usage, this
                could be used, e.g., to request more modern hash
                algorithms when signing with RSA keys.  (No such
                real-world usage is currently implemented.)
            check_if_key_loaded:
                If true, check beforehand (via `list_keys`) if the
                corresponding key has been loaded into the agent.

Returns:
            The binary signature of the payload under the given key.

Raises:
            EOFError:
                The response from the SSH agent is truncated or missing.
            TrailingDataError:
                The response from the SSH agent is too long.
            SSHAgentFailedError:
                The agent failed to complete the request.
            KeyError:
                `check_if_key_loaded` is true, and the `key` was not
                loaded into the agent.

"""
        if check_if_key_loaded:
            loaded_keys = frozenset({pair.key for pair in self.list_keys()})
            if bytes(key) not in loaded_keys:
                msg = 'target SSH key not loaded into agent'
                raise KeyError(msg)
        request_data = bytearray(self.string(key))
        request_data.extend(self.string(payload))
        request_data.extend(self.uint32(flags))
        response_code, response = self.request(
            _types.SSH_AGENTC.SIGN_REQUEST.value, request_data
        )
        if response_code != _types.SSH_AGENT.SIGN_RESPONSE.value:
            raise SSHAgentFailedError(response_code, response)
        return self.unstring(response)

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