feat(protocol): add PSK authentication provider

README.md

@@ -61,6 +61,21 @@ For compatibility, the existing `cert_path` / `key_path` and `cert_pem` /
 They are routed through `CertificateAuth` internally. Do not combine `auth`
 with those legacy arguments.
 
+An existing OCF PSK credential can be supplied through `PskAuth`:
+
+```python
+from smartthings_local.protocol.auth import PskAuth
+
+auth = PskAuth(identity=psk_identity, key=psk_key)
+sess = DtlsCoapSession("192.0.2.100", 49154, auth=auth)
+```
+
+The identity must be the raw 16-byte OCF UUID and cannot contain a NUL byte;
+the key must be exactly 16 or 32 bytes. `PskAuth` selects only
+`ECDHE-PSK-AES128-CBC-SHA256` and does not acquire, derive, provision, rotate,
+or persist credentials. Ownership transfer and credential discovery are
+outside this package.
+
 ### Classified errors
 
 Runtime transport failures use the public types in

smartthings_local/protocol/auth.py

@@ -7,10 +7,15 @@ from os import PathLike
 from pathlib import Path
 from typing import Protocol, runtime_checkable
 
-from OpenSSL import SSL, crypto
+from OpenSSL import SSL, _util, crypto
 
 _OCF_ROOT_CA = str(Path(__file__).with_name("ocf_root_ca.pem"))
 _DTLS_CIPHERS = b"ECDHE-ECDSA-AES128-GCM-SHA256:@SECLEVEL=0"
+_DTLS_PSK_CIPHERS = b"ECDHE-PSK-AES128-CBC-SHA256:@SECLEVEL=0"
+_PSK_CLIENT_CALLBACK_CDEF = (
+    "unsigned int (*)(SSL *, char *, char *, unsigned int, "
+    "unsigned char *, unsigned int)"
+)
 _PEM_CERT_RE = re.compile(
     rb"-----BEGIN CERTIFICATE-----.*?-----END CERTIFICATE-----",
     re.DOTALL,
@@ -45,7 +50,7 @@ class AuthenticationProvider(Protocol):
     """Configure authentication for a newly created DTLS context."""
 
     def configure_context(self, context: SSL.Context) -> None:
-        """Configure trust, verification, ciphers, and client credentials."""
+        """Configure a context while this provider remains session-owned."""
 
 
 class CertificateAuth:
@@ -164,4 +169,78 @@ class CertificateAuth:
             context.check_privatekey()
 
 
-__all__ = ["AuthenticationProvider", "CertificateAuth"]
+class PskAuth:
+    """DTLS authentication using an existing OCF PSK credential.
+
+    The identity must be a raw 16-byte OCF UUID. The key must contain 16 or
+    32 bytes. Credential material is intentionally not exposed as public
+    attributes and is never included in this provider's representation. A
+    configured context must not outlive this provider; ``DtlsCoapSession``
+    enforces that lifetime by retaining its provider.
+    """
+
+    __slots__ = ("_callback",)
+
+    def __init__(self, *, identity: bytes, key: bytes) -> None:
+        if type(identity) is not bytes or type(key) is not bytes:
+            raise TypeError("identity and key must be bytes")
+        if len(identity) != 16:
+            raise ValueError("identity must be a raw 16-byte OCF UUID")
+        if b"\x00" in identity:
+            raise ValueError("identity cannot contain a NUL byte")
+        if len(key) not in (16, 32):
+            raise ValueError("key must be 16 or 32 bytes")
+
+        ffi = _util.ffi
+
+        @ffi.callback(_PSK_CLIENT_CALLBACK_CDEF)
+        def client_callback(
+            _ssl,
+            _identity_hint,
+            identity_buffer,
+            max_identity_length,
+            key_buffer,
+            max_key_length,
+        ):
+            # OpenSSL callbacks cannot propagate Python exceptions. Fail
+            # before touching either destination when a buffer is unavailable
+            # or too small for the complete credential.
+            if (
+                identity_buffer == ffi.NULL
+                or key_buffer == ffi.NULL
+                or len(identity) + 1 > max_identity_length
+                or len(key) > max_key_length
+            ):
+                return 0
+            ffi.memmove(
+                identity_buffer,
+                identity + b"\x00",
+                len(identity) + 1,
+            )
+            ffi.memmove(key_buffer, key, len(key))
+            return len(key)
+
+        object.__setattr__(self, "_callback", client_callback)
+
+    def __setattr__(self, _name: str, _value: object) -> None:
+        raise AttributeError("PskAuth is immutable")
+
+    def __delattr__(self, _name: str) -> None:
+        raise AttributeError("PskAuth is immutable")
+
+    def __repr__(self) -> str:
+        """Return a representation that never includes credential material."""
+        return "PskAuth()"
+
+    def configure_context(self, context: SSL.Context) -> None:
+        """Configure one context for the narrow Samsung OCF PSK profile."""
+        setter = getattr(_util.lib, "SSL_CTX_set_psk_client_callback", None)
+        if setter is None:
+            raise RuntimeError(
+                "the installed OpenSSL binding does not support DTLS PSK"
+            )
+        context.set_cipher_list(_DTLS_PSK_CIPHERS)
+        setter(context._context, self._callback)  # noqa: SLF001
+
+
+__all__ = ["AuthenticationProvider", "CertificateAuth", "PskAuth"]

tests/test_psk_auth.py

@@ -0,0 +1,380 @@
+from __future__ import annotations
+
+import gc
+import traceback
+import weakref
+from concurrent.futures import ThreadPoolExecutor
+from dataclasses import asdict
+from types import SimpleNamespace
+from unittest.mock import MagicMock, patch
+
+import pytest
+from OpenSSL import SSL
+
+from smartthings_local.errors import SessionError
+from smartthings_local.protocol import auth as auth_module
+from smartthings_local.protocol import dtls_session as session_module
+from smartthings_local.protocol.auth import PskAuth
+from smartthings_local.protocol.dtls_session import DtlsCoapSession
+
+_IDENTITY = b"i" * 16
+_KEY = b"k" * 16
+_OTHER_IDENTITY = b"j" * 16
+_OTHER_KEY = b"l" * 32
+
+
+class _BytesSubclass(bytes):
+    pass
+
+
+def _fake_openssl_util(setter):
+    return SimpleNamespace(
+        ffi=auth_module._util.ffi,
+        lib=SimpleNamespace(SSL_CTX_set_psk_client_callback=setter),
+    )
+
+
+def _invoke_callback(callback, identity_size: int, key_size: int):
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", max(identity_size, 1))
+    key_buffer = ffi.new("unsigned char[]", max(key_size, 1))
+    copied = callback(
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        identity_size,
+        key_buffer,
+        key_size,
+    )
+    return (
+        copied,
+        bytes(ffi.buffer(identity_buffer, max(identity_size, 1))),
+        bytes(ffi.buffer(key_buffer, max(key_size, 1))),
+    )
+
+
+@pytest.mark.parametrize("key_length", [16, 32])
+def test_psk_auth_accepts_exact_supported_credential_lengths(key_length):
+    provider = PskAuth(identity=_IDENTITY, key=b"k" * key_length)
+
+    assert repr(provider) == "PskAuth()"
+
+
+@pytest.mark.parametrize(
+    ("identity", "key"),
+    [
+        ("i" * 16, _KEY),
+        (bytearray(_IDENTITY), _KEY),
+        (memoryview(_IDENTITY), _KEY),
+        (_BytesSubclass(_IDENTITY), _KEY),
+        (_IDENTITY, "k" * 16),
+        (_IDENTITY, bytearray(_KEY)),
+        (_IDENTITY, memoryview(_KEY)),
+        (_IDENTITY, _BytesSubclass(_KEY)),
+    ],
+)
+def test_psk_auth_rejects_non_bytes_credentials(identity, key):
+    with pytest.raises(TypeError, match="identity and key must be bytes"):
+        PskAuth(identity=identity, key=key)
+
+
+@pytest.mark.parametrize("identity_length", [0, 15, 17])
+def test_psk_auth_rejects_invalid_identity_lengths(identity_length):
+    with pytest.raises(ValueError, match="raw 16-byte OCF UUID"):
+        PskAuth(identity=b"i" * identity_length, key=_KEY)
+
+
+def test_psk_auth_rejects_identity_with_nul_byte():
+    with pytest.raises(ValueError, match="cannot contain a NUL"):
+        PskAuth(identity=b"i" * 15 + b"\x00", key=_KEY)
+
+
+@pytest.mark.parametrize("key_length", [0, 15, 17, 31, 33])
+def test_psk_auth_rejects_invalid_key_lengths(key_length):
+    with pytest.raises(ValueError, match="16 or 32 bytes"):
+        PskAuth(identity=_IDENTITY, key=b"k" * key_length)
+
+
+def test_psk_auth_is_immutable_and_has_no_public_credential_surface():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    rendered = repr(provider)
+    assert rendered == "PskAuth()"
+    assert str(provider) == rendered
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    assert not hasattr(provider, "identity")
+    assert not hasattr(provider, "key")
+    assert not hasattr(provider, "_identity")
+    assert not hasattr(provider, "_key")
+    with pytest.raises(TypeError):
+        vars(provider)
+    with pytest.raises(TypeError):
+        asdict(provider)
+    with pytest.raises(AttributeError, match="immutable"):
+        provider.identity = _OTHER_IDENTITY
+    with pytest.raises(AttributeError, match="immutable"):
+        del provider._callback
+
+
+def test_psk_auth_identity_equality_does_not_compare_credentials():
+    first = PskAuth(identity=_IDENTITY, key=_KEY)
+    second = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    assert first != second
+    assert len({first, second}) == 2
+
+
+def test_psk_callback_copies_exact_identity_and_key():
+    installed = {}
+
+    def setter(context_handle, callback):
+        installed["context"] = context_handle
+        installed["callback"] = callback
+
+    context_handle = object()
+    context = MagicMock()
+    context._context = context_handle
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        provider.configure_context(context)
+
+    assert installed["context"] is context_handle
+    callback = installed["callback"]
+    copied, identity_bytes, key_bytes = _invoke_callback(callback, 17, 16)
+    assert copied == 16
+    assert identity_bytes == _IDENTITY + b"\x00"
+    assert key_bytes == _KEY
+    context.set_cipher_list.assert_called_once_with(
+        b"ECDHE-PSK-AES128-CBC-SHA256:@SECLEVEL=0"
+    )
+    context.load_verify_locations.assert_not_called()
+    context.set_verify.assert_not_called()
+
+
+@pytest.mark.parametrize(
+    ("identity_size", "key_size"),
+    [(16, 16), (17, 15)],
+)
+def test_psk_callback_rejects_short_buffers_without_partial_copy(
+    identity_size,
+    key_size,
+):
+    installed = {}
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+
+    with patch.object(
+        auth_module,
+        "_util",
+        _fake_openssl_util(
+            lambda _context, callback: installed.setdefault(
+                "callback", callback
+            )
+        ),
+    ):
+        provider.configure_context(context)
+
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", 17)
+    key_buffer = ffi.new("unsigned char[]", 16)
+    ffi.memmove(identity_buffer, b"I" * 17, 17)
+    ffi.memmove(key_buffer, b"K" * 16, 16)
+    copied = installed["callback"](
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        identity_size,
+        key_buffer,
+        key_size,
+    )
+
+    assert copied == 0
+    assert bytes(ffi.buffer(identity_buffer, 17)) == b"I" * 17
+    assert bytes(ffi.buffer(key_buffer, 16)) == b"K" * 16
+
+
+@pytest.mark.parametrize("null_buffer", ["identity", "key"])
+def test_psk_callback_rejects_null_buffers(null_buffer):
+    installed = {}
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+
+    with patch.object(
+        auth_module,
+        "_util",
+        _fake_openssl_util(
+            lambda _context, callback: installed.setdefault(
+                "callback", callback
+            )
+        ),
+    ):
+        provider.configure_context(context)
+
+    ffi = auth_module._util.ffi
+    identity_buffer = ffi.new("char[]", 17)
+    key_buffer = ffi.new("unsigned char[]", 16)
+    ffi.memmove(identity_buffer, b"I" * 17, 17)
+    ffi.memmove(key_buffer, b"K" * 16, 16)
+    if null_buffer == "identity":
+        identity_buffer = ffi.NULL
+    else:
+        key_buffer = ffi.NULL
+
+    copied = installed["callback"](
+        ffi.NULL,
+        ffi.NULL,
+        identity_buffer,
+        17,
+        key_buffer,
+        16,
+    )
+    assert copied == 0
+    if null_buffer == "identity":
+        assert bytes(ffi.buffer(key_buffer, 16)) == b"K" * 16
+    else:
+        assert bytes(ffi.buffer(identity_buffer, 17)) == b"I" * 17
+
+
+def test_psk_auth_unsupported_binding_error_contains_no_credentials():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    context = MagicMock()
+    context._context = object()
+    unsupported_util = SimpleNamespace(
+        ffi=auth_module._util.ffi,
+        lib=SimpleNamespace(),
+    )
+
+    with (
+        patch.object(auth_module, "_util", unsupported_util),
+        pytest.raises(RuntimeError) as captured,
+    ):
+        provider.configure_context(context)
+
+    rendered = (
+        str(captured.value)
+        + repr(captured.value)
+        + "".join(traceback.format_exception(captured.value))
+    )
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    context.set_cipher_list.assert_not_called()
+
+
+def test_psk_auth_configures_real_openssl_context():
+    context = SSL.Context(SSL.DTLS_METHOD)
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+
+    assert provider.configure_context(context) is None
+
+
+def test_distinct_psk_providers_do_not_share_callback_credentials():
+    callbacks = []
+
+    def setter(_context, callback):
+        callbacks.append(callback)
+
+    first = PskAuth(identity=_IDENTITY, key=_KEY)
+    second = PskAuth(identity=_OTHER_IDENTITY, key=_OTHER_KEY)
+    first_context = MagicMock()
+    first_context._context = object()
+    second_context = MagicMock()
+    second_context._context = object()
+
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        first.configure_context(first_context)
+        second.configure_context(second_context)
+
+    assert callbacks[0] is not callbacks[1]
+    with ThreadPoolExecutor(max_workers=2) as executor:
+        first_future = executor.submit(_invoke_callback, callbacks[0], 17, 16)
+        second_future = executor.submit(
+            _invoke_callback,
+            callbacks[1],
+            17,
+            32,
+        )
+        first_result = first_future.result()
+        second_result = second_future.result()
+    assert first_result == (16, _IDENTITY + b"\x00", _KEY)
+    assert second_result == (32, _OTHER_IDENTITY + b"\x00", _OTHER_KEY)
+
+
+def test_session_retains_psk_callback_only_with_provider_lifetime():
+    callback_reference = None
+
+    def setter(_context, callback):
+        nonlocal callback_reference
+        callback_reference = weakref.ref(callback)
+
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession(
+        "appliance.invalid",
+        49154,
+        auth=provider,
+    )
+    context = MagicMock()
+    context._context = object()
+    with patch.object(auth_module, "_util", _fake_openssl_util(setter)):
+        session.auth.configure_context(context)
+
+    del provider
+    gc.collect()
+    assert callback_reference is not None
+    assert callback_reference() is not None
+    assert _invoke_callback(callback_reference(), 17, 16) == (
+        16,
+        _IDENTITY + b"\x00",
+        _KEY,
+    )
+
+    del session
+    gc.collect()
+    assert callback_reference() is None
+
+
+def test_session_accepts_psk_provider_without_legacy_certificate_material():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession("appliance.invalid", 49154, auth=provider)
+
+    assert session.auth is provider
+    assert session.cert_path is None
+    assert session.key_path is None
+    assert session.cert_pem is None
+    assert session.key_pem is None
+
+
+def test_psk_handshake_rejection_does_not_expose_credentials():
+    provider = PskAuth(identity=_IDENTITY, key=_KEY)
+    session = DtlsCoapSession("appliance.invalid", 49154, auth=provider)
+    context = MagicMock()
+    context._context = object()
+    connection = MagicMock()
+    connection.do_handshake.side_effect = SSL.Error()
+    udp_socket = MagicMock()
+    endpoint = SimpleNamespace(sockaddr=("192.0.2.100", 49154))
+
+    with (
+        patch.object(auth_module, "_util", _fake_openssl_util(lambda *_: None)),
+        patch.object(session_module.SSL, "Context", return_value=context),
+        patch.object(session_module.SSL, "Connection", return_value=connection),
+        patch.object(
+            session_module,
+            "open_connected_udp_socket",
+            return_value=(udp_socket, endpoint),
+        ),
+        pytest.raises(SessionError) as captured,
+    ):
+        session.connect()
+
+    rendered = (
+        str(captured.value)
+        + repr(captured.value)
+        + "".join(traceback.format_exception(captured.value))
+    )
+    assert _IDENTITY.decode() not in rendered
+    assert _KEY.decode() not in rendered
+    udp_socket.close.assert_called_once_with()

tests/test_public_api_contract.py

@@ -6,7 +6,11 @@ import inspect
 
 from smartthings_local.ocf.observe_refresh import ObserveRefreshTask
 from smartthings_local.ocf.state_cache import StateCache
-from smartthings_local.protocol.auth import AuthenticationProvider, CertificateAuth
+from smartthings_local.protocol.auth import (
+    AuthenticationProvider,
+    CertificateAuth,
+    PskAuth,
+)
 from smartthings_local.protocol.dtls_session import DtlsCoapSession
 
 
@@ -51,6 +55,18 @@ def test_certificate_auth_is_a_public_authentication_provider():
     assert isinstance(provider, AuthenticationProvider)
 
 
+def test_psk_auth_is_a_public_authentication_provider():
+    provider = PskAuth(identity=b"i" * 16, key=b"k" * 16)
+    assert isinstance(provider, AuthenticationProvider)
+    parameters = inspect.signature(PskAuth).parameters
+    assert list(parameters) == ["identity", "key"]
+    assert all(
+        parameter.kind is inspect.Parameter.KEYWORD_ONLY
+        and parameter.default is inspect.Parameter.empty
+        for parameter in parameters.values()
+    )
+
+
 def test_dtls_session_keeps_current_consumer_methods():
     expected = {
         "close",