import os

from cryptography.exceptions import InvalidTag

from cryptography.hazmat.primitives.ciphers.aead import AESGCM

from mcp.server import MCPServer

from mcp.server.mcpserver import InvalidRequestState, RequestStateSecurity

PREFIX = "kms1." # format version; fed to GCM as associated data, so it is bound under the tag

def unwrap_data_key() -> bytes:

"""One KMS call at process start, kms.decrypt(CiphertextBlob=...); every token after that is local crypto."""

return os.urandom(32) # stand-in for the unwrapped 32-byte data key

class EnvelopeCodec:

def __init__(self, data_key: bytes) -> None:

self._aesgcm = AESGCM(data_key)

def seal(self, payload: bytes) -> str:

nonce = os.urandom(12)

return PREFIX + (nonce + self._aesgcm.encrypt(nonce, payload, PREFIX.encode())).hex()

def unseal(self, token: str) -> bytes:

if not token.startswith(PREFIX):

raise InvalidRequestState("unknown token format")

body = token[len(PREFIX) :]

try:

raw = bytes.fromhex(body)

if raw.hex() != body: # only the exact string seal() produced verifies

raise ValueError("non-canonical hex")

return self._aesgcm.decrypt(raw[:12], raw[12:], PREFIX.encode())

except (ValueError, InvalidTag) as exc:

raise InvalidRequestState("token failed verification") from exc

mcp = MCPServer("Deployer", request_state_security=RequestStateSecurity(codec=EnvelopeCodec(unwrap_data_key())))