使用Python订阅Microsoft Graph在线会议转录文件变更通知时RSA解密失败的问题求助

我现在正在尝试用Python订阅Microsoft Graph的communications/onlineMeetings/getAllTranscripts通知，并用自签名X.509证书解密收到的 payload。目前订阅和接收通知都能成功，但用RSA私钥解密AES密钥的操作一直失败。

这是我用来创建X.509证书的代码：

from cryptography import x509
from cryptography.x509.oid import NameOID
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.primitives import hashes
from datetime import datetime, timedelta, timezone

def create_x509_certificate() -> tuple[x509.Certificate, rsa.RSAPrivateKey]:
    private_key = rsa.generate_private_key(public_exponent=65537, key_size=4096)
    subject = issuer = x509.Name(
        [
            x509.NameAttribute(NameOID.COMMON_NAME, "Self-Signed Certificate"),
        ]
    )
    cert = (
        x509.CertificateBuilder()
        .subject_name(subject)
        .issuer_name(issuer)
        .public_key(private_key.public_key())
        .serial_number(x509.random_serial_number())
        .not_valid_before(datetime.now(timezone.utc))
        .not_valid_after(datetime.now(timezone.utc) + timedelta(days=365))
        .sign(private_key, hashes.SHA256())
    )

    return (cert, private_key)

下面是我用来订阅通知的代码，这里我把证书编码成DER格式：

import base64
import time
import requests

client_state = APP.config.setdefault("client_state", str(int(time.time())))

expiration_time = (datetime.now(timezone.utc) + timedelta(minutes=120)).isoformat()

cert, private_key = create_x509_certificate()
APP.config["private_key"] = private_key

b64_encoded_der_public_key = base64.b64encode(cert.public_bytes(Encoding.DER)).decode()

LOGGER.debug("Public key: '%s'", b64_encoded_der_public_key)

subscription_payload = {
    "resource": "communications/onlineMeetings/getAllTranscripts",
    "changeType": "created",
    "notificationUrl": f"{SERVER_URL}/webhook",
    "lifecycleNotificationUrl": f"{SERVER_URL}/webhook",
    "expirationDateTime": expiration_time,
    "clientState": client_state,
    "includeResourceData": True,
    "encryptionCertificate": b64_encoded_der_public_key,
    "encryptionCertificateId": f"CertificateId-{int(time.time())}",
}

response = requests.post(
    "https://graph.microsoft.com/v1.0/subscriptions",
    headers=_get_auth_headers(),
    json=subscription_payload,
    timeout=(5, 60),
)

收到加密payload后，我尝试用下面的函数解密：

from cryptography.hazmat.primitives.asymmetric.padding import OAEP, MGF1
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.primitives import hashes, hmac
from cryptography.hazmat.primitives.padding import PKCS7
import base64

def _decrypt_payload(
    private_key: rsa.RSAPrivateKey,
    b64_encoded_payload: str,
    b64_encoded_data_key: str,
    b64_encoded_data_signature: str,
) -> str:
    # Decrypt the symmetric key
    symmetric_key = private_key.decrypt(
        base64.b64decode(b64_encoded_data_key),
        OAEP(
            mgf=MGF1(algorithm=hashes.SHA256()),
            algorithm=hashes.SHA256(),
            label=None,
        ),
    )

    encrypted_payload = base64.b64decode(b64_encoded_payload)

    # Create HMAC using decrypted symmetric key
    h = hmac.HMAC(symmetric_key, hashes.SHA256())
    h.update(encrypted_payload)
    calculated_signature = base64.b64encode(h.finalize()).decode()

    # Verify signature
    if b64_encoded_data_signature != calculated_signature:
        raise Exception("Signature is invalid. Possible tampering detected.")

    # Decrypt the content
    iv = symmetric_key[:16]  # First 16 bytes of the symmetric key as IV
    cipher = Cipher(algorithms.AES(symmetric_key), modes.CBC(iv))
    decryptor = cipher.decryptor()
    decrypted_payload = decryptor.update(encrypted_payload) + decryptor.finalize()

    unpadder = PKCS7(algorithms.AES.block_size).unpadder()
    unpadded_data = unpadder.update(decrypted_payload) + unpadder.finalize()
    return unpadded_data.decode()

问题：
解密AES密钥时，private_key.decrypt()操作一直失败。我怀疑问题可能出在订阅时证书的编码方式，或者解密payload时的处理逻辑上。

疑问：

• 我在订阅请求的encryptionCertificate字段中，把证书转成DER格式再base64编码的方式是正确的吗？
• 我解密对称密钥或者验证HMAC签名的方式有没有问题？
• RSA密钥长度（4096位）或者AES密钥的提取逻辑会不会导致问题？

任何建议或见解都非常感谢！

备注：内容来源于stack exchange，提问作者Jims