如何从文件加载RSA私钥

from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.primitives.asymmetric import rsa
from cryptography.hazmat.backends import default_backend

# 从文件中加载私钥
with open("private_key.pem", "rb") as key_file:
    private_key = serialization.load_pem_private_key(
        key_file.read(),
        password=None,
        backend=default_backend()
    )

# 使用私钥进行签名和验证
message = b"Hello, world!"
signature = private_key.sign(message, padding.PSS(
    mgf=padding.MGF1(hashes.SHA256()),
    salt_length=padding.PSS.MAX_LENGTH
))

public_key = private_key.public_key()
try:
    public_key.verify(signature, message, padding.PSS(
        mgf=padding.MGF1(hashes.SHA256()),
        salt_length=padding.PSS.MAX_LENGTH
    ))
    print("Signature is valid.")
except InvalidSignature:
    print("Signature is not valid.")