# digital_signature.py

from cryptography.hazmat.primitives.asymmetric import utils
from cryptography.hazmat.primitives import serialization
from cryptography.hazmat.backends import default_backend
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.asymmetric import padding
import os

def digital_signature(file_path):
    # 读取A的私钥
    with open(os.path.join(os.path.dirname(__file__), 'A_private.txt'), 'r') as file:
        private_key_data = file.read()
        private_key = serialization.load_pem_private_key(
            private_key_data.encode(),
            password=None,
            backend=default_backend()
        )

    # 读取原文
    with open(file_path, 'rb') as file:
        message = file.read()

    # 计算摘要
    hash_object = hashes.Hash(hashes.SHA256(), backend=default_backend())
    hash_object.update(message)
    hash_value = hash_object.finalize()

    # 使用私钥进行签名
    signature = private_key.sign(
        hash_value,
        padding.PSS(
            mgf=padding.MGF1(hashes.SHA256()),
            salt_length=padding.PSS.MAX_LENGTH
        ),
        utils.Prehashed(hashes.SHA256())
    )

    # 将签名保存到文件
    with open(os.path.join(os.path.dirname(__file__), 'signature.txt'), 'wb') as file:
        file.write(signature)

    return hash_value.hex()