RSA加解密（一）

[soapgu]

• 前言

需要进行有关加解密相关的Research，第一反应是比较流行的公钥加密RSA。
先直接跳过理论部分，用什么最快实现

• 快速使用

对于解决方案最快捷方便的语言当然是python。
我用“通义千问”（阿里云的ChatGPT），用下来已经觉得非常不错了。简易我使用pycryptodome。
对于这种AI的回答本能的不信任，我先直接从PYPI里面找模块库，一般来说AI回答大方向出错可能性低，而细节错误率比较高。所以对GPT的使用还是以方向性的入口为主

• PYPI
• PYPI-pycryptodome

看上去AI没骗人

接下来参考相关文档开始使用

1. 安装包

pip install pycryptodome

2. 读取秘钥

这里我们使用固定的秘钥，从磁盘获取

recipient_key = RSA.import_key(open("publicKey").read())
private_key = RSA.import_key(open("privateKey").read())

3. 加密和解密

这里直接给出完整代码

from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP

recipient_key = RSA.import_key(open("publicKey").read())
cipher_rsa = PKCS1_OAEP.new(recipient_key)
message = b'This is a secret message to be encrypted'
ciphertext = cipher_rsa.encrypt(message)

private_key = RSA.import_key(open("privateKey").read())
decipher_rsa = PKCS1_OAEP.new(private_key)
original_message = decipher_rsa.decrypt(ciphertext)
print("Decrypted original message:", original_message.decode('utf-8'))

• RSA的加密长度限制

很快问题就来了，我例子里面用的是很短的字符串，如果我要加密一个文件怎么处理？我加密的长度在哪里？

先试试看把字符串弄长一点

Traceback (most recent call last):
  File "/Users/guhui/GitLabs/RsaDemo/demo.py", line 11, in <module>
    ciphertext = cipher_rsa.encrypt(message)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/Crypto/Cipher/PKCS1_OAEP.py", line 117, in encrypt
    raise ValueError("Plaintext is too long.")
ValueError: Plaintext is too long.

果然报错了

网上说明文的长度=密钥长度（Bytes）-11=1024/8 -11 = 117

实验了一下，果然不靠谱

guhui@guhuideMacBook-Pro RsaDemo % python3 demo.py
使用此密钥进行RSA加密时，最大明文长度是 117 字节
当前长度89
Traceback (most recent call last):
  File "/Users/guhui/GitLabs/RsaDemo/demo.py", line 11, in <module>
    ciphertext = cipher_rsa.encrypt(message)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/homebrew/lib/python3.11/site-packages/Crypto/Cipher/PKCS1_OAEP.py", line 117, in encrypt
    raise ValueError("Plaintext is too long.")
ValueError: Plaintext is too long.

最后还是谷歌一下

• What is the typical block size in RSA?

所以用11是错误的，而是具体情况具体长度。

It depends on the key size and the selected padding. The max block size is (in bytes): key_size_in_bytes - padding_margin

The padding margin is as follows:

RSA/ECB/PKCS1Padding, 11
RSA/ECB/NoPadding, 0
RSA/ECB/OAEPPadding, 42 // Actually it's OAEPWithSHA1AndMGF1Padding
RSA/ECB/OAEPWithMD5AndMGF1Padding, 34
RSA/ECB/OAEPWithSHA1AndMGF1Padding, 42
RSA/ECB/OAEPWithSHA224AndMGF1Padding, 58
RSA/ECB/OAEPWithSHA256AndMGF1Padding, 66
RSA/ECB/OAEPWithSHA384AndMGF1Padding, 98
RSA/ECB/OAEPWithSHA512AndMGF1Padding, 130
RSA/ECB/OAEPWithSHA3-224AndMGF1Padding, 58
RSA/ECB/OAEPWithSHA3-256AndMGF1Padding, 66
RSA/ECB/OAEPWithSHA3-384AndMGF1Padding, 98
RSA/ECB/OAEPWithSHA3-512AndMGF1Padding, 130

我们这里用的应该是42

实验一下果然，一字不差

max_plain_text_len = recipient_key.size_in_bytes() - 42 
print(f"使用此密钥进行RSA加密时，最大明文长度是 {max_plain_text_len} 字节")

guhui@guhuideMacBook-Pro RsaDemo % python3 demo.py
使用此密钥进行RSA加密时，最大明文长度是 86 字节
当前长度86
Ciphertext: b'\x99\xed\x90\x17J\x99v\x0eWjG\xe9\xafm\x0c\xc0@N\x80\x85\x9az\x0c>\xfb\x1b\x18|\xb9 \xb8\tm\xcfe\xc6\xefi\xe8\n\x85\xc3\x02\x7f1\xc7Y\x02\x1b\xaaY\x9392\x19\x84x\x99+c\x83\xb9\x9a\x92b\xbfu\xb4+\x1bP\x86\x86\x1a\x96\x04<\x8c\xf8a\xf5\xf7\x84\x15\x10\xe2\xfd\xfa]\x8f\x1fW\xd2\x93\xa3]1\x06D\xf0\xb1\xa4d\xc7\x0f\x83\x82\x11Mi\xa7.Cx&\x14\xc1\x08\x05\xcc\xca\xa10S\x8b(*\xf5'
128
86
Decrypted original message: This is a secret message to be encrypted,This is a secret message to be encrypted,This

• RSA怎么实现长数据加密

既然有长度限制，就只能分块加密了

下面是分块加密的代码

from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP
import os

recipient_key = RSA.import_key(open("publicKey").read())
max_plain_text_len = recipient_key.size_in_bytes() - 42 
print(f"使用此密钥进行RSA加密时，最大明文长度是 {max_plain_text_len} 字节")
cipher_rsa = PKCS1_OAEP.new(recipient_key)

file_path = "sample.jpg"
output_filename = "cipher.data"
with open(file_path, 'rb') as in_file:
    with open(output_filename, 'wb') as out_file:
        while content := in_file.read(max_plain_text_len):  # 每次读取数据
            #print("---read data---")
            encrypted_chunk = cipher_rsa.encrypt(content)
            out_file.write(encrypted_chunk)

file_length = os.path.getsize(output_filename)
print(f"cipher file size:{file_length}")

分块解密的代码

from Crypto.PublicKey import RSA
from Crypto.Cipher import PKCS1_OAEP

private_key = RSA.import_key(open("privateKey").read())
decipher_rsa = PKCS1_OAEP.new(private_key)
data_length = private_key.size_in_bytes()
print(f"私钥长度:{data_length}")

file_path = "cipher.data"
output_filename = "sample_restore.jpg"

with open(file_path, 'rb') as in_file:
    with open(output_filename, 'wb') as out_file:
        while content := in_file.read(data_length):  # 每次读取数据
            #print("---read data---")
            decrypted_chunk = decipher_rsa.decrypt(content)
            out_file.write(decrypted_chunk)

print("decrypt ok")

• 小结

RSA加密解密的基础技术准备已经完成了，接下来把他做成http服务来做专门测试