Key derivation function with Python

Welcome to the next pikoTutorial!

In the previous article, we learned how to use Python for symmetric data encryption. The final example is about converting a user-supplied password directly into an encryption key. While it works, it's not a recommended practice. Today you will learn the recommended method, the key derivation function.

Key derivation function

Below you can find an extended example of how to use the PBKDF2HMAC key derivation function in Python:

<code class="language-python"># 导入Base64编码的实用程序
import base64
# 导入Fernet
from cryptography.fernet import Fernet
from cryptography.hazmat.primitives.kdf.pbkdf2 import PBKDF2HMAC
from cryptography.hazmat.primitives import hashes
# 导入getpass用于安全输入读取
from getpass import getpass
# 读取纯文本密码
plain_text_password: str = getpass(prompt='密码：')
# 将密码转换为字节
password_bytes = plain_text_password.encode('utf-8')
# 演示的一些salt值，在实践中使用安全的随机值
salt = b'\x00' * 16
# 使用PBKDF2HMAC从密码派生安全密钥
kdf = PBKDF2HMAC(
    algorithm=hashes.SHA256(),
    length=32,
    salt=salt,
    iterations=100000
)
# 使用Base64编码派生的密钥
key = base64.urlsafe_b64encode(kdf.derive(password_bytes))
# 使用派生的密钥创建一个Fernet实例
fernet = Fernet(key)
# 要加密的数据
data = b'一些秘密数据'
# 加密数据
encrypted_data = fernet.encrypt(data)
# 解密数据
decrypted_data = fernet.decrypt(encrypted_data)
# 打印解密后的数据
print(f"解密文本：{decrypted_data.decode()}")</code>

Not only are keys created this way more secure, but they no longer require plain text passwords to be exactly 32 bytes long.

Note for beginners: Remember that salt is necessary to decrypt data!

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