如何用Python解密CryptoJS RC4Drop加密的数据？

现有一段Node.js代码，使用CryptoJS的RC4Drop算法（drop参数为3072/4=768）实现加密解密，运行正常。需要用Python实现对应的解密功能，解密Node.js加密的数据。以下是相关代码及示例：

Node.js 加密解密代码 #

encryptData(data) {
    try {
        var encryptionKey = CryptoJS.enc.Utf8.parse("SecretKey");
        return CryptoJS.RC4Drop.encrypt(JSON.stringify(data), encryptionKey, {
            drop: 3072 / 4
        }).toString();
    } catch (error) {
        console.error(error);
    }
},
decryptData(encrypted) {
    try {
        var encryptionKey = CryptoJS.enc.Utf8.parse("SecretKey");
        const plainPass = CryptoJS.RC4Drop.decrypt(encrypted, encryptionKey, {
            drop: 3072 / 4
        }).toString(CryptoJS.enc.Utf8);
        return plainPass;
    } catch (error) {
        console.error(error);
    }
},

尝试的错误Python代码 #

def rc4_drop(key, data, drop_bytes):
    # RC4 Encryption/Decryption
    cipher = ARC4.new(key)
    decrypted = cipher.decrypt(data)
    
    # Apply drop functionality
    if drop_bytes < len(decrypted):
        return decrypted[drop_bytes:]
    else:
        return b''

def decrypt_data(encrypted_base64, key, drop_bytes):
    try:
        # Convert Base64-encoded encrypted data to bytes
        encrypted_bytes = base64.b64decode(encrypted_base64)
        print("Encrypted bytes:", encrypted_bytes)

        # Convert the key to bytes
        key_bytes = key.encode('utf-8')
        print("Key bytes:", key_bytes)

        # Decrypt data with RC4 and apply drop bytes
        plaintext_bytes = rc4_drop(key_bytes, encrypted_bytes, drop_bytes)
        print("Plaintext bytes after dropping:", plaintext_bytes)
        
        # Convert bytes to string
        return plaintext_bytes.decode('utf-8') if plaintext_bytes else ''
    
    except Exception as e:
        print(f"An error occurred: {e}")
        return None

示例：Node.js加密字符串"Hello"得到Base64输出RlxFVdR/FA==。

正确的Python解密实现 #

错误原因分析 #

核心错误是对RC4Drop逻辑的误解：RC4Drop不是解密后截断数据，而是初始化RC4密钥流后，先丢弃前drop个字节，再用剩余的密钥流对密文进行异或解密。

实现代码 #

依赖pycryptodome库（先安装：pip install pycryptodome）：

import base64
from Crypto.Cipher import ARC4

def rc4_drop_decrypt(key, encrypted_data, drop_bytes):
    # 初始化RC4密码器
    cipher = ARC4.new(key)
    # 先丢弃前drop_bytes个密钥流字节
    cipher.decrypt(b'\x00' * drop_bytes)
    # 用剩余的密钥流解密数据
    return cipher.decrypt(encrypted_data)

def decrypt_data(encrypted_base64, key, drop_bytes):
    try:
        # 解码Base64密文
        encrypted_bytes = base64.b64decode(encrypted_base64)
        # 转换密钥为字节
        key_bytes = key.encode('utf-8')
        # 执行RC4Drop解密
        plaintext_bytes = rc4_drop_decrypt(key_bytes, encrypted_bytes, drop_bytes)
        # 转换为UTF-8字符串
        return plaintext_bytes.decode('utf-8')
    except Exception as e:
        print(f"解密错误: {e}")
        return None

# 测试示例
if __name__ == "__main__":
    encrypted_base64 = "RlxFVdR/FA=="
    key = "SecretKey"
    drop_bytes = int(3072 / 4)  # 768
    result = decrypt_data(encrypted_base64, key, drop_bytes)
    print("解密结果:", result)  # 输出: "Hello"

代码说明 #

1. 初始化ARC4密码器后，调用decrypt(b'\x00' * drop_bytes)：生成并丢弃前drop_bytes个密钥流字节，模拟CryptoJS RC4Drop的drop参数逻辑。
2. 用同一个密码器解密实际密文，此时使用的是丢弃初始字节后的密钥流，与CryptoJS逻辑完全一致。

内容的提问来源于stack exchange，提问作者Vijay Mulani