用 Python 告诉你什么是计时攻击

2023-11-18 05:40:01

什么是计时攻击？

计时攻击是一种针对加密算法的攻击方式，它利用了加密算法在处理不同数据时的执行时间差异来推断加密密钥。计时攻击的原理是，如果攻击者能够控制输入到加密算法的数据，并测量加密算法执行所需的时间，那么攻击者就可以通过分析加密算法的执行时间来推断出加密密钥。

计时攻击的原理

计时攻击的原理可以分为以下几个步骤：

攻击者首先收集加密算法的执行时间数据。这些数据可以通过向加密算法输入不同的数据并测量加密算法执行所需的时间来获得。
攻击者然后分析加密算法的执行时间数据，寻找加密算法执行时间与输入数据之间的关系。
攻击者利用加密算法执行时间与输入数据之间的关系来推断加密密钥。

计时攻击的实现方法

计时攻击可以通过多种方式实现，其中最常见的一种方式是使用称为“侧信道攻击”的技术。侧信道攻击是指攻击者通过分析加密算法执行过程中产生的物理信号来推断加密密钥。侧信道攻击可以用来测量加密算法执行所需的时间，也可以用来测量加密算法执行过程中产生的其他物理信号，例如功耗、电磁辐射等。

如何防御计时攻击？

有许多方法可以防御计时攻击，其中最有效的方法是使用称为“恒定时间加密”的技术。恒定时间加密是指加密算法在处理不同数据时执行所需的时间是相同的。这样，攻击者就无法通过分析加密算法的执行时间来推断加密密钥。

使用 Python 实现计时攻击

计时攻击可以使用 Python 语言轻松实现。以下是一个使用 Python 实现计时攻击的示例代码：

import time

def encrypt(plaintext, key):
    """
    Encrypt plaintext using key.

    Args:
        plaintext: The plaintext to encrypt.
        key: The encryption key.

    Returns:
        The ciphertext.
    """

    # Pad the plaintext to a multiple of the block size.
    plaintext = pad(plaintext)

    # Create an empty ciphertext array.
    ciphertext = []

    # Encrypt each block of plaintext.
    for i in range(0, len(plaintext), block_size):
        block = plaintext[i:i+block_size]
        ciphertext.append(encrypt_block(block, key))

    # Return the ciphertext.
    return ciphertext

def encrypt_block(block, key):
    """
    Encrypt a single block of plaintext.

    Args:
        block: The plaintext block to encrypt.
        key: The encryption key.

    Returns:
        The ciphertext block.
    """

    # Convert the plaintext block to an integer.
    block_int = int.from_bytes(block, byteorder='big')

    # XOR the plaintext block with the key.
    ciphertext_int = block_int ^ key

    # Convert the ciphertext integer to a byte array.
    ciphertext = ciphertext_int.to_bytes(block_size, byteorder='big')

    # Return the ciphertext block.
    return ciphertext

def pad(plaintext):
    """
    Pad plaintext to a multiple of the block size.

    Args:
        plaintext: The plaintext to pad.

    Returns:
        The padded plaintext.
    """

    # Get the block size.
    block_size = 16

    # Calculate the amount of padding needed.
    padding_length = block_size - (len(plaintext) % block_size)

    # Create a padding byte.
    padding_byte = bytes([padding_length])

    # Pad the plaintext with the padding byte.
    plaintext += padding_byte * padding_length

    # Return the padded plaintext.
    return plaintext

def attack(ciphertext):
    """
    Attack the ciphertext to recover the encryption key.

    Args:
        ciphertext: The ciphertext to attack.

    Returns:
        The encryption key.
    """

    # Get the block size.
    block_size = 16

    # Create an empty key array.
    key = []

    # Attack each block of ciphertext.
    for i in range(0, len(ciphertext), block_size):
        block = ciphertext[i:i+block_size]
        key.append(attack_block(block))

    # Return the key.
    return key

def attack_block(block):
    """
    Attack a single block of ciphertext to recover the key byte.

    Args:
        block: The ciphertext block to attack.

    Returns:
        The key byte.
    """

    # Create a list of all possible key bytes.
    possible_key_bytes = list(range(256))

    # Try each possible key byte.
    for key_byte in possible_key_bytes:
        # Decrypt the ciphertext block with the key byte.
        plaintext_block = decrypt_block(block, key_byte)

        # Check if the plaintext block is valid.
        if is_valid_plaintext(plaintext_block):
            # Return the key byte.
            return key_byte

def decrypt_block(block, key_byte):
    """
    Decrypt a single block of ciphertext.

    Args:
        block: The ciphertext block to decrypt.
        key_byte: The key byte to use.

    Returns:
        The plaintext block.
    """

    # Convert the ciphertext block to an integer.
    ciphertext_int = int.from_bytes(block, byteorder='big')

    # XOR the ciphertext block with the key byte.
    plaintext_int = ciphertext_int ^ key_byte

    # Convert the plaintext integer to a byte array.
    plaintext = plaintext_int.to_bytes(block_size, byteorder='big')

    # Return the plaintext block.
    return plaintext

def is_valid_plaintext(plaintext):
    """
    Check if plaintext is valid.

    Args:
        plaintext: The plaintext to check.

    Returns:
        True if plaintext is valid, False otherwise.
    """

    # Check if plaintext contains any non-printable characters.
    for char in plaintext:
        if char < 32 or char > 126:
            return False

    # Check if plaintext contains any null bytes.
    if b'\x00' in plaintext:
        return False

    # Return True if plaintext is valid.
    return True

if __name__ == "__main__":
    # The plaintext to encrypt.
    plaintext = b"Hello, world!"

    # The encryption key.
    key = b"1234567890123456"

    # Encrypt the plaintext.
    ciphertext = encrypt(plaintext, key)

    # Attack the ciphertext to recover the encryption key.
    recovered_key = attack(ciphertext)

    # Print the recovered key.
    print(recovered_key)

在上述代码中，encrypt()函数用于加密plaintext，encrypt_block()函数用于加密一个block的plaintext，pad()函数用于将plaintext填充到block size的倍数，attack()函数用于攻击ciphertext以恢复加密密钥，attack_block()函数用于攻击一个block的ciphertext以恢复密钥字节，decrypt_block()函数用于解密一个block的ciphertext，is_valid_plaintext()函数用于检查plaintext是否有效。