Signal 协议核心技术深度解析：从理论到代码实现

1. Signal 协议架构概述

Signal 协议的核心是建立在密码学原语之上的安全消息传输系统，主要包含以下组件：

1. X3DH 密钥协商协议 - 用于初始会话建立

2. Double Ratchet 算法 - 用于会话中的密钥演进

3. PreKey 机制 - 支持异步通信

4. KDF 链 - 用于密钥派生

下面我们通过实际代码示例来解析这些核心组件。

2. X3DH 密钥交换协议

X3DH (Extended Triple Diffie-Hellman) 是Signal协议的初始握手协议，用于在两个设备之间建立共享密钥。

2.1 数学基础

python

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from cryptography.hazmat.primitives.asymmetric import x25519
from cryptography.hazmat.primitives import hashes
from cryptography.hazmat.primitives.kdf.hkdf import HKDF

# 生成身份密钥对
identity_key_private = x25519.X25519PrivateKey.generate()
identity_key_public = identity_key_private.public_key()

# 生成临时密钥对
ephemeral_key_private = x25519.X25519PrivateKey.generate()
ephemeral_key_public = ephemeral_key_private.public_key()

2.2 X3DH 密钥计算

python

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def x3dh(initiator_identity_priv, initiator_ephemeral_priv, 
         recipient_identity_pub, recipient_signed_prekey_pub, 
         recipient_one_time_prekey_pub=None):
    # DH1 = initiator_identity_priv × recipient_signed_prekey_pub
    dh1 = initiator_identity_priv.exchange(recipient_signed_prekey_pub)
    
    # DH2 = initiator_ephemeral_priv × recipient_identity_pub
    dh2 = initiator_ephemeral_priv.exchange(recipient_identity_pub)
    
    # DH3 = initiator_ephemeral_priv × recipient_signed_prekey_pub
    dh3 = initiator_ephemeral_priv.exchange(recipient_signed_prekey_pub)
    
    # DH4 (optional) = initiator_ephemeral_priv × recipient_one_time_prekey_pub
    dh4 = b""
    if recipient_one_time_prekey_pub:
        dh4 = initiator_ephemeral_priv.exchange(recipient_one_time_prekey_pub)
    
    # 组合所有DH结果
    shared_secret = dh1 + dh2