DTLS-PSK算法抓包解析

最新推荐文章于 2025-05-26 22:28:44 发布

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最新推荐文章于 2025-05-26 22:28:44 发布 · 1.2k 阅读

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本文详细介绍了DTLS-PSK的完整握手流程和会话恢复过程，通过抓包分析加深了对DTLS协议的理解。内容包括ClientHello、HelloVerifyRequest、ServerHello、ClientKeyExchange等步骤的解析，以及会话恢复时的交互过程。同时，提到了Scadium这一DTLS实现，并提供了相关技术参考链接。

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一、DTLS -PSK

PSK 是DTLS 定义的密钥交换方案之一，相对于公钥证书方案(如 ECDHA_RSA) 来说，其具备更加轻量化、高效的优点；
而目前 PSK方案应用也比较广泛。关于DTLS协议可以看看前面的文章 DTLS要点解析

本次通过模拟的DTLS程序，对DTLS-PSK 握手流程进行抓包分析，以期加深对协议本身的理解。

二、完整握手

流程

         Client                                               Server
         ------                                               ------
         1.ClientHello                 -------->

                                     <--------    2..HelloVerifyRequest
                                                   (contains cookie)

         3.ClientHello                  -------->
         (with cookie)
                                                         4.ServerHello
                                      <--------      5.ServerHelloDone
         6.ClientKeyExchange
         7.ChangeCipherSpec
         8.Finished                     -------->
                                                    9.ChangeCipherSpec
                                      <--------             10.Finished

         Application Data             <------->     Application Data

步骤解析

~1. Client 发送ClientHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: localhost/127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 0
Length: 82
Fragment:
    Handshake Protocol
    Type: CLIENT_HELLO (1)
    Peer: localhost/127.0.0.1:5684
    Message Sequence No: 0
    Fragment Offset: 0
    Fragment Length: 70
    Length: 70
        Version: 254, 253
        Random: 
            GMT Unix Time: Mon Jan 30 22:45:32 CST 2017
            Random Bytes: 4B 8B 3C CF 0F 62 57 99 94 E9 86 0A 46 68 BF 44 00 D1 34 45 FC 81 C3 AC BC 55 7E DB
        Session ID Length: 0
        Cookie Length: 0
        Cipher Suites Length: 4
        Cipher Suites (2 suites)
            Cipher Suite: TLS_PSK_WITH_AES_128_CCM_8
            Cipher Suite: TLS_PSK_WITH_AES_128_CBC_SHA256
        Compression Methods Length: 1
        Compression Methods (1 method)
            Compression Method: NULL
        Extensions Length: 24
            Extension: elliptic_curves (10)
                Length: 8
                Elliptic Curves Length: 6
                Elliptic Curves (3 curves):
                    Elliptic Curve: secp256r1 (23)
                    Elliptic Curve: secp384r1 (24)
                    Elliptic Curve: secp521r1 (25)
            Extension: ec_point_formats (11)
                Length: 2
                EC point formats length: 1
                Elliptic Curves Point Formats (1):
                    EC point format: uncompressed (0)
            Extension: server_certificate_type (20)
                Server certificate type: RAW_PUBLIC_KEY

===============================================================

此时没有携带Cookie，SessionID未生成；
Cipher Suites 携带了用于协商的算法集。

~2. Server 回复HelloVerifyRequest

    Handshake Protocol
    Type: HELLO_VERIFY_REQUEST (3)
    Peer: localhost/127.0.0.1:5684
    Message Sequence No: 0
    Fragment Offset: 0
    Fragment Length: 35
    Length: 35
        Server Version: 254, 253
        Cookie Length: 32
        Cookie: 77 25 7E 96 9E BD 39 42 94 5F 27 6C 8A 6D 9D D2 1A C9 A3 B8 62 1A 34 86 76 1D D7 AA F4 28 98 6D

Request中携带了32字节的Cookie。

~3. Client 再次发送ClientHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: localhost/127.0.0.1:5684

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