DTLS-PSK算法抓包解析

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
Version: 254, 253
Epoch: 0
Sequence Number: 1
Length: 114
Fragment:
	Handshake Protocol
	Type: CLIENT_HELLO (1)
	Peer: localhost/127.0.0.1:5684
	Message Sequence No: 1
	Fragment Offset: 0
	Fragment Length: 102
	Length: 102
		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: 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
		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

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

本次携带了服务端返回的Cookie值。

~4. Server 回复ServerHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:64688
Version: 254, 253
Epoch: 0
Sequence Number: 1
Length: 82
Fragment:
	Handshake Protocol
	Type: SERVER_HELLO (2)
	Peer: /127.0.0.1:64688
	Message Sequence No: 1
	Fragment Offset: 0
	Fragment Length: 70
	Length: 70
		Server Version: 254, 253
		Random: 
			GMT Unix Time: Mon Jan 30 22:45:32 CST 2017
			Random Bytes: AB AB 69 55 C4 2E 1F B0 8D B7 FE 7F EA 36 E5 18 6A FD 4D C8 19 4C 73 63 D3 19 B5 E0
		Session ID Length: 32
		Session ID: 58 8F 51 8C 2A 2A B5 DC 14 9C AB D3 F2 EE BA 25 78 80 47 25 A7 93 35 34 00 D5 CD 53 2C EC B3 D4
		Cipher Suite: TLS_PSK_WITH_AES_128_CCM_8
		Compression Method: NULL
		Extensions Length: 0
===============================================================

此时生成了32字节的 SessionID；
CipherSute 指明了即将采用的密钥算法集；

~5. Server 回复ServerHelloDone

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:64688
Version: 254, 253
Epoch: 0
Sequence Number: 2
Length: 12
Fragment:
	Handshake Protocol
	Type: SERVER_HELLO_DONE (14)
	Peer: /127.0.0.1:64688
	Message Sequence No: 2
	Fragment Offset: 0
	Fragment Length: 0
	Length: 0

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

Server 通知ServerHello结束。

~6. Client 发送ClientKeyExchange

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: localhost/127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 2
Length: 23
Fragment:
	Handshake Protocol
	Type: CLIENT_KEY_EXCHANGE (16)
	Peer: localhost/127.0.0.1:5684
	Message Sequence No: 2
	Fragment Offset: 0
	Fragment Length: 11
	Length: 11
		PSK Identity: 012345678

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

PSK Identity 用于标识终端ID，后续双方根据该ID找到终端预置的PSK用于计算密钥；

~7. Client 发送ChangeCipherSpec

==[ DTLS Record ]==============================================
Content Type: Change Cipher Spec (20)
Peer address: localhost/127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 3
Length: 1
Fragment:
	Change Cipher Spec Message

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

Client 表示已经确认算法和密钥。

~8. Client 发送Finish

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: localhost/127.0.0.1:5684
Version: 254, 253
Epoch: 1
Sequence Number: 0
Length: 40
Fragment:
	Handshake Protocol
	Type: FINISHED (20)
	Peer: localhost/127.0.0.1:5684
	Message Sequence No: 3
	Fragment Offset: 0
	Fragment Length: 12
	Length: 12
		Verify Data: BC 00 D7 F6 6F E5 A4 B4 0D 8B 5C 8A

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

Client 表示握手完成，其中Verify Data是由动态密钥计算得出的摘要，用于Server端验证。

~9. Server 发送ChangeCipherSpec

==[ DTLS Record ]==============================================
Content Type: Change Cipher Spec (20)
Peer address: /127.0.0.1:64688
Version: 254, 253
Epoch: 0
Sequence Number: 3
Length: 1
Fragment:
	Change Cipher Spec Message

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

Server 表示已经确认算法和密钥。

~10. Server 发送Finish

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:64688
Version: 254, 253
Epoch: 1
Sequence Number: 0
Length: 40
Fragment:
	Handshake Protocol
	Type: FINISHED (20)
	Peer: /127.0.0.1:64688
	Message Sequence No: 3
	Fragment Offset: 0
	Fragment Length: 12
	Length: 12
		Verify Data: EC C9 5E 4E 24 BE 77 78 CB F5 20 54

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

Server 表示握手完成，其中Verify Data是由动态密钥计算得出的摘要，用于Client端验证。

至此，完整的PSK 握手流程已经结束，接下来执行会话恢复的场景。

三、会话恢复

流程

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

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

         3.ClientHello                 -------->
         (with cookie)
                                                         4.ServerHello
                                                         5.ChangeCipherSpec
                                       <--------         6.Finished

         7.ChangeCipherSpec
         8.Finished                    -------->

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

步骤解析

~1. Client 发送ClientHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 0
Length: 115
Fragment:
	Handshake Protocol
	Type: CLIENT_HELLO (1)
	Peer: /127.0.0.1:5684
	Message Sequence No: 0
	Fragment Offset: 0
	Fragment Length: 103
	Length: 103
		Version: 254, 253
		Random: 
			GMT Unix Time: Tue Jan 31 00:02:05 CST 2017
			Random Bytes: B6 28 F7 76 FE C1 B9 7A 87 CE D9 81 2D C3 9A AA 07 F8 69 2D 36 A3 B3 A2 1F 47 E1 FF
		Session ID Length: 32
		Session ID: 58 8F 63 71 DE B4 87 9A C0 0B 67 BB 16 7F 33 1C B6 FF E2 74 74 D9 EB 58 D4 78 44 BA 4C 22 42 38
		Cookie Length: 0
		Cipher Suites Length: 4
		Cipher Suites (2 suites)
			Cipher Suite: TLS_NULL_WITH_NULL_NULL
			Cipher Suite: TLS_PSK_WITH_AES_128_CCM_8
		Compression Methods Length: 2
		Compression Methods (2 method)
			Compression Method: NULL
			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SessionID已经存在，但Cookie为空，仍然需要经过一次HelloVerify。

~2.Server 发送HelloVerifyRequest

	Handshake Protocol
	Type: HELLO_VERIFY_REQUEST (3)
	Peer: /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: 67 6B 86 62 06 F6 A4 3D 31 59 B1 82 80 39 23 76 C6 2C E2 FC E1 7F 41 E8 EE 13 6C 12 A6 76 7B C5

Server 发送了32字节的Cookie值

~3. Client 再次发送ClientHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 1
Length: 147
Fragment:
	Handshake Protocol
	Type: CLIENT_HELLO (1)
	Peer: /127.0.0.1:5684
	Message Sequence No: 1
	Fragment Offset: 0
	Fragment Length: 135
	Length: 135
		Version: 254, 253
		Random: 
			GMT Unix Time: Tue Jan 31 00:02:05 CST 2017
			Random Bytes: B6 28 F7 76 FE C1 B9 7A 87 CE D9 81 2D C3 9A AA 07 F8 69 2D 36 A3 B3 A2 1F 47 E1 FF
		Session ID Length: 32
		Session ID: 58 8F 63 71 DE B4 87 9A C0 0B 67 BB 16 7F 33 1C B6 FF E2 74 74 D9 EB 58 D4 78 44 BA 4C 22 42 38
		Cookie Length: 32
		Cookie: 67 6B 86 62 06 F6 A4 3D 31 59 B1 82 80 39 23 76 C6 2C E2 FC E1 7F 41 E8 EE 13 6C 12 A6 76 7B C5
		Cipher Suites Length: 4
		Cipher Suites (2 suites)
			Cipher Suite: TLS_NULL_WITH_NULL_NULL
			Cipher Suite: TLS_PSK_WITH_AES_128_CCM_8
		Compression Methods Length: 2
		Compression Methods (2 method)
			Compression Method: NULL
			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Cookie和SessionID都不为空。

~4. Server 发送ServerHello

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:54595
Version: 254, 253
Epoch: 0
Sequence Number: 1
Length: 82
Fragment:
	Handshake Protocol
	Type: SERVER_HELLO (2)
	Peer: /127.0.0.1:54595
	Message Sequence No: 1
	Fragment Offset: 0
	Fragment Length: 70
	Length: 70
		Server Version: 254, 253
		Random: 
			GMT Unix Time: Tue Jan 31 00:02:05 CST 2017
			Random Bytes: 6B 21 0D B0 A3 33 A3 49 65 0E D9 D1 DB 0E 62 74 51 EE 1B E1 CC 37 1E FD 8C 67 39 00
		Session ID Length: 32
		Session ID: 58 8F 63 71 DE B4 87 9A C0 0B 67 BB 16 7F 33 1C B6 FF E2 74 74 D9 EB 58 D4 78 44 BA 4C 22 42 38
		Cipher Suite: TLS_PSK_WITH_AES_128_CCM_8
		Compression Method: NULL
===============================================================

~5. Server 发送ChangeCipherSpec

==[ DTLS Record ]==============================================
Content Type: Change Cipher Spec (20)
Peer address: /127.0.0.1:54595
Version: 254, 253
Epoch: 0
Sequence Number: 2
Length: 1
Fragment:
	Change Cipher Spec Message

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

Server 表示已经确认算法和密钥。

~6. Server 发送Finish

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:54595
Version: 254, 253
Epoch: 1
Sequence Number: 0
Length: 40
Fragment:
	Handshake Protocol
	Type: FINISHED (20)
	Peer: /127.0.0.1:54595
	Message Sequence No: 2
	Fragment Offset: 0
	Fragment Length: 12
	Length: 12
		Verify Data: D7 8F CA EC 97 B7 96 A3 CD 5E 5C 97

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

Server 表示握手完成，其中VerifyData 用于Client端验证。

~7. Client 发送ChangeCipherSpec

==[ DTLS Record ]==============================================
Content Type: Change Cipher Spec (20)
Peer address: /127.0.0.1:5684
Version: 254, 253
Epoch: 0
Sequence Number: 2
Length: 1
Fragment:
	Change Cipher Spec Message

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

Client 表示已经确认算法和密钥。

~8. Client 发送Finish

==[ DTLS Record ]==============================================
Content Type: Handshake (22)
Peer address: /127.0.0.1:5684
Version: 254, 253
Epoch: 1
Sequence Number: 0
Length: 40
Fragment:
	Handshake Protocol
	Type: FINISHED (20)
	Peer: /127.0.0.1:5684
	Message Sequence No: 2
	Fragment Offset: 0
	Fragment Length: 12
	Length: 12
		Verify Data: 3F 86 FC 45 D8 41 A6 BE BD 54 C2 7A

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

Client 表示握手完成，其中VerifyData 用于Server端验证。

至此，会话恢复(Resuming)流程分析完毕。