Lecture 7: Transport Level Security – SSL/TLS CS 336/536: Computer Network Security Fall 2013 Nitesh Saxena Adopted from previous lecture by Tony Barnard

Course Admin HW/Lab 1 – Graded; scores posted; to be returned today – Solution was provided ( ed) HW/Lab 2 posted – Covers Lecture 5 (network mapping and attacks) – Due Oct 25 Questions? 24/30/2015Lecture 7 - SSL/TLS

Course Admin Mid-Term Exam – Oct 23 – In-class, class timing (2 hrs?) – Covers Lecture 1-7 – Review Oct 16 4/30/2015Lecture 7 - SSL/TLS3

Outline SSL/TLS – Protocol – Messages and Message Formats – Secure Data Exchange Exposition borrowed from Stephen Thomas (a book on SSL) 4/30/2015Lecture 7 - SSL/TLS4

5 SSL: Secure Sockets Layer Widely deployed security protocol –Supported by almost all browsers and web servers –https –Tens of billions $ spent per year over SSL Originally designed by Netscape in 1993 Number of variations: –TLS: transport layer security, RFC 2246 Provides –Confidentiality –Integrity –Authentication Original goals: –Had web e-commerce transactions in mind –Encryption (especially credit- card numbers) –Web-server authentication –Optional client authentication –Minimum hassle in doing business with new merchant Available to all TCP applications –Not just web –e.g., (IMAP, SMTP), FTP 4/30/2015Lecture 7 - SSL/TLS

SSL in Action Let us see some examples… –Gmail (uses SSL) –Wells fargo (uses SSL) –Blazernet (uses SSL) –Uab (no SSL) HTTPS: HTTP over SSL (or TLS) – Typically on port 443 (regular http on port 80) 4/30/2015Lecture 7 - SSL/TLS6

77 Which Layer to Add Security to? Relative Location of Security Facilities in the TCP/IP Protocol Stack 4/30/2015Lecture 7 - SSL/TLS

88 SSL and TLS SSL 2.0 was developed and patented by Netscape in TLS is the non-proprietary Internet standard development (RFC 2246, 1999) TLS 1.0 was an upgrade of SSL 3.0, so TLS 1.0 is sometimes referred to as SSL 3.1 Latest standard is TLS 1.2, sometimes referred to as SSL 3.3 4/30/2015Lecture 7 - SSL/TLS

99 SSL Main Components 1.Handshake 1.Negotiation of protocol algorithms, versions and parameters 2.Authentication of communicating parties 3.Agreement of session keys 2.Secure Session Communication 4/30/2015Lecture 7 - SSL/TLS

10 1 or more SSL Record Layer units 443 4/30/2015Lecture 7 - SSL/TLS

11 Establishing Secure Communications First, establish TCP connection from client to port 443 on server Secure channel established – proceed to use

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18 Secure channel established

19 ClientHello Current versions: SSL 3.3, TLS 1.2 Also used as a nonce to repel replay attacks 4/30/2015Lecture 7 - SSL/TLS

20 ServerHello Server selects from menu submitted by client Server decides 4/30/2015Lecture 7 - SSL/TLS

21 ServerKeyExchange Server sends its public key certificate ServerHelloDone Server has completed initial negotiation. ClientKeyExchange Client generates “premaster secret,” and sends it encrypted with the server’s public key. Server decrypts the premaster secret using the corresponding private key. Both sides can compute necessary keys. Change Cipher Spec Preliminary negotiations are complete and client tells server “I’m going to begin using the agreed cipher suite.”

22 ChangeCipherSpec “Since the transition to secured communication is critical, and both sides have to get it exactly right, the SSL specification is very precise in describing the process.” “The SSL specification also recognizes that some of the information (in particular, the key material) will be different for each direction of communication. In other words, one set of keys will secure data the client sends to the server, and a different set of keys will secure data the server sends to the client.” “For a given system, whether it is a client or a server, SSL defines a write state and a read state. The write state defines the security information for data that the system sends, and the read state defines the security information for data that the system receives.” 4/30/2015Lecture 7 - SSL/TLS

23 ChangeCipher Spec

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25 Finished “Immediately after sending their ChangeCipherSpec messages, each system sends a Finished message. The Finished messages allow both systems to verify that negotiation has been successful and that security has not been compromised. Two aspects of the Finished message contribute to this security.” “First … the Finished message itself is subject to the negotiated cipher suite … If the receiving party cannot successfully decrypt and verify the message, then clearly something has gone awry with the security negotiation.” “The contents of the Finished message also serves to protect the security of the SSL negotiation. Each Finished message contains a cryptographic keyed hash (MAC) of important information about the just-finished negotiation … This protects against an attacker who manages to insert fictitious messages into, or remove legitimate messages from, the communication.” 4/30/2015Lecture 7 - SSL/TLS

26 Authenticating the Server By now in this course we’re familiar with the need to authenticate the server’s identity. In the usual situation in which SSL is deployed (ordering from Amazon.com) we do not need to authenticate the client – SSL has an option to do so, but we will skip this. No surprise: we will insist on the server sending the client an X.509 certificate – browser will automatically check validity, using its library of CA public keys. 4/30/2015Lecture 7 - SSL/TLS

27 Authenticating the Server’s Identity – continued New: replaces ServerKeyExchange 4/30/2015Lecture 7 - SSL/TLS

28 ClientKeyExchange Encryption of the “pre-master secret” with the public key sent in the Certificate message means that the server must actually possess the corresponding private key to decrypt the pre- master secret. Both sides can compute necessary keys. Darth Sends amazon.com certificate

29 Message Formats Transport Requirements Record Layer ChangeCipherSpec Protocol Alert Protocol Severity Level Alert Description Handshake Protocol ClientHello ServerHello Certificate ServerHelloDone ClientKeyExchange- include RSA only Finished Securing Messages Message Authentication Code Encryption Creating Cryptographic Keys

30 1 or more SSL Record Layer units 443 4/30/2015Lecture 7 - SSL/TLS

31 Transport Requirements 4/30/2015Lecture 7 - SSL/TLS

32 Record Layer 4/30/2015Lecture 7 - SSL/TLS

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34 Figure 5.3 SSL Record Protocol Operations 4/30/2015Lecture 7 - SSL/TLS

35 HTTP 4/30/2015Lecture 7 - SSL/TLS

36 ChangeCipherSpec Protocol Record Layer Header 4/30/2015Lecture 7 - SSL/TLS

37 Alert Protocol The Alert Protocol signals an error. Some error messages are cautionary, others fatal. TLS removes some of the error categories in SSL and adds some new ones. 4/30/2015Lecture 7 - SSL/TLS

38 Alert Protocol Description

39 Handshake Protocol Purposes: 1. negotiate cipher suite to be used ClientHello message ServerHello message 2. authenticate I/D of server Certificate message ClientKeyExchange message 3. generate collection of shared secret information Premaster secret (ClientKeyExchange) Master secret Keying material MAC key Encryption key IV

40 Record Layer Header protocol = 22 In practice they are not! Format of Handshake message 4/30/2015Lecture 7 - SSL/TLS

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43 ClientHello Record Layer Header protocol = 22

44 There are more of these in SSL; TLS removes some and adds others.

45 Secure Socket Layer TLSv1 Record Layer: Handshake Protocol: Client Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 92 Handshake Protocol: Client Hello Handshake Type: Client Hello (1) Length: 88 Version: TLS 1.0 (0x0301) Random gmt_unix_time: Oct 10, :54: random_bytes: 751AB9DCEBF3014D799038D27E24E6409C8397FE6E1A Session ID Length: 0 Cipher Suites Length: 24 Cipher Suites (12 suites) Cipher Suite: TLS_DHE_RSA_WITH_AES_256_CBC_SHA (0x0039) Cipher Suite: TLS_DHE_DSS_WITH_AES_256_CBC_SHA (0x0038) Cipher Suite: TLS_RSA_WITH_AES_256_CBC_SHA (0x0035) Cipher Suite: TLS_DHE_RSA_WITH_AES_128_CBC_SHA (0x0033) Cipher Suite: TLS_DHE_DSS_WITH_AES_128_CBC_SHA (0x0032) Cipher Suite: TLS_RSA_WITH_RC4_128_MD5 (0x0004) Cipher Suite: TLS_RSA_WITH_RC4_128_SHA (0x0005) Cipher Suite: TLS_RSA_WITH_AES_128_CBC_SHA (0x002f) Cipher Suite: TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA (0x0016) Cipher Suite: TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA (0x0013) Cipher Suite: SSL_RSA_FIPS_WITH_3DES_EDE_CBC_SHA (0xfeff) Cipher Suite: TLS_RSA_WITH_3DES_EDE_CBC_SHA (0x000a) Compression Methods Length: 1 Compression Methods (1 method) Compression Method: null (0) Client can handle up to TLS 1.0 (SSL 3.1) Remarkable range of capabilities in browser!

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47 ServerHello 4/30/2015Lecture 7 - SSL/TLS

48 Secure Socket Layer TLSv1 Record Layer: Handshake Protocol: Server Hello Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 74 Handshake Protocol: Server Hello Handshake Type: Server Hello (2) Length: 70 Version: TLS 1.0 (0x0301) Random gmt_unix_time: Oct 10, :00: random_bytes: C7B2A2F58454A2C2A0DE667781E C86C8FF724069E... Session ID Length: 32 Session ID: 77987B601B5544C111C3FCB1DF96F7A8970D1EFD39630F3F... Cipher Suite: TLS_RSA_WITH_RC4_128_MD5 (0x0004) Compression Method: null (0) Server to client:

49 Certificate

50 Secure Socket Layer TLSv1 Record Layer: Handshake Protocol: Certificate Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 2468 Handshake Protocol: Certificate Handshake Type: Certificate (11) Length: 2464 Certificates Length: 2461 Certificates (2461 bytes) Certificate Length: 1271 Certificate (id-at-commonName= Certificate Length: 1184 Certificate (id-at-commonName=VeriSign Class 3 Secure Server CA Secure Socket Layer TLSv1 Record Layer: Handshake Protocol: Server Hello Done Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 4 Handshake Protocol: Server Hello Done Handshake Type: Server Hello Done (14) Length: 0 Server to client:

51 Certificate #1: Issued to: Issuer: VeriSign Class 3 Secure Server CA Certificate #2: Issued to: VeriSign Class 3 Secure Server CA Issuer: VeriSign Class 3 Public Primary Certification Authority Example “Certificate” message from Amazon.com contains a chain of public key certificates: 4/30/2015Lecture 7 - SSL/TLS

52 ServerHelloDone 4/30/2015Lecture 7 - SSL/TLS

53 Both sides know algorithms, client generates “pre-master secret” and can use it to compute all necessary keys (session key, MAC key). Client encrypts pre-master secret with server public key and sends. Server has received encrypted pre-master secret, decrypts with its private key and uses pre-master secret to compute all necessary keys. Both sides know all keys.

54 ClientKeyExchange Chronologically, ChangeCipherSpec comes here, but it’s not part of the Handshake Protocol. 4/30/2015Lecture 7 - SSL/TLS

55 Finished 4/30/2015Lecture 7 - SSL/TLS

56 Secure Socket Layer TLSv1 Record Layer: Handshake Protocol: Client Key Exchange Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 134 Handshake Protocol: Client Key Exchange Handshake Type: Client Key Exchange (16) Length: 130 TLSv1 Record Layer: Change Cipher Spec Protocol: Change Cipher Spec Content Type: Change Cipher Spec (20) Version: TLS 1.0 (0x0301) Length: 1 Change Cipher Spec Message TLSv1 Record Layer: Handshake Protocol: Encrypted Handshake Message Content Type: Handshake (22) Version: TLS 1.0 (0x0301) Length: 32 Handshake Protocol: Encrypted Handshake Message The 3 messages from the client: 4/30/2015Lecture 7 - SSL/TLS

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58 Creating Cryptographic Parameters Where did the various keys come from? Calculation of the Master Secret: 48 bytes

We need this secret information

Creation of the secret information (key material) TLS does this somewhat differently

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62 Both sides know algorithms, client generates “pre-master secret” and can use it to compute all necessary keys (session key, IV, MAC key). Client encrypts pre-master secret with server public key and sends. Server receives encrypted pre-master secret, decrypts with its private key and uses pre-master secret to compute all necessary keys. Then both sides have computed identical keys. Review: repeat of a previous slide : We need to have an agreed test message.

63 Return to Finished “Finished” message carries the agreed test message, MD5 and SHA hashes of the previous handshake messages. Here’s the SHA: TLS uses a slightly different hash calculation. Inner and outer hash remind us of HMAC Keyed, not signed

64 Finished 4/30/2015Lecture 7 - SSL/TLS

65 Securing Messages (Application) Handshake finally over! Ready to do useful work.

66 The inner and outer hash used here in SSL reminds us of HMAC (RFC 2104). This is slightly different, but TLS uses HMAC exactly. 4/30/2015Lecture 7 - SSL/TLS

67 Session Resumption Full handshake is expensive: CPU time and number of RTTs If the client and server have already communicated once, they can skip handshake and proceed directly to data transfer –For a given session, client and server store session_id, master_secret, negotiated ciphers Client sends session_id in ClientHello Server then agrees to resume in ServerHello –New key_block computed from master_secret and client and server random numbers

Further Reading SSL and TLS Essentials, Stephen Thomas Stallings Chapter 6 4/30/2015Lecture 7 - SSL/TLS68