Lecture 7: Transport Level
Security – SSL/TLS
CS 336/536: Computer Network Security
Fall 2014
Nitesh Saxena
Adopted from previous lecture by Tony BarnardCourse Admin
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HW/Lab 1
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Graded; scores posted; to be returned today
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Solution was provided (emailed)
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HW/Lab 2 posted
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Covers Lecture 5 and 6 (network mapping and
attacks)
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Due Oct 20
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Labs active this Friday
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Questions?
Course Admin
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Mid-Term Exam
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Oct 22
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In-class, class timing (2 hrs?)
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Covers Lecture 1-7
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Closed Book/Notes
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Review Oct 15
10/8/2014 Lecture 7 - SSL/TLS 3Outline
SSL/TLS
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Protocol
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Messages and Message Formats
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Secure Data Exchange
Exposition borrowed from Stephen Thomas (a book
solely focusing on SSL)
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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., email (IMAP, SMTP), FTP
10/8/2014 Lecture 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)
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Which Layer to Add Security to?
Relative Location of Security Facilities in the TCP/IP Protocol Stack
10/8/2014 Lecture 7 - SSL/TLS
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SSL and TLS
SSL 2.0 was developed and patented by Netscape in 1994.
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
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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
10/8/2014 Lecture 7 - SSL/TLS
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1 or more SSL Record Layer units 443
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Establishing Secure Communications First, establish TCP connection from client to
port 443 on server
Secure channel established – proceed to use
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13 13 10/8/2014 Lecture 7 - SSL/TLS 14 14 10/8/2014 Lecture 7 - SSL/TLS
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19 19 ClientHello Current versions: SSL 3.3, TLS 1.2 Also used as a nonce to repel replay
attacks 10/8/2014 Lecture 7 - SSL/TLS 20 20 ServerHello Server selects from menu submitted by client Server decides 10/8/2014 Lecture 7 - SSL/TLS
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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.”
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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.”
23 23 ChangeCipher Spec 24 24
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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.”
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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.
27 27 Authenticating the Server’s Identity – continued New: replaces ServerKeyExchange 10/8/2014 Lecture 7 - SSL/TLS 28 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 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
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1 or more SSL Record Layer units 443
31 31 Transport Requirements 10/8/2014 Lecture 7 - SSL/TLS 32 32 Record Layer 10/8/2014 Lecture 7 - SSL/TLS
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Figure 5.3 SSL Record Protocol Operations
35 35 HTTP 10/8/2014 Lecture 7 - SSL/TLS 36 36 ChangeCipherSpec Protocol
Record Layer Header
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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.
10/8/2014 Lecture 7 - SSL/TLS
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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 40
Record Layer Header protocol = 22
In practice they are not! Format of Handshake message
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ClientHello Record Layer Header
protocol = 22
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There are more of these in SSL; TLS removes some and adds others.
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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, 2008 10:54:18.000000000 random_bytes: 751AB9DCEBF3014D799038D27E24E6409C8397FE6E1A7553... 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! 46 46 10/8/2014 Lecture 7 - SSL/TLS
47 47 ServerHello 10/8/2014 Lecture 7 - SSL/TLS 48 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, 2008 11:00:13.000000000 random_bytes: C7B2A2F58454A2C2A0DE667781E2773544C86C8FF724069E... Session ID Length: 32
Session ID: 77987B601B5544C111C3FCB1DF96F7A8970D1EFD39630F3F... Cipher Suite: TLS_RSA_WITH_RC4_128_MD5 (0x0004)
Compression Method: null (0) Server to client:
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Certificate
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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=www.amazon.com, 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
51 Certificate #1:
Issued to:www.amazon.com
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:
10/8/2014 Lecture 7 - SSL/TLS
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ServerHelloDone
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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.
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ClientKeyExchange
Chronologically, ChangeCipherSpec comes here, but it’s not part of the Handshake Protocol.
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Finished
10/8/2014 Lecture 7 - SSL/TLS 56 56Secure 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:
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Creating Cryptographic Parameters Where did the various keys come from?
Calculation of the Master Secret:
We need this secret information Creation of the secret information (key material) TLS does this somewhat differently
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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
:
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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
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Finished
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Securing Messages (Application)
Handshake finally over! Ready to do useful work.
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The inner and outer hash used here in SSL reminds
us of HMAC (RFC 2104). This is slightly different, but
TLS uses HMAC exactly.
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Session Resumption
• Full handshake is expensive: CPU time and amount of
communication
• 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 serverrandom numbers