lecture 7: transport level security – ssl/tls

Lecture 7: Transport Level Security – SSL/TLS

CS 336/536: Computer Network SecurityFall 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 (emailed)

• HW/Lab 2 posted – Covers Lecture 5 (network mapping and attacks)– Due Oct 25

• Questions?

204/19/23 Lecture 7 - SSL/TLS

Course Admin

• Mid-Term Exam– Oct 23– In-class, class timing (2 hrs?)– Covers Lecture 1-7– Review Oct 16

04/19/23 Lecture 7 - SSL/TLS 3

Outline

SSL/TLS– Protocol– Messages and Message Formats– Secure Data Exchange

Exposition borrowed from Stephen Thomas (a book on SSL)


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., email (IMAP, SMTP),

FTP


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)


77

Which Layer to Add Security to?

Relative Location of Security Facilities in the TCP/IP Protocol Stack


88

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


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


1010

1 or more SSL Record Layer units

443


1111

Establishing Secure Communications

First, establish TCP connection from client to port 443 on server

Secure channel established – proceed to use

1818Secure channel established

1919

ClientHello

Current versions: SSL 3.3, TLS 1.2

Also used as a nonce to repel replay

attacks


2020

ServerHello

Server selects from menu

submitted by client

Server decides


2121

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.”

2222

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.”


2323

ChangeCipherSpec

2525

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.”


2626

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.


2727

Authenticating the Server’s Identity –

continued

New: replaces ServerKeyExchange


2828

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

2929

Message Formats

Transport Requirements

Record Layer

ChangeCipherSpec Protocol

Alert ProtocolSeverity LevelAlert Description

Handshake ProtocolClientHelloServerHelloCertificateServerHelloDoneClientKeyExchange- include RSA onlyFinished

Securing MessagesMessage Authentication CodeEncryptionCreating Cryptographic Keys

3030

1 or more SSL Record Layer units

443


3131

Transport Requirements


3232

Record Layer


3434

Figure 5.3 SSL Record Protocol Operations


3535

HTTP


3636

ChangeCipherSpec Protocol

Record Layer Header


3737

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.


3838

Alert Protocol Description

3939

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 keyEncryption keyIV

4040

Record Layer Header protocol = 22

In practice they are not!

Format of Handshake message


4343

ClientHello Record Layer Header protocol = 22

4444There are more of these in SSL; TLS removes some and adds others.

4545

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!

4747

ServerHello


4848

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:

4949

Certificate

5050

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

Server to client:

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:


http://www.amazon.com/

5252

ServerHelloDone


5353

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.

5454

ClientKeyExchange

Chronologically, ChangeCipherSpec comes here, but it’s not part of the Handshake Protocol.


5555

Finished


5656

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:


5858

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

6262

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.

6363

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

6464

Finished


6565

Securing Messages (Application)

Handshake finally over! Ready to do useful work.

6666

The inner and outer hash used here in SSL reminds us of HMAC (RFC 2104).

This is slightly different, but TLS uses HMAC exactly.


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


lecture 7: transport level security – ssl/tls

Documents

covers lecture

previous lecture

tls ssl

upgrade of ssl

ssl record layer units443

transport layer security

secure communicationsfirst

premaster secret