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WEB Security

Behzad AkbariFall 2009

This slides are based in parts on the slides of Prof Keith Ross (Polytechnic University)

In the Name of the Most High

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Outline

Secure Socket Layer (SSL) and Transport Layer Security (TLS)

Secure Electronic Transaction (SET) Recommended Reading and WEB Sites

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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 Secure socket interface

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SSL and TCP/IP

Application

TCP

IP

Normal Application

Application

SSL

TCP

IP

Application with SSL

• SSL provides application programming interface (API) to applications• C and Java SSL libraries/classes readily available

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Could do something like PGP:

• But want to send byte streams & interactive data• Want a set of secret keys for the entire connection• Want certificate exchange part of protocol: handshake phase

H) (. KA) (.-

+

KA(H(m))-

m

KA-

m

KS) ( .

KB) (.+

+

KB(KS )+

KS

KB+

Internet

KS

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Toy SSL: a simple secure channel

Handshake: Alice and Bob use their certificates and private keys to authenticate each other and exchange shared secret

Key Derivation: Alice and Bob use shared secret to derive set of keys

Data Transfer: Data to be transferred is broken up into a series of records

Connection Closure: Special messages to securely close connection

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Toy: A simple handshake

MS = master secret EMS = encrypted master secret

hello

certificate

KB+(MS) = EMS

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RECALL: Certification Authorities When Alice wants Bob’s public key:

gets Bob’s certificate (Bob or elsewhere). apply CA’s public key to Bob’s certificate, get

Bob’s public key

Bob’s public

key K B+

digitalsignature

(decrypt)

CA public

key K CA+

K B+

Bob’s Certificate

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Toy: Key derivation

Considered bad to use same key for more than one cryptographic operation Use different keys for message authentication code (MAC)

and encryption

Four keys: Kc = encryption key for data sent from client to server

Mc = MAC key for data sent from client to server

Ks = encryption key for data sent from server to client

Ms = MAC key for data sent from server to client

Keys derived from key derivation function (KDF) Takes master secret and (possibly) some additional random

data and creates the keys

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Recall MACm

ess

ag

e

H) (

s

mess

ag

e

mess

ag

e

s

H) (

compare

s = shared secret

Recall that HMAC is a standardized MAC algorithm SSL uses a variation of HMAC TLS uses HMAC

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Toy: Data Records Why not encrypt data in constant stream as we write

it to TCP? Where would we put the MAC? If at end, no message

integrity until all data processed. For example, with instant messaging, how can we do

integrity check over all bytes sent before displaying? Instead, break stream in series of records

Each record carries a MAC Receiver can act on each record as it arrives

Issue: in record, receiver needs to distinguish MAC from data Want to use variable-length records

length data MAC

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Toy: Sequence Numbers

Attacker can capture and replay record or re-order records

Solution: put sequence number into MAC: MAC = MAC(Mx, sequence||data) Note: no sequence number field

Attacker could still replay all of the records Use random nonce

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Toy: Control information

Truncation attack: attacker forges TCP connection close segment One or both sides thinks there is less data than

there actually is. Solution: record types, with one type for

closure type 0 for data; type 1 for closure

MAC = MAC(Mx, sequence||type||data)

length type data MAC

Short Question:

In the SSL record, there is a field for SSL seq. numbers? True or False?

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False. Both sides maintain Seq. no independently .

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Toy SSL: summary

hello

certificate, nonce

KB+(MS) = EMS

type 0, seq 1, datatype 0, seq 2, data

type 0, seq 1, data

type 0, seq 3, data

type 1, seq 4, close

type 1, seq 2, close

en

cryp

ted

bob.com

Question

Suppose Seq. number is NOT used.

Question: In an SSL session, Can Trudy (woman-in-the-middle) delete a TCP segment?

Question: What effect will it have?

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Question

Suppose Seq. number is USED.

In an SSL session, an attacker inserts a bogus TCP segment into a packet stream with correct TCP checksum and seq. no.

Question: Will SSL at the receiving side accept bogus packet and pass the payload to upper-level?

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Toy SSL isn’t complete

How long are the fields? What encryption protocols? No negotiation

Allow client and server to support different encryption algorithms

Allow client and server to choose together specific algorithm before data transfer

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Most common symmetric ciphers in SSL DES – Data Encryption Standard: block 3DES – Triple strength: block RC2 – Rivest Cipher 2: block RC4 – Rivest Cipher 4: stream

Public key encryption RSA

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SSL Cipher Suite

Cipher Suite Public-key algorithm Symmetric encryption algorithm MAC algorithm

SSL supports a variety of cipher suites Negotiation: client and server must agree on

cipher suite Client offers choice; server picks one

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Real SSL: Handshake (1)

Purpose

1. Server authentication

2. Negotiation: agree on crypto algorithms

3. Establish keys

4. Client authentication (optional)

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Real SSL: Handshake (2)

1. Client sends list of algorithms it supports, along with client nonce

2. Server chooses algorithms from list; sends back: choice + certificate + server nonce

3. Client verifies certificate, extracts server’s public key, generates pre_master_secret, encrypts with server’s public key, sends to server

4. Client and server independently compute encryption and MAC keys from pre_master_secret and nonces

5. Client sends a MAC of all the handshake messages6. Server sends a MAC of all the handshake messages

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Real SSL: Handshaking (3)

Last 2 steps protect handshake from tampering Client typically offers range of algorithms,

some strong, some weak Man-in-the middle could delete the stronger

algorithms from list Last 2 steps prevent this

Last two messages are encrypted

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Real SSL: Handshaking (4) Why the two random nonces? Suppose Trudy sniffs all messages between

Alice & Bob. Next day, Trudy sets up TCP connection with

Bob, sends the exact same sequence of records,. Bob (Amazon) thinks Alice made two separate

orders for the same thing. Solution: Bob sends different random nonce for

each connection. This causes encryption keys to be different on the two days.

Trudy’s messages will fail Bob’s integrity check.

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Handshake types

All handshake messages (with SSL header) have 1 byte type field: Types ClientHello ServerHello Certificate ServerKeyExchange CertificateRequest ServerHelloDone CertificateVerify ClientKeyExchange Finished

SSL Protocol Stack

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SSL Record Protocol

data

data fragment

data fragment

MAC MAC

encrypteddata and MAC

encrypteddata and MAC

recordheader

recordheader

record header: content type; version; length

MAC: includes sequence number, MAC key Mx

Fragment: each SSL fragment 214 bytes (~16 Kbytes)

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SSL Record Format

contenttype SSL version length

MAC

data

1 byte 2 bytes 3 bytes

Data and MAC encrypted (symmetric algo)

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Content types in record header application_data (23) alert (21)

signaling errors during handshake handshake (22)

initial handshake messages are carried in records of type “handshake”

Hankshake messages in turn have their own “sub” types

change_cipher_spec (20) indicates change in encryption and authentication

algorithms

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handshake: ClientHello

handshake: ServerHello

handshake: Certificate

handshake: ServerHelloDone

handshake: ClientKeyExchangeChangeCipherSpec

handshake: Finished

ChangeCipherSpec

handshake: Finished

application_data

application_data

Alert: warning, close_notify

Real Connection

TCP Fin follow

Everythinghenceforth

is encrypted

Short Question

In which step of SSL handshake, can Alice discover that she is not talking with Bob?

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handshake: ClientHello

handshake: ServerHello

Bob’s certificateAlice

TrudyBobhandshake: ClientKeyExchangeChangeCipherSpec

handshake: Finished

ChangeCipherSpec

handshake: Finished

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a) Packet 112 sent by client or server? b) Server IP and port?c) What is the seq. no of the next TCP segment sent by client?

P19.

Client

216.75.194.220 443(https)

79( seq + )204 (len) = 283

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d) Does packet 112 contain a Master Secret? e) Assume HandShake type field is 1 byte, each length field is 3

bytes, what are the values of the first / last bytes of Master Secret?

P19.

Yes

First: bc; Last: 29

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Key derivation

Client nonce, server nonce, and pre-master secret input into pseudo random-number generator. Produces master secret

Master secret and new nonces inputed into another random-number generator: “key block”

Key block sliced and diced: client MAC key server MAC key client encryption key server encryption key client initialization vector (IV) server initialization vector (IV)

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RECALL: Cipher Block Chaining (CBC) CBC generates its own random numbers

Have encryption of current block depend on result of previous block

c(i) = KS( m(i) c(i-1) )

m(i) = KS( c(i)) c(i-1)

How do we encrypt first block? Initialization vector (IV): random block = c(0) IV does not have to be secret

Change IV for each message (or session) Guarantees that even if the same message is sent repeatedly,

the ciphertext will be completely different each time

Short Question:

Suppose an SSL session employs a block cipher with CBC (cipher block chaining).

The server sends Initialization Vector (VI) in clear-text? True or False?

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False. Each side can derive it.

Short Question:

What is the purpose of random nonces in SSL handshake?

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Defend against Playback Attack

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SSL Performance Big-number operations in public-key crypto are CPU

intensive Server handshake

Typically over half SSL handshake CPU time goes to RSA decryption of the encrypted pre_master_secret

Client handshake Public key encryption is less expensive Server is handshake bottleneck

Data transfer Symmetric encryption MAC calculation Neither as CPU intensive as public-key decryption

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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

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Client authentication

SSL can also authenticate client Server sends a CertificateRequest message

to client

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SSL Record Protocol Payload

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Transport Layer Security

The same record format as the SSL record format. Defined in RFC 2246.

Similar to SSLv3. Differences in the:

version number message authentication code pseudorandom function alert codes cipher suites client certificate types certificate_verify and finished message cryptographic computations padding

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Secure Electronic Transactions (SET) An open encryption and security specification. Protect credit card transaction on the Internet. Companies involved:

MasterCard, Visa, IBM, Microsoft, Netscape, RSA, Terisa and Verisign

Not a payment system. Set of security protocols and formats.

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SET Services

Provides a secure communication channel in a transaction.

Provides trust by the use of X.509v3 digital certificates.

Ensures privacy.

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SET Overview

Key Features of SET: Confidentiality of information Integrity of data Cardholder account authentication Merchant authentication

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SET Participants

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Sequence of events for transactions

1. The customer opens an account.2. The customer receives a certificate.3. Merchants have their own certificates.4. The customer places an order.5. The merchant is verified.6. The order and payment are sent.7. The merchant request payment authorization.8. The merchant confirm the order.9. The merchant provides the goods or service.10. The merchant requests payments.

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Dual Signature

H(OI))]||)(([ PIHHEDScKR

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Payment processing

Cardholder sends Purchase Request

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Payment processing

Merchant Verifies Customer Purchase Request

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Payment processing

Payment Authorization: Authorization Request Authorization Response

Payment Capture: Capture Request Capture Response

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Recommended Reading and WEB sites Drew, G. Using SET for Secure Electronic

Commerce. Prentice Hall, 1999 Garfinkel, S., and Spafford, G. Web Security &

Commerce. O’Reilly and Associates, 1997 MasterCard SET site Visa Electronic Commerce Site SETCo (documents and glossary of terms)