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SSL/TLS

IT443 – Network Security AdministrationInstructor: Bo Sheng

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What Layer?

OS

App

l.

TCPIPSec

IPLAN layer

TCPSSL

IPLAN layer

ApplicationApplication

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History• SSLv2 proposed and deployed in Netscape 1.1 (1995)

• PCT (Private Communications Technology) by Microsoft

• SSLv3: most commonly used (1995)– was developed with public review

• TLS proposed by the IETF based on SSLv3 but not compatible (1996)• Uses patent free DH and DSS instead of RSA which patent

didn’t expire yet

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SSL vs. IPsec• SSL:

– Avoids modifying “TCP stack” and requires minimum changes to the application

– Mostly used to authenticate servers

• IPsec– Transparent to the application and requires modification of the

network stack– Authenticates network nodes and establishes a secure channel

between nodes– Application still needs to authenticate the users

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SSL Architecture

HTTP and other applications

SSLHandshake

Protocol

SSLChange Cipher

Protocol

SSLAlert

Protocol

SSLAPI

SSL Record Protocol

TCP

IP…

Relies on TCP for reliable communication

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SSL Architecture• Handshake protocol: establishment of a session key

• Change Cipher protocol: start using the previously-negotiated encryption / message authentication

• Alert protocol: notification (warnings or fatal exceptions)

• Record protocol: protected (encrypted, authenticated) communication between client and server

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Connections and Sessions• SSL Session

– an association between peers– created through a handshake, negotiates security

parameters, can be long-lasting• SSL Connection

– a type of service (i.e., an application) between a client and a server

– transient• Multiple connections can be part of a single

session

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Basic Protocols• Goal: application independent security

– Originally for HTTP, but now used for many applications– Each application has an assigned TCP port, e.g., https (HTTP

over SSL) uses port 443

• Messages– A -> B: I want to talk, ciphers I support, RA

– B -> A: certificates, cipher I choose, RB

– A -> B: {S}B+, {keyed hash of handshake msgs}– B -> A: {keyed hash of handshake msgs}– A <-> B: data encrypted and integrity checked with keys derived from K– Keyed hashes use K = f(S, RA, RB)

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Basic Protocols• How do you make sure that keyed hash in message 3 is

different from B’s response?– Include a constant CLNT/client finished (in SSL/TLS) for A and

SRVR/server finished for B

• Keyed hash is sent encrypted and integrity protected for no real reason

• Keys: derived by hashing K and RA and RB– 3 keys in each direction: encryption, integrity and IV– Write keys (to send: encrypt, integrity protect)– Read keys (to receive: decrypt, integrity check)

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Session Resumption• Many secure connections can be derived from the session

– Cheap: how?

• Session initiation: modify message 2– B -> A: session_id, certificate, cipher, RB

• A and B remember: (session_id, master key)

• To resume a session: A presents the session_id in message 1– A -> B: session_id, ciphers I support, RA

– B -> A: session_id, cipher I choose, RB, {keyed hash of handshake msgs}– A -> B: {keyed hash of handshake msgs}– A <-> B: data encrypted and integrity checked with keys derived from K

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Negotiating Cipher Suites• A cipher suite is a complete package:

– (encryption algorithm, key length, integrity checksum algorithm, etc.)

• Cipher suites are predefined:– Each assigned a unique value (contrast with IKE)– SSLv2: 3 bytes, SSLv3: 2 bytes => up to 65000 combinations

• 30 defined, • 256 reserved for private use: FFxx (risk of non-interoperability)

• Selection decision: – In v3 A proposes, B chooses– In v2 A proposes, B returns acceptable choices, and A chooses

• Suite names examples:– SSL_RSA_EXPORT_WITH_DES40_CBC_SHA– SSL2_RC4_128_WITH_MD5

SSL Record Protocol

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Protocol Steps1. Fragment data stream into records

– each with a maximum length of 214 (=16K) bytes

2. Compress each record

3. Create message authentication code for each record

4. Encrypt each record

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Protocol Steps

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Application Data

Fragment

Compress

Add MAC

Encrypt

Add SSL Hdr

SSL Record Format

• There is, unfortunately, some version number silliness between v2 and v3; see text for (ugly) details

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RecordType SSL Version Payload

Length

Application Data(optionally compressed)

Optional MAC (16 or 20 bytes)

Encr

ypte

d

SSL Handshake Protocol

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Phases of ProtocolI. Establish security capabilities

• version of SSL to use• cipher + parameters to use

II. Authenticate server (optional), and perform key exchange

III. Authenticate client (optional), and perform key exchange

IV. Finish up

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All the Messages

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I. Establish Security Capabilities

• Messages marked with * are mandatory19

Client ServerClient_Hello*

Server_Hello*

Client_Hello Message• Transmitted in plaintext

• Contents– highest SSL version understood by client– RC: a 4-byte timestamp + 28-byte random number– session ID: 0 for a new session, non-zero for a

previous session– list of supported cryptographic algorithms– list of supported compression methods

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Server_Hello Message• Also transmitted in plaintext

• Contents– minimum of (highest version supported by server,

highest version supported by client)– RS: 4-byte timestamp and 28-byte random number– session ID– a cryptographic choice selected from the client’s list– a compression method selected from the client’s list

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II. Server Auth. / Key Exchange

• The Server_Certificate message is optional, but almost always used in practice

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Client Server

Server_Certifica

te

Server_Key_Excha

nge

Client_Certifica

te_Request

Server_Handshake

_Done*

Server_Certificate Message• Contains a certificate with server’s public

key, in X.509 format– or, a chain of certificates if required

• The server certificate is necessary for any key exchange method except for anonymous Diffie-Hellman

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Authenticating the Server

• Step #4: Domain name in certificate must match domain name of server (not part of SSL protocol, but clients should check this)

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source: sun.com

Client_Certificate_Request Msg.• Normally not used, because in most applications

– only the server is authenticated– client is authenticated at the application layer, if

needed

• Two parameters– certificate type accepted, e.g., RSA/signature only,

DSS/signature only, …– list of certificate authorities recognized (i.e., trusted

third parties)

III. Client Auth. / Key Exchange

Client ServerClient_CertificateClient_Key_Exchange*Client_Certificate_Verify

Client_Certificate_Verify Msg• Proves the client is the valid owner of a

certificate (i.e., knows the corresponding private key)

• Only sent following any client certificate that has signing capability

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IV. Finish Up

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Client ServerChange_Cipher_Spec*

Change_Cipher_Sp

ec*

Client_Finished*

Server_Finished*

Switch to the negotiated cipherfor all remaining (application)

messages

Change_Cipher_Spec Msg• Confirms the change of the current state of

the session to a newly-negotiated set of cryptographic parameters

• Finished Messages– keyed hash of the previous handshake

messages to prevent man-in-the-middle-attacks from succeeding

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Alert Protocol Examples• Type 1: Warning

– ex.: No_Certificate, Close_Notify

• Type 2: Fatal_Alert– ex.: Unexpected_Message, Bad_MAC, etc.– connection is immediately terminated

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Summary

1.SSL is the de facto authentication/encryption protocol standard for HTTP– becoming popular for many other protocols as

well

2.Allows negotiation of cryptographic methods and parameters

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