SSL & TLS ArchitectureBy Avirot M. LiangsiriSenior Technical SpecialistProfessional Computer Co., Ltd.

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Web Security Essential

• Web now widely used by business, government, individuals for multiple application

• But Internet & Web are vulnerable• Have a variety of threats

• integrity• confidentiality• denial of service• authentication

• Need added security mechanisms

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Security Architecture• ITU-T Recommendation X.805 Security

architecture for systems providing end‑to‑end communications had been developed by ITU-T SG 17 (ITU-T Lead Study Group on Telecommunication Security) and was published in October 2003.

• The group has developed a set of the well-recognized Recommendations on security. Among them are X.800 Series of Recommendations on security and X.509 v3 - Public-key and Attribute Certificate Frameworks.

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ITU-T X.800 Threat Model(simplified)

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X

X1 - Destruction (an attack on availability):

– Destruction of information and/or network resources

2 - Corruption (an attack on integrity):

– Unauthorized tampering with an asset

3 - Removal (an attack on availability):

– Theft, removal or loss of information and/or other resources

4 - Disclosure (an attack on confidentiality):

– Unauthorized access to an asset

5 - Interruption (an attack on availability):

– Interruption of services. Network becomes unavailable or unusable

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Access Control

Authentication

Non-repudiation

Data Confidentiality

Communication Security

Data Integrity

Availability

Privacy

• Limit & control access to network elements, services & applications

• Examples: password, ACL, firewall

• Prevent ability to deny that an activity on the network occurred

• Examples: system logs, digital signatures

• Ensure information only flows from source to destination

• Examples: VPN, MPLS, L2TP

• Ensure network elements, services and application available to legitimate users

• Examples: IDS/IPS, network redundancy, BC/DR

• Provide Proof of Identity• Examples: shared secret,

PKI, digital signature, digital certificate

• Ensure confidentiality of data • Example: encryption

• Ensure data is received as sent or retrieved as stored

• Examples: MD5, digital signature, anti-virus software

• Ensure identification and network use is kept private

• Examples: NAT, encryption

ITU-T X.800 Eight Security Dimensions Address the Breadth of Network

Vulnerabilities

Eight Security Dimensions applied to each Security Perspective (layer and plane)

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ITU-T X.800 Three Security Layers

• Each Security Layer has unique vulnerabilities, threats• Infrastructure security enables services security enables applications security

Infrastructure Security

Applications Security

Services Security

THREATS

VULNERABILITIES

ATTACKS

Destruction

Disclosure

Corruption

Removal

Infrastructure Security

Applications Security

Services SecurityVULNERABILITIES

InterruptionVulnerabilities Can ExistIn Each Layer

1 - Infrastructure Security Layer:• Fundamental building blocks of networks

services and applications• Examples:

– Individual routers, switches, servers– Point-to-point WAN links– Ethernet links

2 - Services Security Layer:• Services Provided to End-Users• Examples:

– Frame Relay, ATM, IP– Cellular, Wi-Fi,– VoIP, QoS, IM, Location services– Toll free call services

3 - Applications Security Layer:• Network-based applications accessed by

end-users• Examples:

– Web browsing– Directory assistance– Email– E-commerce

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ITU-T X.800 Applying Security Planes to Network Protocols

End User Security Plane Activities

•End-user data transfer•End-user – application interactions

Protocols• HTTP, RTP, POP, IMAP• TCP, UDP, FTP• IPsec, TLS

Control/Signaling Security Plane Activities

•Update of routing/switching tables•Service initiation, control, and teardown•Application control

Protocols

• BGP, OSPF, IS-IS, RIP, PIM

• SIP, RSVP, H.323, SS7.• IKE, ICMP• PKI, DNS, DHCP, SMTP

Management Security Plane

•Operations•Administration•Management•Provisioning

Activities Protocols•SNMP•Telnet•FTP•HTTP

SSL (Secure Socket Layer)

• transport layer security service• originally developed by Netscape• version 3 designed with public input• subsequently became Internet standard known as

TLS (Transport Layer Security)• uses TCP to provide a reliable end-to-end service• SSL has two layers of protocols

Where SSL Fits

HTTP SMTP POP3

80 25 110

HTTPS SSMTP SPOP3

443 465 995

Secure Sockets Layer

Transport

Network

Link

Uses Public Key Scheme

• Each client-server pair uses• 2 public keys

• one for client (browser)• created when browser is installed on client machine

• one for server (http server)• created when server is installed on server hardware

• 2 private keys• one for client browser• one for server (http server)

SSL Architecture

SSL Architecture• SSL session

• an association between client & server• created by the Handshake Protocol• define a set of cryptographic parameters• may be shared by multiple SSL connections (by using

same session symmetric key)• SSL connection

• a transient, peer-to-peer, communications link• associated with 1 SSL session

SSL Record Protocol

• confidentiality• using symmetric encryption with a shared secret key

defined by Handshake Protocol• IDEA, RC2-40, DES-40, DES, 3DES, Fortezza, RC4-40,

RC4-128• message is compressed before encryption

• message integrity• using a MAC (Message Authentication Code) created

using a shared secret key and a short message

SSL Alert Protocol

• conveys SSL-related alerts to peer entity• severity

• warning or fatal

• specific alert• unexpected message, bad record mac, decompression failure,

handshake failure, illegal parameter• close notify, no certificate, bad certificate, unsupported

certificate, certificate revoked, certificate expired, certificate unknown

• compressed & encrypted like all SSL data

SSL Handshake Protocol

• allows server & client to:• authenticate each other• to negotiate encryption & MAC algorithms• to negotiate cryptographic keys to be used

• comprises a series of messages in phases• Establish Security Capabilities• Server Authentication and Key Exchange• Client Authentication and Key Exchange• Finish

SSL Handshake Protocol

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Changes from SSL 3.0 to TLS

• Fortezza removed• Additional Alerts added• Modification to hash calculations• Protocol version 3.1 in ClientHello,

ServerHello

TLS (Transport Layer Security)• IETF standard RFC 2246 similar to SSLv3• with minor differences

• in record format version number• uses HMAC for MAC• a pseudo-random function expands secrets• has additional alert codes• some changes in supported ciphers• changes in certificate negotiations• changes in use of padding

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TLS:Key Exchange

• Need secure method to exchange secret key• Use public key encryption for this

• “key pair” is used - either one can encrypt and then the other can decrypt

• slower than conventional cryptography• share one key, keep the other private

• Choices are RSA or Diffie-Hellman

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TLS: Integrity

• Compute fixed-length Message Authentication Code (MAC)• Includes hash of message• Includes a shared secret• Include sequence number

• Transmit MAC with message

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TLS: Integrity

• Receiver creates new MAC• should match transmitted MAC

• TLS allows MD5, SHA-1

A B

Message’

MAC’

MAC

=?

Message

MAC

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TLS: Authentication

• Verify identities of participants• Client authentication is optional• Certificate is used to associate identity with

public key and other attributes

A

Certificate

B

Certificate

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TLS: Overview

• Establish a session • Agree on algorithms• Share secrets• Perform authentication

• Transfer application data• Ensure privacy and integrity

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TLS: Architecture

• TLS defines Record Protocol to transfer application and TLS information

• A session is established using a Handshake Protocol

TLS Record Protocol

Handshake Protocol

Alert Protocol

ChangeCipher Spec

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TLS: Record Protocol

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TLS: Handshake

• Negotiate Cipher-Suite Algorithms• Symmetric cipher to use• Key exchange method• Message digest function

• Establish and share master secret• Optionally authenticate server and/or client

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Handshake Phases

• Hello messages• Certificate and Key Exchange messages• Change CipherSpec and Finished messages

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TLS: Hello

• Client “Hello” - initiates session• Propose protocol version• Propose cipher suite• Server chooses protocol and suite

• Client may request use of cached session• Server chooses whether to honor request

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TLS: Key Exchange

• Server sends certificate containing public key (RSA) or Diffie-Hellman parameters

• Client sends encrypted “pre-master” secret to server using Client Key Exchange message

• Master secret calculated • Use random values passed in Client and Server

Hello messages

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Public Key Certificates

• X.509 Certificate associates public key with identity

• Certification Authority (CA) creates certificate• Adheres to policies and verifies identity• Signs certificate

• User of Certificate must ensure it is valid

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Validating a Certificate

• Must recognize accepted CA in certificate chain• One CA may issue certificate for another CA

• Must verify that certificate has not been revoked• CA publishes Certificate Revocation List (CRL)

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X.509: Certificate Content• Version• Serial Number• Signature Algorithm Identifier

• Object Identifier (OID)• e.g. id-dsa: {iso(1) member-

body(2) us(840) x9-57 (10040) x9algorithm(4) 1}

• Issuer (CA) X.500 name• Validity Period (Start,End)

• Subject X.500 name• Subject Public Key

• Algorithm• Value

• Issuer Unique Id (Version 2 ,3)• Subject Unique Id (Version

2,3) • Extensions (version 3)

• optional

• CA digital Signature

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Subject Names

• X.500 Distinguished Name (DN)• Associated with node in hierarchical directory

(X.500)• Each node has Relative Distinguished Name

(RDN)• Path for parent node• Unique set of attribute/value pairs for this

node

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Example Subject Name

• Country at Highest Level (e.g. US)• Organization typically at next level (e.g. CertCo)• Individual below (e.g. Common Name

“Elizabeth” with Id = 1)DN = {

• C=US; • O=CertCo; • CN=Elizabeth, ID=1}

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Version 3 Certificates

• Version 3 X.509 Certificates support alternative name formats as extensions• X.500 names• Internet domain names• e-mail addresses• URLs

• Certificate may include more than one name

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Certificate Signature

• RSA Signature• Create hash of certificate• Encrypt using CA’s private key

• Signature verification • Decrypt using CA’s public key• Verify hash

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TLS: ServerKeyExchangeClient

ClientHello

Server

ServerHello Certificate ServerKeyExchange

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TLS: Certificate RequestClient

ClientHello

Server

ServerHello Certificate ServerKeyExchange CertificateRequest

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TLS: Client CertificateClient

ClientHello

ClientCertificateClientKeyExchange

Server

ServerHello Certificate ServerKeyExchange CertificateRequest

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TLS: Change Cipher Spec, FinishedClient

[ChangeCipherSpec]Finished

Application Data

Server

[ChangeCipherSpec] Finished

Application Data

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TLS: Change Cipher Spec/Finished• Change Cipher Spec

• Announce switch to negotiated algorithms and values

• Finished• Send copy of handshake using new session• Permits validation of handshake

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TLS: Using a SessionClient

ClientHello (Session #)

[ChangeCipherSpec]Finished

Application Data

Server

ServerHello (Session #)[ChangeCipherSpec] Finished

Application Data

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TLS: HTTP Application

• HTTP most common TLS application• https://

• Requires TLS-capable web server• Requires TLS-capable web browser

• Netscape Navigator• Internet Explorer• Cryptozilla

• Netscape Mozilla sources with SSLeay

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X.509 Certificate Issues

• Certificate Administration is complex• Hierarchy of Certification Authorities• Mechanisms for requesting, issuing, revoking

certificates• X.500 names are complicated• Description formats are cumbersome (ASN.1)

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X.509 Alternative: SDSI

• SDSI: Simple Distributed Security Infrastructure (Rivest, Lampson)• Merging with IETF SPKI: Simple Public-Key

Infrastructure in SDSI 2.0• Eliminate X.500 names - use DNS and text• Everyone is their own CA• Instead of ASN.1 use “S-expressions” and simple

syntax• Name and Authorization certificates

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TLS “Alternatives”

• S-HTTP: secure HTTP protocol, shttp://• IPSec: secure IP• SET: Secure Electronic Transaction

• Protocol and infrastructure for bank card payments

• SASL: Simple Authentication and Security Layer (RFC 2222)

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Summary

• SSL/TLS addresses the need for security in Internet communications• Privacy - conventional encryption• Integrity - Message Authentication Codes• Authentication - X.509 certificates

• SSL in use today with web browsers and servers• Equivalent to TLS