Chapter 11 and 12

Cryptography and Security Services: Mechanisms and Applications

Manuel [email protected]

M. Mogollon – 1

Chapter 11 and 12Chapter 11 and 12VPNs, IPsec, and TLS/SSL

M. Mogollon – 2VPN IPsec IKE v2 TLS

Session 9 – Contents

• VPNs

• Tunneling— IPsec— Layer 2 Tunneling Protocol (L2TP)

• TLS/SSL


TCP/IP Stack and Security Related Protocols

Application Layer

Transport Layer

Network Layer

Data Layer

SMTP, Telnet, FTP, Gopher

TCP UDP

IP

Ethernet, Token-Ring, FDDI, X.25, Wireless, Async, ATM, SNA...Data Layer

ARP RARP

• S/MIME• S-HTTP• PGP• SET

• IPsec (ISAKMP)

• SOCKS V5• TLS/SSL

• IPsec (AH, ESP)

• Packet Filtering• Tunneling Protocols

• PPP-EAP, IEEE 802.1X, CHAP, PAP, MS-CHAP


What is a Virtual Private Network?

VPNs / Private data communication channels that use a public IP network, i.e., Internet, as the basic transport for connecting corporate data centers, remote offices, mobile employees, telecommuters, customers, suppliers, and business partners. The public network is used as a wide area communications network, and it offers the appearance, functionality, and usefulness of a dedicated private network.


E-Commerce, E-Procurement, E-Care

Headquarters

Business Partners

Mobile Workforce

Suppliers Customers

TelecommutersContractors

Internet

But, the Internet is a public network and it doesn’t have any security!


Secure VPNs

• Security is implemented in all products that offer VPNs.

• Secure VPNs are revolutionizing the way the Internet is used.

• IETF has standardized IPsec (IP Security) for secure VPN applications that have the following features:—are transparent to all TCP/IP applications—can be implemented in any LAN/WAN environment using TCP/IP—can secure any business communication over the Internet.

• IPsec is a mandatory part of the forthcoming IPv6 standard.


Implementation of VPNs

• Located at the carrier’s network— In the first scenario, the service provider provides a service similar to

the public switched Frame Relay or ATM service, and the customer trusts that packets will not be misdirected, modified in transit, or subjected to traffic analysis by unauthorized parties.

• On the customer’s premises— In the second scenario, the customer does not trust the service

provider and implements a VPN using CPE equipment that provides firewall functionality and security.

• Any devices with microprocessors, such as routers, servers, firewalls or even PCs, can perform VPN functions, such as creating tunnels and encrypting packets.


Secure VPN

Headquarters

Business Partners

Mobile Workforce

Suppliers Customers

Telecommuters VPNsContractors

Internet

With Secure VPNs,• I am sure to whom I am talking.• I know my message has not been modified.• I know that only authorized persons have seen my message.• I know that the message recipient can’t deny receiving my message.


VPN Applications:Extranets and Remote Access

Internet

Tunnel Mode

Protected Subnet

Security Policy Server

Security Policy Server

Certificate Authority

Gateway

Mobile Workforce withIPsec Client Software

Transport Mode

Protected Subnet

• Tunnel ModeAuthentication is provided between a client and a corporate VPN device, or between two VPN devices.

• Transport ModeAuthentication is provided directly between a client and a server or between two work stations.

Server


Virtual Private Networks (VPN)

• Network of virtual circuits for carrying private traffic.

• VPN Protocols

• PPP and L2TP are aimed at remote access use.

• IPsec is used for connecting LANs.

PPP L2TP IPsec

Mode Client-server Client-server Host-to-Host

Purpose Remote access Remote access Intranets, extranets,via tunneling via tunneling remote access via

tunnelingOSI layer Layer 2 Layer 2 Layer 3

TCP/IP Layer Data Data Network

Protocol IP, IPX, IP, IPX, IPAppleTalk, etc AppleTalk, etc


VPN Benefits

• Ease of use – Facilitating electronic communications makes corporations more efficient and productive.

• Cost — Eliminating long-haul leased lines, 800 numbers or long distance fees,

modem banks, and multiple access connections results in significant savings.— Voice over IP reduces long distance phone call expenses.— Savings of up to 65% on monthly circuit costs by moving from a FR and ATM

environment to an IP VPN— Teleworker lower connection costs by 20%-25% per month over traditional

dial up & ISDN.

• Use of Standard Protocols – Internet Protocol IP and IPsec provide needed standardization.

• Simplification of Maintenance and Support – Reducing scalability issues and management complexity simplifies network operation.


What is IPsec?

IPsec / (1) A suit of security protocols standardized by the Internet Engineering Task Force (IETF) that address data privacy, integrity, authentication, and key management, as well as, tunneling to TCP/IP networks. (2) A secure architecture that supports several applications that encrypt and/or authenticate all traffic at the IP level.


Why IPsec

• IPsec-compliant products allow secure Virtual Private Networks in any existing IP-based network.

• IPsec is based on several strong encryption standards.

• IPsec provides security services such as: data origin authentication, access control, confidentiality (encryption), connectionless integrity, rejection of replayed packets (a form of partial sequence integrity), and limited traffic flow confidentiality.

• IPsec has government and industry support.

• IPsec allows corporations to select security services according to internal security policies.


Internet Protocol (IP) – Security Threats

• The Internet protocol has no security.— Source/destination address &

port

Various IP Header Fields

Source IP Address

Destination IP Address

Upper Protocol Header (i.e., TCP,

UDP, ICMP)Data

DataIP Header TCP

IP Packet

• Attacks include:— IP Spoofing— Packet Sniffing— Session Hijacking— Man-in-the-Middle

IP Header Payload Data


IPsec Interlocking Technologies

Cryptographic Security Mechanisms for IP• Authentication Header (AH)

— Provides integrity and authentication without confidentiality to IP datagrams.

— Available even in locations where the export, import or use of encryption to provide confidentiality is regulated.

• Encapsulation Security Payload (ESP)— Provides integrity, authentication, and confidentiality to IP datagrams.

Key Management• Internet Key Exchange IKEv2

— Allows users to agree on authentication methods, encryption methods, keys to use, and key duration.

— Key exchange could be manual or automated.


IP Security Architecture

• IPsec Databases— Security Policy Database (SPD)— Security Association Database (SAD)— Peer Authorization Database (PAD)

• Security Associations— Information shared between two

Gateways on how to secure communications.

IP Packets

ESP/AH Engine

IPsec Databases (SPD, SAD, PAD)

Authentication Header Protocol

Encapsulation Security

Payload Protocol

EncryptionAlgorithm

Authentication& IntegrityAlgorithms

Key Management

• Security Protocols— AH is used to authenticate.— ESP is used to encrypt and to

authenticate.

• Algorithms for encryption and authentication

— Symmetric encryption algorithms.— Keyed hash algorithms.

• Key Management Protocols— Manual and Automated


Security Protocols

• IPsec provides mechanisms to provide security services to IP and upper layer protocols (e.g., UDP or TCP).

• IPsec protect IP datagrams by defining a method in a SA.

• The SA associated with a connection could be Encapsulating Security Payload (ESP), or Authentication Header (AH), but not both.

• If both AH and ESP protection are applied to a connection, then two (or more) SAs are created to provide protection to the connection.

• To secure typical, bi-directional communication between two hosts, or between two security gateways, two Security Associations (one in each direction) are required.

• Both ESP and AH security protocols support two modes of operation: transport or tunnel mode.


IPsec Negotiation

1

2

3

4

5

6

Applications

TCP/IP

NegotiatorEngine

Unprotect-Protect EngineSecurity Policy

Database

Outbound IPsec Packet

Applications

TCP/IP

Unprotect-Protect Engine

1

2

SPI

SA Attributes

Inbound IPsec Packet

IPsec Databases(SAD, PAD)

IPsec Databases (SAD, PAD)

NegotiatorEngine

Security Policy Database


IP Security ArchitectureRFC 4301

AH ProtocolRFC 4302

Encryption AlgorithmsRPC 3602 (AES-CBC (128-Bit)RFC 3686 (AES-CTR)RFC 2451 (Triple DES-CBC)

Authentication AlgorithmsRFC 3566 (AES-XCBC-MAC-96)RFC 2404 (HMAC-SHA1-96)RFC 2403 (HMAC-MD5-96)

IKE v2RFC 4306

Key ManagementRFC 4120 (Kerberos)RFC 2093 (GKMP)RFC 2412 (OAKLEY)

ESP ProtocolRFC 4303

IPsec Document Roadmap


AH and ESP Modes of Operation

TunnelMode

Authentication / Integrity

Confidentiality

New IPHeader

HeaderESP

OriginalIP Header


Confidentiality

OriginalIP Header

HeaderESP


New IPHeader

HeaderAH

OriginalIP Header

OriginalIP Header

HeaderAH


TransportMode

ESPAH

PayloadData

PayloadData

PayloadData

PayloadData

Outer IP Header

Inner IP Header

Inner IP Header

Inner IP Header

Outer IP Header

VPN Device VPN DeviceServer Client

TunnelTransport


Authentication Header (AH)• Data Integrity: Undetected modification to a

packet’s content in transit is not possible• Authentication: Enables a network device to

authenticate a user.• Anti-replay service (optional)

Authentication Data Algorithms• HMAC-SHA-1-96 (Must be supported)• AES-XCBC-MAC-96 (Should be

supported)• HMAC-MD5-96 (May be supported)

Security Parameters Index (SPI)

Next Header ReservedAH Payload Length

Sequence Number

Payload DataIP Header

32 bits

AH

Authentication

Integrity Check Value –ICV (variable)

8 bits

Word 1

Word 2

Word 3

Word 4 -

16 bits8 bits


Encapsulation Security Payload (ESP)• Data Integrity + Authentication (optional)• Anti-replay Service (optional)• Confidentiality (optional)

8 bits

Security Parameters Index (SPI)

Sequence Number

32 bits

Integrity Check Value –ICV (variable)

Original IP Header ESP Header

Authentication

Encryption

ESP ICVESP TrailerPayload Data

Payload Data (variable)

Next HeaderPad Length

Padding (0 – 255 bytes)

8 bits


Internet Key Exchange (IKE v2)

• IPsec security services use symmetric encryption.— Source and destination need to agree to the mechanisms used to

share the secret keys and the keys that are used for authentication/integrity and encryption services.

• IPsec supports both manual and automatic distribution of keys.

• Public Key is used for automatic key management, but other automated key distribution techniques may be used.

• IKE v2 defines procedures and packet formats to establish, negotiate, modify, and delete Security Associations (SA).


Negotiating a Security Association using IKE

IKE Security Association (IKE SA) proposes the following:

• Type of protection to use, either ESP or AH.

• Authentication algorithms and keys for signing data.

• Encryption algorithms and keys to protect data.

• Hash algorithms to reduce data for signing.

• Information about a group over which to do a Diffie-Hellman exchange.

• A pseudo-random function (prf) to hash certain values during the key exchange.


Security Association

Security Parameters• Encryption and authentication

algorithms— Encapsulation Security Payload (ESP)— Authentication Header (AH)

• Crypto keys• Initialization values• Protocol mode• Source and destination IP addresses• Source and destination IDs• Key lifetimes

I would like to establish a secure IP communication, and since we haven’t talked before, let’s agree on all the security parameters we need by creating an

SA.

Source DestinationOnce we finish, let’s assign an index to the SA, (Security Parameter Index) and store the

information in our Security Policy Databases. By doing this, we will not have to create another SA

when we communicate again.


Internet Key Exchange (IKE)

• First Message Exchange— IKE Security Association

– In IKE_SA_INIT, the initiator and responder negotiate the use of encryption algorithms by establishing an IKE_SA. The agreed keys are used to protect the IKE_AUTH exchange.

– In IKE_AUTH, the initiator and responder authenticate each other using authentication mechanisms such as digital signatures (exchanging certificates), Extensible Authentication Protocol (EAP), or pre-shared keys.

— Child Security Association– In IKE_AUTH, the first IKE_SA and associated IPsec SA, called child SA,

are created.

• Second Message Exchange— CREATE_CHILD_SA exchange is used to create new CHILD_SAs and to

rekey IKE_SAs and CHILD_SAs.— All messages are cryptographically protected using the encryption

algorithms and keys negotiated in IKE_SA_INIT and IKE_SA_AUTH.


IKE First Message Exchange

AUTH – Authentication CERT – Certificate CERTREQ – Certificate RequestHDR – IKE Header i, r – Initiator, Responder IDi - Initiator IdentificationIDr – Responder Identification KEi – Initiator DH gi KEr – Responder DH gi Ni, Nr – Nonce SA - Security Association TSi, TSr – Traffic SelectorSAi1 , SAr1 – Used to create IKE_SASAi2, SAr2 – Used to create the first CHILD_SASK{….} – Payload is encrypted and integrity protected using SK_e and SK_a.

SAi1 HDR

SAr1HDR

1

2

3

4

I would like to establish an IKE security association and a child security association.

InitiatorNetworking Device with

IPsecEnd -system or Gateway

environment

Responder Networking Device with


environment KEiNi

[CERTREQ]NrKEr

SK{IDi, [CERT], [CERTREQ], [IDr], AUTH, SAi2, TSi, TSr}

HDR

SK{IDr, [CERT], AUTH, SAr2, TSi, TSr}

HDR


IKE Second Message Exchange

HDR – IKE Header i, r – Initiator, Responder[KE] – Optional Key Exchange [N+] – Optional Notify Ni, Nr – Nonce SA - Security AssociationTSi, TSr – Traffic SelectorSK{….} – Payload is encrypted and integrity protected using SK_e and SK_a.

5

6

I would like to generate a new Child_SA or rekey IKE SA and/or a previous Child_SA.

InitiatorNetworking Device with


environment

Responder Networking Device with


environment SK{ [N+], SA, Ni, [KEi], TSi, TSr} HDR

SK{ [N+], SA, Nr, [KEr], TSi, TSr} HDR


IKE v2 Header

• Initiator’s SPI (8 Octets) – A value selected by the initiator to identify a unique IKE security association.• Responder’s SPI (8 Octets) – A value selected by the responder initiator to identify a unique IKE security

association. This value is zero in the first message of the IKE_INIT.• Next Payload (1 Octet) – the type of payload that follows the header.• Major Version (4 bits) – The major version of the IKE protocol used.• Minor Version (4 bits) – The minor version of the IKE protocol used.• Exchange Type (1 Octet) – The type of exchange being use, IKE_INIT, IKE_AUTH, CREATE_CHILD_SA, or

INFOTMATIONAL.• Flags (1 Octet) – Indicates specific options that are set for the message.• Message ID (4 Octets) – Message identifier used to control retransmision of lost packets. It is used to prevent

message replay attacks.• Length (4 Octets) – Length of total message (header and payload)

IKE_SA Initiator’s Security Parameters Index (SPI)

IKE_SA Responder’s Security Parameter Index (SPI)

Next Payload FlagsExchange TypeMjVer MjVer

Message ID

Length


Generating Key Material in IKE_SA

• In IKEv2, Diffie-Hellman is the only key exchange algorithm used.

• Key material for all of the cryptographic algorithms used in both IKE_SA and CHILD_SA is always derived as the output of a prf algorithm.

• Diffie-Hellman exchange has the following three components: a generator g, the modulo p, and a secret that in IKEv2 terminology is called i or r.

• During IKE_INIT, in KEi and KEr, the Initiator and Responder exchange Diffie-Hellman information, gi and gr, as well as nonces Ni and Nr

• The shared key, SKEYSEED, is calculated by both the Initiator and Responder from the nonces exchanged and the Diffie-Hellman shared secret key generated, gi and gr, according to the following formula: ),|( irgNrNiprfSKEYSEED


IKE v2 DH Key Agreement

g =12 p = 47I Secret = i = 3

Nonce = Ni = 11

g = 12 p = 47R Secret = r =5Nonce = Nr = 7

18 18

36, 11 14, 7

g and p do not need to be secret

Both ends use 11, 7, and 18, as the secret and seed to calculate SKEYSEED

In the security association, the initiator and responder agreed on the same group or pair of g and p.

18)47(mod143 rig 18)47(mod365 rig

Initiator Responder

),|( irgNrNiprfSKEYSEED )( seed secret,prfSKEYSEED

36)47(mod123 ig 14)47(mod125 rg


Diffie-Hellman Groups in IKE

• Three distinct group representations can be used with IKE. — Modular Exponentiation Groups (named MODP)— Elliptic Curve Groups over the field GF [2n] (named EC2N)— Elliptic Curve Groups over GF [P] (named ECP).

• Groups Identifiers supported in IKE— Group 0: No group (used as a placeholder and for non-DH

exchanges)— Group 1: A modular exponentiation group with a 768 bit modulus— Group 2: A modular exponentiation group with a 1024 bit modulus— Group 4: An elliptic curve group over GF [2^155]— Group 5: A modular exponentiation group with a 1536 bit modulus— Group 14: A modular exponentiation group with a 2048 bit modulus— Group 15 A modular exponentiation group with a 3072 bit modulus.— Group 16 A modular exponentiation group with a 4096 bit modulus.— Group 17 A modular exponentiation group with a 6144 bit modulus.— Group 18 A modular exponentiation group with a 8192 bit modulus.


TCP/IP Stack and Security Related Protocols

Application Layer

Transport Layer

Network Layer

Data Layer

SMTP, Telnet, FTP, Gopher

TCP UDP

IP

Ethernet, Token-Ring, FDDI, X.25, Wireless, Async, ATM, SNA...Data Layer

ARP RARP

• S/MIME• S-HTTP• PGP• IPsec (ISAKMP)

• SOCKS V5• TLS/SSL

• IPsec (AH, ESP)• Packet Filtering• Tunneling Protocols

• PPP-EAP, IEEE 802.1X, CHAP, PAP, MS-CHAP


TLS and SSL

• TLS and SSL protocols are used to secure the communication between a client (Web browser) and a server (Web Server) over the Internet.

• TLS versions 1.1, 1.0, and SSL 3.1 and 3.0 are very similar. TLS and SSL clients are built into all web browsers.

• TLS and SSL provide mutual authentication (digital signature), confidentiality (data encryption), and data integrity (hash algorithms).

• A secure client-server communication requires:— Which protocol and version (TLS 1.0, 1,1, SSL2 or SSL3) to use and which

cryptographic algorithm will be used.— Whether or not to authenticate each other. Server and client authentication.— The type of cryptographic key exchange where both parties agree on a pre-master

secret key — The creation of session keys to encipher the message. — The encryption technique to the enciphering of data using keys generated from the pre-

master key.


TLS Architecture

• Session— A TLS session is an association between a client and a server.

Sessions are created by the handshake protocol.— Sessions define a set of cryptographic security parameters, which

can be shared among multiple connections.— Sessions are used to avoid the negotiation of new security

parameters for each connection.

• Connection— A connection is a transport (in the OSI layering model definition) that

provides a suitable type of service.— For TLS, such connections are peer-to-peer relationships.— A connections is transient. Every connection is associated with one

session.


Session Parameters

• Session identifier— An arbitrary byte sequence chosen by the server to identify an active or resumable

session state.

• Peer certificate— An X509.v3 certificate of the peer. This element of the state may be null.

• Compression method— The algorithm used to compress data prior to encryption.

• Cipher spec— Specifies the data symmetric encryption algorithm (such as null, DES, etc.) and a MAC

algorithm (such as MD5 or SHA). It also defines cryptographic attributes such as the hash_size.

• Master secret— A 48-byte secret shared between the client and server.

• Is Resumable— A flag indicating whether the session can be used to initiate new connections.


Connection Parameters

• Server and client random— Byte sequences that are chosen by the server and client for each connection.

• Server write MAC secret— The secret key used in MAC operations on data written by the server.

• Client write MAC secret— The secret key used in MAC operations on data written by the client.

• Server write key— The symmetric cipher key used by the server to encipher data and by the client to

decipher it.

• Client write key— The symmetric cipher key used by the client to encipher data and by the server to

decipher it.

• Initialization vectors— When a block cipher in CBC mode is used, an initialization vector (IV) is maintained for

each key.

• Sequence numbers— Sequence numbers maintained by each party for transmitted and received messages.


TLS Record Protocol

• The TLS Record Protocol provides connection security that has four basic properties: — The connection is private. Symmetric encryption (e.g., AES, DES, RC4, etc.)

is used for data encryption, after an initial handshake in which a pre-master secret key is defined.

— The negotiation of a shared secret is secure.– No attacker can modify the negotiation communication without being

detected by the parties to the communication.— The peer's identity can be authenticated using asymmetric or public key

cryptography (e.g., RSA, DSS, etc.).— The connection is reliable.

– Message transport includes a message integrity check using a keyed MAC (HMAC).

– HMAC can be used with a variety of different hash algorithms, but TLS uses MD5 and SHA-1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret, data).


TLS Record Protocol

Compression (Optional)

HMAC

HMAC

Compressed Cleartext Message

Stream Cipher

Enciphered [Compressed

Cleartex Message ║

HMAC]Block CipherPadding

SSL Header

Message Block Mn

Blocks of equal size such that the final SSL Record is not bigger

than 214 bytes.

M1M2 … Mn .. Key

Key

Key Exchange

Key Exchange

HMAC-SHA-1HMAC-RSA

Data Encryption

Stream Ciphers: RC4 40-bit or 128-bit key.Block Ciphers: DES 56-bit, 3DES 168-bit, or AES-128

The Record Protocol is responsible for coordinating the client and server sessions.


Handshake Protocol (Session State)

Client Web Server

Exchange client and server security capabilities:secure ID, compression method, and initial random number.

Client_Hello

Server_hello

Phase 1 Establishing Security Capabilities

Phase 2 and 3 Server & Client Authentication and Key Exchange

Server_Key_Exchange

Server and client exchange authentication, type of key exchange,

and public-key parameters.

Client_Key_Exchange

Server-Shared Master Key

Server and client create the shared master key and the cryptographic parameters.

Client-Shared Master Key

Generating the Master Secret Keys

Server Finish

Client and server exchange Finish Message and a hash of the

Finish Message

Client FinishPhase 4 Finish Message


Phase 1 Handshake Protocol

ClientWeb Server

Client_Hello

Server_hello

Phase 1 Establishing Security Capabilities

1. A ClientHello.random number (28 bytes), which is used later in the protocol;

2. A CipherSuite list containing the combinations of cryptographic algorithms supported by the client (in order of the client's preference, first choice first);

3. A list of the compression methods supported by the client, sorted by client preference.

When the client sends a client_hello message, the server must respond with a server_hello message, or else a fatal error will occur

and the connection will fail.

When the client sends a client_hello message, the server must respond with a server_hello message, or else a fatal error will occur

and the connection will fail.

1. A ServerHello.random number (28 bytes), different from the one sent by the client;

2. A CipherSuite list containing the combinations of cryptographic algorithms supported by the server (in order of the server's preference, first choice first);

3. A list of the compression methods supported by the server, sorted by the server.



ClientWeb ServerServer Authentication and Key Exchange

1. Server sends its authentication certificate, using a X.509.v3 certificate.

2. Information about the type of key exchange the server is proposing. — RSA: The secret key is encrypted with the server’s private key.— Fixed Diffie-Hellman: The server’s certificate has the Diffie-Hellman

parameters, signed by a Certificate Authority (CA).— Ephemeral Diffie-Hellman: The Diffie-Hellman parameters are signed using the

server’s RSA or DSA.— Anonymous Diffie-Hellman: The Diffie-Hellman parameters are not signed.

Key Exchange Parameters for RSA or Diffie-Hellman— RSA: The modulo of the server's temporary RSA key and the public exponent

of the server's temporary RSA key.— Diffie-Helman:

– The prime modulus p used for the Diffie-Hellman operation.– The generator g used for the Diffie-Hellman operation.– The server's Diffie-Hellman public value y (y = gx mod p).

3. A message requesting a client certification (optional);

4. A message indicating that the handshake of phase 2 is complete.

Key Exchange Parameters Signing =ESPriv

[Hash(ClientHello.random ║ ServerHello.random ║ ServerParams)]



ClientWeb Server

Client Authentication and Key Exchange

1. Client verifies whether or not the server’s certificate is valid.

2. Client sends certificate, if the server has requested it. — Client must send either the certificate message or a no_certificate alert; this

alert is only a warning. If client authentication is required, the server may respond with a fatal handshake failure alert.

3. Pre-master key exchange— RSA: A 48-byte pre-master secret key, encrypted with the server’s RSA public

key.— Diffie-Helman: Both client and server perform the Diffie-Hellman calculation to

create a pre-master key.

4. Master Key generation— Once the pre-master key has been created, either from RSA or from Diffie-

Hellman, the master key is computed as follows:

Master_Key = PRF(pre_master_secret, "master secret", ClientHello.random + ServerHello.random)Master_Key = PRF(pre_master_secret, "master secret", ClientHello.random + ServerHello.random)

PRF = Pseudo Random Function. See slide 20



ClientWeb Server

Finish

1. Client and server update the cipher_spec with the new, agreed-upon encryption algorithms, keys, and hash functions.

2. Client sends a “finished message” using the just negotiated encryption algorithms, hash functions, and symmetric encrypting keys to verify that the key exchange and authentication processes were successful.

3. The finished message is hashed as follows:— MD5[master_secret ║ pad2 ║ MD5(handshake_messages

║ Sender ║ master_secret ║ pad1)]— SHA[master_secret ║ pad2 ║ SHA(handshake_messages

║ Sender ║ master_secret ║ pad1)] Pad1 and pad 2 are the values defined in the MAC Handshake refers to all handshake messages exchanged Sender is a code that identifies that the sender is a client

(0x434C4E54) or a server (0x53525652).

Client and server may begin sending confidential data immediately after sending the Finish message. The master secret is used as an entropy

source to generate random values for the export and non-export MACS, secret keys, and initialization values (IV) required to encipher the data.

Client and server may begin sending confidential data immediately after sending the Finish message. The master secret is used as an entropy

source to generate random values for the export and non-export MACS, secret keys, and initialization values (IV) required to encipher the data.


TLS Alert Protocol

• Alert messages convey information about the status of the connection.• There are two types of alerts: Fatal and Warning.

Fatal Alert: Indicates that the connection is so bad that it needs to be terminated immediately.

Warning Alert: Indicates that there are some problems in the connection.

• Error Alerts unexpected_message: An inappropriate message was received. Fatal. bad_record_mac: This alert is returned if a record is received with an incorrect MAC. Fatal. decompression_failure: The decompression function received improper input. Fatal. handshake_failure: Reception of a handshake_failure alert message indicates that the sender

was unable to negotiate an acceptable set of security parameters given the options available. Fatal. illegal_parameter: A field in the handshake was out of range or inconsistent with other fields.

Fatal. no_certificate: A no_certificate alert message may be sent in response to a certification request if

no appropriate certificate is available. bad_certificate: A certificate was corrupt, contained signatures were not verifiable. unsupported_certificate: A certificate was of an unsupported type. certificate_revoked: A certificate was revoked by its signer. certificate_expired: A certificate has expired or is not currently valid. certificate_unknown: Some other (unspecified) issue arose in processing the certificate,

rendering it unacceptable.


Key Calculation - Pre Master Key Generation

Method 1 RSA – 48-byte Generated by the Client

Encipher RSA

Decipher RSA

Server’sSecret Key

Method 2: Diffe-Hellman

Diffie-Hellman Key Exchange

Server’s Certificate

Server’s Public Key

PreMaster Key

PreMaster Key

Diffie-Hellman Key Exchange

PreMaster Key

PreMaster Key

Web ServerClient


Key Calculation – Key and MAC Secrets

Exchange (wrap / transport ) or agree on (Diffie-Hellman) a pre-master key.

Encipher Decipher

Client Web Server

Master_Key Generation

Pre_Master_Key

Pre_Master_Key

Client MACServer MAC

Key_Block prf Expansion

Client Key, IVServer Key, IV

Symmetric Block

Encryption

Master_Key Generation

Client MACServer MAC

Client Key, IVServer Key, IV

Symmetric Block

Encryption

Integrity

Confidentiality

Key_Block prf Expansion


TLS – Pseudo Random Function

The PRF is created by splitting the secret key into two and using one half to generate data with P_MD5 and the other half to generate data with P_SHA-1. Then, the outputs of these two expansion functions together are XORed.

• The label is an ASCII string. For example, the label "plano tx" would be processed by hashing the following bytes (hex): 70 6C 61 6E 6F 20 74 78.

• The P_Hash data expansion function is used to create a pseudo random function (PRF).

Secret Label (Password)

S1 S2

PRF(secret, label, seed) = P_MD5 (S1, label ║ seed) XOR P_SHA-1 (S2, label ║ seed)


TLS – P_hash (secret, seed)

P_hash(secret, seed) = HMAC_hash (secret, A(1) ║ seed) ║

HMAC_hash (secret, A(2) ║ seed) ║

HMAC_hash (secret, A(3) ║ seed) ║ ...

HMAC

HMAC

HMAC

║

║

HMAC

║

Seed

Secret

Secret

Secret

Secret

A1

A2

A3

HMAC(secret, A(1) ║ seed) HMAC(secret, A(2) ║ seed) HMAC(secret, A(3) ║ seed)


SSL VPN

• Provides secure remote access to corporate applications. • Uses SSL & TTL as the underlying transport to establish a secure session

between any web browser and the proxy server in the SSL VPN Gateway.• Presents users with a web portal containing links to applications.• Functions as a proxy for both client (web browser) and server (web server) –

there is never a direct connection to the private network.• Ensures that authorized users have access only to specific resources as

allowed by the company security policy implemented by the proxy server and integrated traffic management.

ProxySocks

Address Translation

Applications

Internet

Kiosk

• Web Applications

• Client/Server• Telnet• SSH• Email• File Transfer

SSL (TLS) Secure ConnectionSSL

SSL VPN Gateway


SSL VPN Threats

• User passwords may remain on public-computers after users log off.—User passwords are stored by the browser.

• Sensitive data, such as browser cache entries, URL entries, cookies, and any historical information created during the session, may remain on public computers after users complete their SSL VPN sessions.

• Downloaded files are stored in the public computer’s “Temporary Folder.”

• Users forget to logout.—Next public computer user may have access to applications.

• Worms and viruses may be transferred from the public computers to the corporate internal network.


IPsec and TLS: Complementary Solutions

Solutions SSL IP Sec Comments

TelecommuterEg. Employee working from Home Office

IPsec provides secure access to all network resources and applications.SSL requires legacy applications to be first ‘translated’ into HTTP or will give access to SSL-enabled apps only.

Site-to-Site VPNEg. Remote branch office requires connectivity to corporate WAN

IPsec provides a secure tunnel between permanent locations

Remote Webmail Eg. Outlook Web Access, Lotus iNotes

SSL allows secure access from any web browser.

Internal Application SecurityEg. HR Self-service

SSL provides application layer security within VPNs

Partner ExtranetEg. Supplier access to inventory system

SSL doesn’t require installation of software on partner’s equipment but no control on workstation securityIPsec might require firewall reconfiguration – need to police access but closer control on workstation security

Web Application PortalsEg. iPlanet, web-enabled enterprise apps, custom web apps.

SSL is simplest choice for native web apps.IPsec is overkill

VOIP securityEg. Either site-to-site or telecommuter VOIP security.

VOIP can’t be carried by SSLIPsec is the solution for VOIP encryption

Wireless LAN securityEg. Securing WLAN access to the network by using stronger authentication and encryption.

IPsec provides secure access to all network resources and applications.SSL requires legacy applications to be first ‘translated’ into HTTP or will give access to web apps only

Ideal InappropriateOverkill / ComplexAppropriate


To Probe Further

• Atkinson, R. (1995). IP Encapsulating Security Payload (ESP). RFC 1827.

• Gleeson B., Lin A., Heinanen J, Armitage G., Malis A. (2000). A Framework for IP Based Virtual Private Networks. RFC 2764.

• C. Kaufman, Ed. (2005). The Internet Key Exchange (IKEv2). RFC 4306.

• Kent, S., Seo K. (2005). Security Architecture for the Internet Protocol. RFC 4301.

• Kent, S. (2005). IP Authentication Header. RFC 4302.• Kent, S. (2005). IP Encapsulating Security Payload (ESP). RFC 4303.• Madson, C., Glenn, R. (1998). The Use of HMAC-SHA-1-96 within

ESP and AH. RFC 2404.• Orman, H. (1998). The OAKLEY Key Determination Protocol. RFC

2412.


To Probe Further

• Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., & Wright, T. (2006). Transport Layer Security (TLS) Extensions. RFC 4366, IETF. http://www.ietf.org/rfc/rfc4366.txt?number=4366

• Dierks, T., Rescorla, E. (2006). 4346 The Transport Layer Security (TLS) Protocol Version 1.1. RFC 4346, IETF. http://www.ietf.org/rfc/rfc4346.txt?number=4346

• Freier, A., Karlton, P., & Kocher, P. The SSL Protocol Version 3.0. Internet-Draft, November 1996.

• Santesson, S. (2006). TLS Handshake Message for Supplemental Data. RFC 4680, IETF. http://www.ietf.org/rfc/rfc4680.txt?number=4680

• Santesson, S., Medvinsky, A., & Ball, J. (2006). TLS User Mapping Extension. RFC 4681, IETF. http://www.ietf.org/rfc/rfc4681.txt?number=4681

http://www.ietf.org/rfc/rfc4366.txt?number=4366




Documents

Chapter 11 and 12