lecture 24 secure communications cs 450/650 fundamentals of integrated computer security slides are...

Lecture 24

Secure Communications

CS 450/650

Fundamentals of Integrated Computer Security

Slides are modified from Ian Goldberg

Who Attacks Networks

• Hackers break into organizations from the outside– Challenge– Fame– Money & Espionage– Ideology

• However, most security breaches are internal, by employees and ex-employees

Threat Precursors

• Port Scan• Social Engineering

– Reconnaissance– Bulletin Board / Chat– Docs

• Packet Sniffers (telnet/ftp in cleartext)

Network Security Threats

• Interception– If interceptor cannot read, have confidentiality

(privacy)– If cannot modify without detection, have message

integrity


• Impostors (Spoofing/ Masquerade)– Claim to be someone else– Need to authenticate the sender--prove that they

are who they claim to be

TruePerson

Impostor


• Remotely Log in as Root User– Requires cracking the root login password– Then control the machine– Read and/or steal information– Damage data (erase hard disk)– Create backdoor user account that will let them in

easily later

Root Login Command

Security Threats

• Content Threats– Application layer content may cause problems

• Viruses• In many ways, most severe security problem in

corporations today• Must examine application messages

Replay Attack

• First, attacker intercepts a message– Not difficult to do

Replay Attack

• Later, attacker retransmits (replays) the message to the original destination host– Does not have to be able to read a message to

replay it

Replay Attack

• Why replay attacks?– To gain access to resources by replaying an

authentication message– In a denial-of-service attack, to confuse the

destination host

Thwarting Replay Attacks

• Put a time stamp in each message to ensure that the message is “fresh”– Do not accept a message that is too old

• Place a sequence number in each message– Do not accept a duplicated message

Message

SequenceNumber

TimeStamp

Thwarting Replay Attacks

• In request-response applications,– Sender of request generates a nonce (random

number)– Places the nonce in the request– Server places the nonce in the response– Neither party accepts duplicate nonces

Nonce Nonce

Request Response


• Denial of Service (DOS) Attacks– Overload system with a flood of messages– Or, send a single message that crashes the

machine

Denial of Service (DOS) Attacks

• Transmission Failure• Connection Flooding

– Echo-Chargen– Ping of Death– Smurf– Syn Flood– Traffic Redirection– DNS Attacks

• Distributed Denial of Service

VPNs

• IETF developing IPsec security standards– IP security– At the internet layer– Protects all messages at the transport and

application layers

IPsec

TCP UDP

E-Mail, WWW, Database, etc.

VPNs

• IPsec Transport Mode– End-to-end security for hosts

LocalNetwork

Internet LocalNetwork

Secure Communication

VPNs

• IPsec Tunnel Mode– IPsec server at each site– Secure communication between sites

LocalNetwork



IPsecServer

VPNs

• IPsec Modes Can be Combined– End-to-end transport mode connection– Within site-to-site tunnel connection

LocalNetwork


Tunnel Mode Transport Mode

VPNs

• Another Security System for VPNs is the Point-to-Point Tunneling Protocol (PPTP)– For dial-up connections, based on PPP– Connects user with securely to a remote access

server at a site


Remote Access Server

Dial-UpConnection

PPTP Connection

PKIs

• To use public key methods, an organization must establish a comprehensive Public Key Infrastructure (PKI)– A PKI automates most aspects of using public key

encryption and authentication– Uses a PKI Server

PKIServer

PKIs

• PKI Server Creates Public Key-Private Key Pairs– Distributes private keys to applicants securely– Often, private keys are embedded in delivered

software

PKIServer

Private Key

PKIs

• PKI Server Provides CRL Checks– Distributes digital certificates to verifiers– Checks certificate revocation list before sending

digital certificates

PKIServer

Digital Certificate

PKIs

• CRL (Certificate Revocation List) Checks– If applicant gives verifier a digital certificate,– The verifier must check the certificate revocation

list

PKIServer

OK?

OK or Revoked

CRL

Integrated Security System

• When two parties communicate …– Their software usually handles the details– First, negotiate security methods– Then, authenticate one another– Then, exchange symmetric session key– Then can communicate securely using symmetric

session key and message-by-message authentication

SSL Integrated Security System

• SSL– Secure Sockets Layer– Developed by Netscape

• TLS (now)– Netscape gave IETF control over SSL– IETF renamed it TLS (Transport Layer Security)– Usually still called SSL

Location of SSL

• Below the Application Layer– IETF views it at the transport layer– Protects all application exchanges– Not limited to any single application

• WWW transactions, e-mail, etc.

SSL SSL

E-Mail WWW E-Mail WWW

SSL Operation

• Browser & Webserver Software Implement SSL– User can be unaware

SSL Operation

• SSL ISS Process– Two sides negotiate security parameters– Webserver authenticates itself– Browser may authenticate itself but rarely does– Browser selects a symmetric session key, sends to

webserver– Adds a digital signature and encrypts all messages

with the symmetric key

Importance of SSL

• Supported by Almost All Browsers– De facto standard for Internet application security

• Problems– Relatively weak security– Does not involve security on merchant server– Does not validate credit card numbers– Viewed as an available but temporary approach to

consumer security

Other ISSs

• SSL is merely an example integrated security system

• Many other ISSs exist– IPsec – PPP and PPTP– Etc.

Other ISSs

• All ISSs have the same general steps– Negotiate security parameters– Authenticate the partners– Exchange a session key– Communicate with message-by-message privacy,

authentication, and message integrity

IPsec

• IPsec (IP security)• Security for transmission over IP networks

– The Internet– Internal corporate IP networks– IP packets sent over public switched data

networks (PSDN)

LocalNetwork


IPsec

• Why do we need IPsec?– IP has no security– Add security to create a virtual private network

(VPN) to give secure communication over the Internet or another IP network

LocalNetwork


IPsec

• Genesis– Being created by the Internet Engineering Task

Force– For both IP version 4 and IP version 6

IPsec

• Two Modes of operation• Tunnel Mode

– IPsec server at each site– Secures messages going through the Internet

LocalNetwork



IPsecServer

IPsec

• Tunnel Mode– Hosts operate in their usual way

• Tunnel mode IPsec is transparent to the hosts

– No security within the site networks

LocalNetwork



IPsecServer

IPsec

• Two Modes of operation• Transport Mode

– End-to-end security between the hosts– Security within site networks as well – Requires hosts to implement IPsec

LocalNetwork



IPsec

• Transport Mode– Adds a security header to IP packet– After the main IP header– Source and destination addresses of hosts can be

learned by interceptor– Only the original data field is protected

ProtectedOriginal Data Field

OriginalIP Header

TransportSecurityHeader

IPsec

• Tunnel Mode– Adds a security header before the original IP

header– Has IP addresses of the source and destination

IPsec servers only, not those of the source and destination hosts

– Protects the main IP header

ProtectedOriginal Data Field

ProtectedOriginal

IP Header

TunnelSecurityHeader

IPsec

• Can combine the two modes– Transport mode for end-to-end security– Plus tunnel mode to hide the IP addresses of the

source and destination hosts during passage through the Internet

LocalNetwork


Tunnel Mode Transport Mode

IPsec• Two forms of protection• Encapsulating Security Protocol (ESP) security

provides confidentiality as well as authentication

• Authentication Header (AH) security provides authentication but not confidentiality– Useful where encryption is forbidden by law– Provides slightly better authentication by providing

authentication over a slightly larger part of the message, but this is rarely decisive

IPsec

• Modes and protection methods can be applied in any combination

Tunnel Tunnel ModeMode

Transport Transport ModeMode

ESPESP SupportedSupported SupportedSupported

AHAH SupportedSupported SupportedSupported

IPsec

• Security Associations (SAs) are agreements between two hosts or two IPsec servers, depending on the mode

• “Contracts” for how security will be performed

• Negotiated• Governs subsequent transmissions

Host A Host B

NegotiateSecurity Association

IPsec

• Security Associations (SAs) can be asymmetrical– Different strengths in the two directions– For instance, clients and servers may have

different security needs

Host A Host B

SA for messagesFrom A to B

SA for messagesFrom B to A

IPsec

• Policies may limit what SAs can be negotiated– To ensure that adequately strong SAs for the

organization’s threats– Gives uniformity to negotiation decisions

Host A Host B

Security AssociationNegotiations Limited

By Policies

IPsec

• First, two parties negotiate IKE (Internet Key Exchange) Security Associations– IKE is not IPsec-specific– Can be used in other security protocols

Host A Host BCommunication

Governed byIKE SA

IPsec

• Under the protection of communication governed by this IKE SA, negotiate IPsec-specific security associations

Host A Host BCommunication

Governed byIKE SA

IPsec SA Negotiation

IPsec

• Process of Creating IKE SAs (and other SAs)– Negotiate security parameters within policy

limitations– Authenticate the parties using SA-agreed methods– Exchange a symmetric session key using SA-agreed

method– Communicate securely with confidentiality,

message-by-message authentication, and message integrity using SA-agreed method

IPsec

• IPsec has mandatory security algorithms– Uses them as defaults if no other algorithm is

negotiated– Other algorithms may be negotiated– But these mandatory algorithms MUST be

supported

IPsec

• Diffie-Hellman Key Agreement– To agree upon a symmetric session key to be used

for confidentiality during this session– Also does authentication

Party A Party B

IPsec

• Diffie-Hellman Key Agreement– Each party sends the other a nonce (random

number)– The nonces will almost certainly be different – Nonces are not sent confidentially

Party A Party BNonce B

Nonce A

IPsec

• Diffie-Hellman Key Agreement– From the different nonces, each party will be able

to compute the same symmetric session key for subsequent use

– No exchange of the key; instead, agreement on the key

Party A Party B

Symmetric Key Symmetric KeyFrom nonces,

independently computesame symmetric

session key

Multi-Layer Security

• Security Can be Applied at Multiple Layers Simultaneously– Application layer security for database, e-mail, etc.– Transport layer: SSL– Internet layer: IPsec – Data link layer: PPTP, L2TP– Physical layer: locks

Multi-Layer Security

• Applying security at 2 or more layers is good– If security is broken at one layer, the

communication will still be secure

• However,– Security slows down processing– Multi-Layer security slows down processing at

each layer

Total Security

• Network Security is Only Part• Server Security

– Hackers can take down servers with denial-of-service attack

– Hacker can log in as root user and take over the server

– Steal data, lock out legitimate users, etc.

Total Security

• Server Security– Occasionally, weakness are discovered in server

operating systems– This knowledge is quickly disseminated– Known security weaknesses

Total Security

• Server Security– Server operating system (SOS) vendors create

patches– Many firms do not download patches– This makes them vulnerable to hackers, who

quickly develop tools to probe for and then exploit known weaknesses

Total Security

• Client PC Security– Known security weaknesses exist but patches are

rarely downloaded– Users often have no passwords or weak passwords

on their computer– Adversaries take over client PCs and can therefore

take over control over SSL, other secure communication protocols

Total Security

• Application Software– May contain viruses

• Must filter incoming messages

– Database and other applications can add their own security with passwords and other protections

Total Security

• Managing Users– Often violate security procedures, making

technical security worthless– Social engineering: attacker tricks user into

violating security procedures

Defense in Depth

• Firewalls• Antivirus• Intrusion Detection Systems• Intrusion Protection Systems

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