Lecture 22 Network Security CPE 401 / 601 Computer Network Systems slides are modified from Dave...

Lecture 22

Network Security

CPE 401 / 601Computer Network Systems

slides are modified from Dave Hollinger

Terminology

Authentication: identifying someone (or something) reliably. Proving you are who you say you are.

Authorization: permission to access a resource.

Netprog: Security 2

Terminology

Encryption: Scramble data so that only someone with a secret can make sense of the data.

Decryption: Descrambling encrypted data.

DES: Data Encryption Standard: secret key cryptographic function standardized by NBS (NIST).

Netprog: Security 3

Terminology (cont.)

Secret Key Cryptography: a cryptographic scheme where the same key is used to encrypt and decrypt.

Public Key Cryptography: a cryptographic scheme where different keys are used for encryption and decryption.

Netprog: Security 4

Terminology (more!)

Firewall: a network component that separates two networks and (typically) operates in the upper layers of the OSI reference model (Application layer).

Screening Router: a discriminating router that filters packets based on network layer (and sometimes transport layer) protocols and addresses.

Netprog: Security 5

Unix Network Security

Some basic approaches:

1. Do nothing and assume requesting system is secure.

2. Require host to identify itself and trust users on known hosts.

3. Require a password (authentication) every time a service is requested.

Netprog: Security 6

Traditional Unix Security (BSD) Based on option 2 – trust users on

trusted hosts. if the user has been authenticated by a

trusted host, we will trust the user.

Authentication of hosts based on IP address! (doesn’t deal with IP spoofing)

Netprog: Security 7

Reserved Ports

Trust only clients coming from trusted hosts with source port less than 1024. Only root can bind to these ports.

We trust the host. The request is coming via a trusted service (a reserved port) on the host.

Netprog: Security 8

Potential Problem

Anyone who knows the root password can replace trusted services.

Not all Operating Systems have a notion of root or reserved ports!

It’s easy to impersonate a host that is down.

Netprog: Security 9

Services that use the BSD security model

lpd – line printing daemon.

rshd – remote execution.

rexec – another remote execution.

rlogin – remote login.

Netprog: Security 10

BSD Config Files

/etc/hosts.equiv – list of trusted hosts.

/etc/hosts.lpd – trusted printing clients.

~/.rusers – user defined trusted hosts and users.


lpd security

check client's address for reserved portand

check /etc/hosts.equiv for client IP

orcheck /etc/hosts.lpd for client IP


rshd, rexecd, rlogind security

As part of a request for service a username is sent by the client.

The username must be valid on the server!


rshd security

1. check client’s address for reserved port

if not a reserved port – reject request.

2. check for password entry on server for specified user.

if not a valid username – reject request.


rshd security (cont.)

3. check /etc/hosts.equiv for client’s IP address.if found – process request.

4. check users ~/.rhosts for client's IP address.if found – process request, otherwise reject.


rexecd security

client sends username and password to server as part of the request (plaintext).

1. check for password entry on server for user name.

2. encrypt password and check for match.

rexecd is rarely used!


rlogind security

Just like rshd.

If trusted host (user) not found – prompts for a password.


Special Cases

If username is root requests are treated as a special case:

look at /.rhosts

often disabled completely.


TCP Wrapper

TCP wrapper is a simple system that provides some firewall-like functionality.

A single host (really just a few services) is isolated from the rest of the world.

Functionality includes logging of requests for service and access control.


TCP Wrapper Picture


TCP basedServers

TCPPorts

The World

TCP wrapper

(tcpd)

Single Host

tcpd

The tcpd daemon checks out incoming TCP connections before the real server gets the connection.

tcpd can find out source IP address and port number (authentication).


tcpd (cont.)

A log message can be generated indicating the service name, client address and time of connection.

tcpd can use client addresses to authorize each service request.


Typical tcpd setup

inetd (the ) is told to start tcpd instead of the real server.

tcpd checks out the client by calling getpeername on descriptor 0.

tcpd decides whether or not to start the real server (by calling exec).


tcpd configuration

The configuration files for tcpd specify which hosts are allowed/denied which services.

Entire domains or IP networks can be permitted or denied easily.

tcpd can be told to perform RFC931 lookup to get a username.


Secret Key Cryptography

• Single key used to encrypt and decrypt.

• Key must be known by both parties.

• Assuming we live in a hostile environment (otherwise - why the need for cryptography?), it may be hard to share a secret key.

Netprog: Cryptgraphy 26

Public Key Cryptography(a.k.a. asymmetric cryptography)• Relatively new field - 1975 (as far as we

know, the NSA is not talking).

• Each entity has 2 keys: private key (a secret) public key (well known).


• Private keys are used for decrypting.

• Public keys are used for encrypting.

encryptionplaintext ciphertext

public key

decryptionciphertext plaintext

private key


Using Keys

Digital Signature

• Public key cryptography is also used to provide digital signatures.

signingplaintext signed message

private key

verificationsigned message plaintext

public key


Transmitting over an insecure channel.Alice wants to send Bob a private

message.

Apublic is Alice’s public key.

Aprivate is Alice’s private key.

Bpublic is Bob’s public key.

Bprivate is Bob’s private key.



Hello Bob,Wanna get together?

AliceAlice BobBob

encrypt using Bpublic decrypt using Bprivate

OK Alice,Your place or mine?


AliceAlice BobBob

decrypt using Aprivate encrypt using Apublic

Bob’s Dilemma

• Nobody can read the message from Alice, but anyone could produce it.

• How does Bob know that the message was really sent from Alice?

• Bob may be comforted to know that only Alice can read his reply.


Alice can sign her message!

• Alice can create a digital signature and prove she sent the message (or someone with knowledge of her private key).

• The signature can be a message digest encrypted with Aprivate.


Message Digest

• Also known as “hash function” or “one-way transformation”.

• Transforms a message of any length and computes a fixed length string.

• We want it to be hard to guess what the message was given only the digest. Guessing is always possible.


Alice’s Signature

• Alice feeds her original message through a hash function and encrypts the message digest with Aprivate.

• Bob can decrypt the message digest using Apublic.

• Bob can compute the message digest himself.

• If the 2 message digests are identical, Bob knows Alice sent the message.



AliceAlice BobBob

Sign with Aprivate check signature using Apublic

encrypt using Bpublic decrypt using Bprivate

Revised Scheme

Why the digest?

• Alice could just encrypt her name, and then Bob could decrypt it with Apublic.

• Why wouldn’t this be sufficient?


Implications

• Suppose Alice denies she sent the message?

• Bob can prove that only someone with Alice’s key could have produced the message.


Another possible problem

• Suppose Bill receives a message from Alice including a digital signature.

“meet me at the library tonight”

• Bill sends the same message to Joe so that it looks like the message came from Alice.

• Bill includes the digital signature from the message Alice sent to him.

• Joe is convinced Alice sent the message!


Solution?

• Always start your messages with: Dear Bill,

• Create a digest from the encrypted message and sign that digest.

• There are many other schemes as well.


Speed

• Secret key encryption/decryption algorithms are much faster than public key algorithms.

• Many times a combination is used: use public key cryptography to share a

secret key. use the secret key to encrypt the bulk of the

communication.


Secure Protocols

• There are a growing number of applications for secure protocols: email electronic commerce electronic voting homework submission


Secure Protocols

• Many application protocols include the use of cryptography as part of the application level protocol. The cryptographic scheme employed is part

of the protocol. If stronger cryptographic tools become

available we need to change the protocol.


SSL and TLS

• Secure Sockets Layer (SSL) is a different approach - a new layer is added that provides a secure channel over a TCP only link.

• TLS is Transport Layer Security (IETF standard based on SSL).


SSL layer


Application

SSL

TCP

IP

Application

SSL

TCP

IP

Advantages of SSL/TLS

• Independent of application layer

• Includes support for negotiated encryption techniques. easy to add new techniques.

• Possible to switch encryption algorithms in the middle of a session.


HTTPS Usage

• HTTPS is HTTP running over SSL. used for most secure web transactions. HTTPS server usually runs on port 443. Include notion of verification of server via a

certificate. Central trusted source of certificates.


Kerberos

• Part of project Athena (MIT).• Trusted 3rd party authentication

scheme.• Assumes that hosts are not trustworthy.• Requires that each client (each request

for service) prove it’s identity.• Does not require user to enter password

every time a service is requested!

Netprog: Kerberos 50

Kerberos Design

• User must identify itself once at the beginning of a workstation session (login session).

• Passwords are never sent across the network in cleartext (or stored in memory)


Kerberos Design (cont.)

• Every user has a password.

• Every service has a password.

• The only entity that knows all the passwords is the Authentication Server.



ServerServerServerServerServerServerServerServer

ServerServerServerServerServerServerServerServer

KerberosKerberosDatabaseDatabase

Ticket GrantingTicket Granting ServerServer

Ticket GrantingTicket Granting ServerServer

AuthenticationAuthentication ServerServer

AuthenticationAuthentication ServerServer

WorkstationWorkstationWorkstationWorkstation

Kerberos Key Distribution ServiceKerberos Key Distribution Service

Secret Key Cryptography

• The encryption used by current Kerberos implementations is DES, although Kerberos V5 has hooks so that other algorithms can be used.

encryption plaintextciphertext

keyciphertext plaintext

decryption


Tickets

• Each request for a service requires a ticket.

• A ticket provides a single client with access to a single server.


Tickets (cont.)

• Tickets are dispensed by the “Ticket Granting Server” (TGS), which has knowledge of all the encryption keys.

• Tickets are meaningless to clients, they simply use them to gain access to servers.


Tickets (cont.)

• The TGS seals (encrypts) each ticket with the secret encryption key of the server.

• Sealed tickets can be sent safely over a network - only the server can make sense out of it.

• Each ticket has a limited lifetime (a few hours).


Ticket Contents

• Client name (user login name)

• Server name

• Client Host network address

• Session Key for Client/Server

• Ticket lifetime

• Creation timestamp


Session Key

• Random number that is specific to a session.

• Session Key is used to seal client requests to server.

• Session Key can be used to seal responses (application specific usage).


Authenticators

• Authenticators prove a client’s identity.

• Includes: Client user name. Client network address. Timestamp.

• Authenticators are sealed with a session key.


Bootstrap

• Each time a client wants to contact a server, it must first ask the 3rd party (TGS) for a ticket and session key.

• In order to request a ticket from the TGS, the client must already have a TG ticket and a session key for communicating with the TGS!


Authentication Server

• The client sends a plaintext request to the AS asking for a ticket it can use to talk to the TGS.

• REQUEST: login nameTGS name

Since this request contains only well-known names, it does not need to be sealed.


Authentication Server

• The AS finds the keys corresponding to the login name and the TGS name.

• The AS creates a ticket: login nameTGS name client network addressTGS session key

• The AS seals the ticket with the TGS secret key.


Authentication Server Response• The AS also creates a random session

key for the client and the TGS to use.• The session key and the sealed ticket

are sealed with the user (login name) secret key.


TGS session key

Ticket:login nameTGS namenet addressTGS session key

Sealed with user keySealed with user key

Sealed with TGS keySealed with TGS key

Accessing the TGS

• The client decrypts the message using the user’s password as the secret key.

• The client now has a session key and ticket that can be used to contact the TGS.

• The client cannot see inside the ticket, since the client does not know the TGS secret key.


Accessing a Server

• When a client wants to start using a server (service), the client must first obtain a ticket.

• The client composes a request to send to the TGS:


TGS Ticket

Authenticator

Server Name

sealed withsealed withTGS keyTGS key

sealed withsession key

TGS response

• The TGS decrypts the ticket using it’s secret key. Inside is the TGS session key.

• The TGS decrypts the Authenticator using the session key.

• The TGS check to make sure login names, client addresses and TGS server name are all OK.

• TGS makes sure the Authenticator is recent.


TGS Response

Once everything checks out - the TGS:

• builds a ticket for the client and requested server. The ticket is sealed with the server key.

• creates a session key

• seals the entire message with the TGS session key and sends it to the client.


Client accesses Server

• The client now decrypts the TGS response using the TGS session key.

• The client now has a session key for use with the new server, and a ticket to use with that server.

• The client can contact the new server using the same format used to access the TGS.


Kerberos Summary

• Every service request needs a ticket.• Tickets come from the TGS (except the

ticket for the TGS!).• Workstations cannot understand tickets,

they are encrypted using the server key.

• Every ticket has an associated session key.

• Tickets are reusable.


Kerberos Summary (cont.)

• Tickets have a finite lifetime.

• Authenticators are only used once (new connection to a server).

• Authenticators expire fast !

• Server maintains list of authenticators (prevent stolen authenticators).

• There is a lot more to Kerberos!!!


Lecture 22 Network Security CPE 401 / 601 Computer Network Systems slides are modified from Dave...

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