Netprog: Security 1

Security

Terminology

Traditional Unix Security

TCP Wrapper

Cryptography

Kerberos

Netprog: Security 2

Terminology

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

Authorization: permission to access a resource.

Netprog: Security 3

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 4

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 5

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 6

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 7

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 8

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 9

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 10

Services that use the BSD security model

lpd – line printing daemon.

rshd – remote execution.

rexec – another remote execution.

rlogin – remote login.

Netprog: Security 11

BSD Config Files

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

/etc/hosts.lpd – trusted printing clients.

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

Netprog: Security 12

lpd security

check client's address for reserved port

and

check /etc/hosts.equiv for client IP

orcheck /etc/hosts.lpd for client IP

Netprog: Security 13

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!

Netprog: Security 14

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.

Netprog: Security 15

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.

Netprog: Security 16

rexecd securityclient 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!

Netprog: Security 17

rlogind security

Just like rshd.

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

Netprog: Security 18

Special Cases

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

– look at /.rhosts

– often disabled completely.

Netprog: Security 19

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.

Netprog: Security 20

TCP basedServers

TCPPorts

The World

TCP wrapper

(tcpd)

Single Host

TCP Wrapper Picture

Netprog: Security 21

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).

Netprog: Security 22

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.

Netprog: Security 23

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).

Netprog: Security 24

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.

Netprog: Security 25

Cryptography

Reference:

Network SecurityPRIVATE Communication in a PUBLIC World.

by Kaufman, Perlman & Speciner.

Netprog: Security 26

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: Security 27

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).

Netprog: Security 28

• Private keys are used for decrypting.

• Public keys are used for encrypting.

encryptionplaintext ciphertext

public key

decryptionciphertext plaintext

private key

Using Keys

Netprog: Security 29

Digital Signature• Public key cryptography is also used to

provide digital signatures.

signingplaintext signed message

private key

verificationsigned message plaintext

public key

Netprog: Security 30

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.

Netprog: Security 31

Hello Bob,Wanna get together?

AliceAlice BobBob

encrypt using Bpublic decrypt using Bprivate

Netprog: Security 32

OK Alice,Your place or mine?

AliceAlice BobBob

decrypt using Aprivate encrypt using Apublic

Netprog: Security 33

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.

Netprog: Security 34

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.

Netprog: Security 35

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.

Netprog: Security 36

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.

Netprog: Security 37

AliceAlice BobBob

Sign with Aprivate check signature using Apublic

encrypt using Bpublic decrypt using Bprivate

Revised Scheme

Netprog: Security 38

Why the digest?

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

• Why wouldn’t this be sufficient?

Netprog: Security 39

Implications

• Suppose Alice denies she sent the message?

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

Netprog: Security 40

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!

Netprog: Security 41

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.

Netprog: Security 42

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.

Netprog: Security 43

Secure Protocols

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

Netprog: Security 44

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.

Netprog: Security 45

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).

Netprog: Security 46

SSL layer

Application

SSL

TCP

IP

Application

SSL

TCP

IP

Netprog: Security 47

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.

Netprog: Security 48

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.