COEN 350: Network Security
Authentication
Authentication


Between human and machine
Between machine and machine
Human Machine
Authentication

Authentication protocols are based on

What you know.


What you have.


Physical key, smart card.
What you are.


E.g. password, pass-phrase, (secret key, private
key).
Biometrics.
Where you are.

E.g. trusted machine, access to room, …
Authentication

Passwords


Predate computers.
As do some attacks (stealing, guessing)




Older cell phone technology transmits
originating number with a password.
Password good, call goes through.
Eavesdropper receives phone number –
password combination.
Eavesdropper can now clone the phone.
Authentication

Password Attacks

Guessing

On-line





Off-line



Time consuming.
Authentication attempts are usually logged.
Can detect attack long before it is likely to succeed.
Can disrupt the attack.
Attacker needs to steal relevant data from which password(s) can
be determined.
Attacker can use arbitrary amount of computing power.
Capturing Passwords


Eavesdropping
Login Trojan Horse
Authentication

Passwords are stored


On each server Alice uses.
Centrally: Authentication Storage Node:


Each server retrieves the information when it
wants to authenticate Alice.
Centrally: Authentication Facilitator Node:

Each server takes Alice’s data and password
and goes to the AFN.
Authentication

Password can be stored

Unencrypted



Implicitly as hashes of passwords



Simple
Dangerous
As in UNIX, VMS
Encrypted
Hashed and Encrypted
Authentication


Example: Network Information Service
(Yellow Pages)



Directory service is the authentication storage
node.
Stores hashed passwords of users.
Typically, hashed passwords list is world readable


Access by claiming to be a server.
NIS authentication storage node does not
authenticate itself to users.

Allows impersonation of authentication service.
Authentication

Passwords for machine – machine
communication can be made difficult to
guess.



Arbitrary length
Truly random choice of characters.
Human-machine passwords


Guessable
Subject to dictionary attack.
Authentication

Dictionary attack






Most passwords are natural language words.
Or derived from natural language words.
Guess the language.
Use a dictionary to try out all words in the
language.
Start with common passwords first.
Replace a single character in a word, attach a
random character, etc.
Authentication


Brute-Force Attack
Generate all possible password.

Sometimes make assumptions on the
alphabet


only printable character
characters on a key-board
Authentication

Salting


Protects hashed passwords against an offline
attack.
Brute Force attack attacks all passwords in
password file simultaneously.
Authentication





Salting
Store a salt with each password
Hash depends on salt and password.
Use different salts for different
passwords.
Store salt with password.
Authentication

Salting

Brute force attack, dictionary attack can
only attack a single password.
Authentication

Rainbow tables

Simple idea:



precompute hashes for all possible passwords,
do a look-up in your precomputed table
To be useful: Lookup table needs to be simple


Use a “reduce” function that maps hashes to passwords
Create a chain:





Starts with password pwd.
Next element is hash(reduce(hash(pwd))
Next one is hash(reduce(hash(reduce(hash(pwd)))))
…
Do this a few thousand times and only store pwd and last
element in chain
Authentication

Rainbow tables

To be useful: Lookup table needs to be simple
(continued)

Assume now a hash of an unknown password


Check whether it is a terminal hash




hash(pwd)
If not, calculate hash(reduce(hash(pwd))
Check whether it is a terminal hash
Repeat until you find a terminal hash with password orgpwd.
You know that


pwd  {orgpwd, hash(reduce(hash(orgpwd)), … }
Authentication

Passwords are compromised:

By obtaining password file.

Safeguard by



Hashing and Salting
Encryption
By eavesdropping on an exchange

Use one-way passwords:

Lamport Hash
Authentication

Address Based

Common in early UNIX

Rtools:


.rhosts
 In user home directory
 (Computer, Account) pairs
 These pairs are allowed access to the user’s account
/etc/hosts.equiv


List of network addresses of “equivalent” machines
Account name on A is equivalent to account name on B.
 Users have to have identical account names.
Authentication

Addressed based authentication
threatened by

Access escalation



Attacker gains access to one hosts.
Access cascades to equivalent hosts / rhosts.
Spoofing addresses


Very easy to spoof source address.
Harder to intercept traffic back.
Authentication

Ethernet network address
impersonation




Easy on the same link.
Hubs do not protect.
Switches can be spoofed through the ARP
protocol.
Routers are harder to fool, but can be
attacked and provided with misleading
routing data.
Authentication

Cryptographic authentication

Alice proves her identity to Bob by proving
to Bob that she knows a secret.



Hashes
Secret key cryptography
Public key cryptography.
Human Machine
Authentication

Initial password distribution to humans

Pre-expired, strong passwords


Through mail
Derivable from common knowledge

Student ID
Human Machine
Authentication

Authentication Token

Possession of the token proves right to access.

Magnetic stripe as on credit cards.




Demand special hardware
Can be lost or stolen


Harder to reproduce
“Impossible” to guess
Add pin or password protection
Are not safe against communication eavesdropping and
forging
Human Machine
Authentication

Authentication Token

Smart Card.


Needs to be inserted in a smart card reader.
Card authenticates to the smart card reader.


PIN protected smart cards.
 Stops working after a number of false PINs.
Cryptographic challenge / response cards
 Card contains a cryptographic key.
 Authenticating computer issues a challenge.
 Card solves the challenge after PIN is entered.
 Harder to crack than PIN protected smart cards
because key is never revealed.
Human Machine
Authentication

Authentication Token

Smart Card.

Readerless smart card (Cryptographic
calculator)





Communicates with owner through mini-keyboard
and display.
Authenticating computer issues a challenge to Alice.
Alice types in challenge into readerless smart card.
Readerless smart card solves the challenge.
 After Alice puts in her password.
Alice transfers the answer to the computer.
Human Machine
Authentication

Biometrics








Retinal scanner
Fingerprint reader
Face recognition
Iris scanner
Handprint readers
Voiceprints
Keystroke timing
Signatures