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.

E.g. password, pass-phrase, (secret key, private key).

What you have. Physical key, smart card.

What you are. 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

Time consuming. Authentication attempts are usually logged. Can detect attack long before it is likely to succeed. Can disrupt the attack.

Off-line 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

Simple Dangerous

Implicitly as hashes of passwords As in UNIX, VMS

Encrypted Hashed and Encrypted

Authentication

Example: Network Information Service Directory service is the authentication

storage node. Stores hashed passwords of users. Typically, hashed passwords list is

world readable (by claiming to be a server).

NIS authentication storage node is

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

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. Harder to reproduce “Impossible” to guess

Demand special hardware Can be lost or stolen

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

Strong Passwords

Goal: Eavesdropper does not obtain enough

information to do an off-line verification of password guesses.

Strong Passwords

Bob (Machine) and Alice (Human) share a “weak” secret W. W is a hash of Alice’s password. Bob knows W because he stores it.

Strong Passwords: EKEAlice Bob

Alice and Bob share a weak secret W = f (password)Alice chooses a random number a.

She sends:

Alice, W{ga}

Bob chooses a random b and a challenge C1.

He sends

W {gb, C1}

Both Bob and Alice use their knowledge of W to encrypt their mutual messages. They both calculate K = gab.Alice then proves her knowledge of W by her ability to calculate

K. She also picks a challenge C2 and sends K { C1, C2 } to Bob.

Bob encrypts this message and finds that Alice has solved his challenge C 1. Finally, Bob authenticates himself to Alice. He proves his knowledge of W by his knowledge of K, which he proves by being able to correctly encrypt Alice’s challenge C2. He sends

K {C2 } to Alice.

Strong Passwords: EKE A bad implementation of EKE allows an

eavesdropper to exclude passwords. Assume that we calculate in the field of number

modulo p, p a prime. Then ga and gb are both m bit numbers smaller than p. Attacker maintains a dictionary of possible passwords

and observes many authentication rounds. If W is in the dictionary, he encrypts Alice’s round 1

message M. If W -1{M } > p, then attacker excludes W. Chance of excluding a false password W is 2m – p / p. If this chance is about 80%, then 50 rounds determine

the password out of a normal dictionary.

Nr of Exchanges Chance of false password surviving

1 80%

2 64%

5 33%

10 10%

20 1.2%

50 0.0014%

Strong Passwords

Simple Password Exponential Key Exchange: SPEKE Like EKE, but

Transmit W a and W b and agree on key K. Has a related vulnerability:

W shares algebraic properties with W a. Generator, perfect square

Strong Passwords

PDM: Password Derived Moduli Key Idea: Pick the field based on the

password. Diffie Hellman exchange based on

p = f (password) with base 2:

Alice to Bob: Alice, 2a mod p. Bob to Alice: 2b mod p. Prove knowledge of K = 2ab mod p.

Strong Passwords

Augmented Strong Password Protocols Prevent someone who has stolen the

server data base to impersonate a user. Server does not store the password, but a

quantity sufficient to evaluate it. Augmented EKE:

http://citeseer.ist.psu.edu/bellovin93augmented.html

Strong Passwords

Strong Passwords

Augmented PDM Server Information Creation

Alice has password pssw Alice sends to Bob

p = f (pssw) [this is a prime] W = hash (pssw) [one-way hash]

Bob stores: Alice, p, W,

Strong Passwords:Augmented PDMAlice creates random number a.

She recomputes W and p from her password.

Alice 2a mod p Bob

Bob chooses a random number b.

Bob calculates 2b mod p.

Bob sends 2b, hash1 (2ab mod p , 2bW mod p) to Alice

Alice knows that Bob is Bob because Bob proves that he knows 2bW. Alice now sends

hash2 (2ab mod p , 2bW mod p)

Bob knows that Alice is Alice because she proves to him that she knows W. If Alice had just broken into the server, she would have to calculate 2bW from 2W mod p.

Strong Passwords

Secure Remote Password RFC 2945 Bob stores {Alice, gW mod p}, where

W = f (passwd).

Strong PasswordsSRP

Alice creates random a and sends ga to Bob.Bob creates random b, challenge CBOB and 32b number u.

Bob sends gb + gW mod p, u, CBOB to Alice.Both calculate K = g b(a+uW) mod p

Alice sends K {CBob}, CAlice to Bob.

Bob sends K {CAlice} to Bob.