Tuomas AuraT-110.4206 Information security technology

User authentication

Aalto University, autumn 2011

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Outline1. Passwords2. Physical security tokens and

two-method authentication3. Biometrics

Common mantra:User authentication can be based on – something you know– something you have– something you are

PASSWORDS

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Username and password Passwords are used for entity authentication– Needed for access control and auditing:

access control = authentication + authorization– Entity authentication vs. message authentication

Password is a shared secret between the user and computer system– Limitations arise from the reliance on of human

memory and input What attacks are there against passwords?

Sniffing and key loggers Password sniffing on the local network used to

be a major problem; mostly solved by cryptographic authentication:– SSH, SSL, HTTP Digest Authentication, MS-CHAPv2

Key logger: software or hardware that stores all key strokes (including passwords) typed on a computer– Particular danger in public-access

computers e.g. at libraries and cafes– Why do some bank web sites ask you to use the

mouse to enter the PIN code?5

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Password recovery Humans are prone to forget things need a

process for recovering from password loss What are the advantages and disadvantages of

the following recovery mechanisms?– Security question or memorable secret, e.g. birth

place, mother’s maiden name, pet’s name– Emailing password to another user account– Physical visit to helpdesk– Yellow sticker on the back of the keyboard– USB key or CD with a password recovery file

Password reuse How many different user accounts and passwords do you have?

Ever used the same password on two accounts? Using the same or related passwords on multiple accounts

means that one corrupt sysadmin or compromised account can lead to compromise of the other accounts

Administrative countermeasures:– Passwords chosen by the service, not set by users– Exotic password format requirements

Personal countermeasures:– Generating service-specific passwords from one master

password– Password wallet (e.g. on phone) encrypted with a master

password7

Shoulder surfing Keyboards and screens are highly visible others may see what you are typing

Password and PIN prompts usually do not show the characters– Does this make sense for all secrets?

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Password guessing Intelligent guessing vs. brute-force guessing– dictionary attack

Countermeasures– Limit the number or rate of login attempts– Minimum password length and complexity, password

quality check– Preventing reuse of old passwords– System-generated random passwords– Password aging i.e. mandatory periodic password

changes (typically every three months)9

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Password entropy Entropy = the amount of information the attacker is missing about the

passwordEntropy = - ∑ x passwords∈ P(x) ⋅ log2 P(x)≤ log2(number of possible passwords)

Examples:– Random 8-character 7-bit passwords have 56 bits of entropy– Random 8-character alphanumeric passwords have at most

8 × log2(26+26+10) ≈ 48 bits– 4-digit PIN codes have about 13 bits of entropy

Human-chosen passwords have less entropy than random ones because some passwords are more common than others Do password quality checks increase entropy?

Passwords rely on human memory entropy cannot grow over time any system that relies on high password entropy to beat brute-force attacks will eventually fail

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Online and offline guessing attacks Offline attack: the attacker obtains a hash (or other function) of the

password and tries to guess the password offline– Attacker who has the hash values from the password database– Older challenge-response network authentication, e.g. MS-CHAPv2 or HTTP

digest authentication (without SSL) Online guessing: attacker tries to login with different passwords

– Login prompt at the console; PIN code on a phone – Network login to an authenticated server over SSH or SSL– Firewall blocks client IP address after some failed login attempts

In offline attack, the attacker can perform an exhaustive brute-force search; in online attack, target system can limit the number of guesses

Big difference in the required password entropy:– Online guessing success probability

≈ number of allowed guesses / number of possible passwords– Offline attack requires cryptographic strength, e.g. 128-bit entropy

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Password database storage Safer to assume that the database is public

– Unix /etc/password is traditionally world readable– Attacks on web servers often manage to dump any file or database

on the server; e.g. SQL injection How to store passwords in a public file?

– Store a hash (i.e. one-way function) of the password– When user enters a password, hash and compare– Use a slow hash (many iterations of a hash function) to make brute-

force cracking more difficult– Include random account-specific “salt”:

slow_hash( password | salt)to prevent simultaneous brute-force cracking of many passwords, precomputation attacks and equality comparison between passwords

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Password hashing Password-based key derivation function PBKDF2

[PKCS#5,RFC2898]*– Good practical guide; uses any standard hash function, at least 64-bit

salt, any number of iterations Unix crypt(3) [Morris and Thompson 1978]*

– Historical function for storing passwords in /etc/passwd

aura:lW90gEpaf4wuk:19057:100:Tuomas Aura:/home/aura:/bin/zsh

– Eight 7-bit characters = 56-bit DES key– Encrypt a zero block 25 times with modified DES– 12-bit salt used to modify DES key schedule– Stored value includes the salt and encryption result– Replaced by more modern hash functions and shadow passwords

(stored in /etc/shadow, which is only readable to root)

DF2PBK PBKDF2 (P, S, c, dkLen)

P = passwordS = saltc = iteration countdkLen = length of the resultPRF = keyed pseudorandom function

F (P, S, c, i) = U1 xor U2 xor ... xor Uc U1 = PRF (P, S || i) U2 = PRF (P, U1)... Uc = PRF (P, Uc-1) Repeat for i=1,2,3... until dkLen output bytes produced

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Function for slow hashing of passwords

Iterations to make the computation slower

Used in WPA2-Personal for deriving keys from password

Could also be used for storing password hashes

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Botnets and online guessing 10 banks, each with 106 customer accounts– 4-digit PIN or one-time code required to log in– Client IP address blocked after 3 failed login attempts

Attacker has a botnet of 105 computers– Each bot makes one login attempt to one account in each bank

every day 106 login attempts in a day ~100 successful break-ins in a day

Countermeasures:– Make user IDs hard to guess; long, different from account

numbers, and not assigned sequentially– Ask a “salt” question, e.g. memorable word, in addition to user

ID and PIN increased entropy reduces attacker success rate

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One-time passwords Use each password only once to thwart password sniffers and key

loggers Lamport hash chain:

H1 = hash (secret seed); Hi+1= hash (Hi)

– Server stores initially H100 and requires user to enter H99. Next stores H99 and requires H98, and so on.

Unix S/KEY or OTP [RFC1760/1938] 1: HOLM BONG VARY TIP JUT ROSY 2: LAIR MEMO BERG DARN ROWE RIG 3: FLEA BOP HAUL CLAD DARK ITS 4: MITT HUM FADE CREW SLOG HAST

Hash-based one-time passwords HOTP [RFC4226]HOTP(K,C) = HMAC-SHA-1(K,C) mod 10D

– Produces a one-time PIN code of D decimal digits Time-based one-time passwords

– E.g. RSA SecurID: one of many commercial products Which attacks are prevented by one-time passwords and which are not?

Spoofing attacks Attacker could spoof the login dialog; how do you

know when it is safe to type in the password?

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Trusted path Attacker could spoof the login dialog; how do you

know when it is safe to type in the password? Trusted path is a mechanism that ensures direct

and secure communication between the user and a specific part of the system– Crtl+Alt+Del in Windows takes to a security screen that

cannot be spoofed– Web browser shows the URL in the address bar in a

way that cannot be spoofed by the web server With malware and virtualization, it is increasingly

hard to know what is real19

Other threats No system is perfectly secure:

system designers have a specific threat model in mind, but the attacker can break these rules– “The attacker does not agree with the

threat model.” (Bruce Christianson) Other attacks against PINs and

passwords:– Phishing and social engineering– Heat camera can detect recently pressed

keys– Acoustic emanations from the keyboard

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PHYSICAL SECURITY TOKENS AND TWO-METHOD AUTHENTICATION

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Physical security tokens Smart card is a typical physical security token– Holds cryptographic keys to prove its identity– Tamperproof: secret keys will stay inside

Used for door keys, computer login, ATM PIN entry is often also required two-method authentication– Attacker needs to both steal the card and learn the

PIN clear qualitative increase in security Other security token implementations: smart

button, USB stick, mobile phone 22

Issues with security tokens Physical tokes require distribution Computers (or doors etc.) must have readers It is not easy to integrate cryptographic tokens to all

systems– E.g. applications that require a password cached on the client

or on a proxy server Process needed for recovering from the loss of tokens Are smart card + PIN really two factors? One alternative is two-channel authentication:

– Confirmation via telephone: callback– Sending a second secret to a known address: text message,

email, post23

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BIOMETRICS

Biometric authentication Biometric authentication means verifying some

physical feature of the user– Physiological characteristic: photo, signature, face

geometry, fingerprint, iris scan, DNA– Behavioral characteristic: voice, typing, gait

Biometrics are not 100% reliable:– False acceptance rate FAR– False rejection rate FRR– Equal error rate EER

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FAR FRR50%

EER

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Issues with biometrics Biometrics require enrollment and readers Unsupervised vs. supervised readers have a big

difference in security– E.g. fingerprints, face recognition

Suitability for security architectures:– Are biometric characteristics secrets? – Can they be copied?– How to revoke biometrics?

What if enrollment fails?– Some people have no fingerprints, or no fingers

Reading material Dieter Gollmann: Computer Security, 2nd ed.,

chapter 3 Matt Bishop: Introduction to computer

security, chapter 11 Ross Anderson: Security Engineering, 2nd ed.,

chapters 2, 15 Edward Amoroso: Fundamentals of Computer

Security Technology, chapters 18-19

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Exercises Why do you need both the username and password? Would not just one secret

identifier (password) be sufficient for logging in? What effect do strict guidelines for password format (e.g. 8 characters, at least 2

capitals, 2 digits, 1 special symbol) have on the password entropy? What is the probability of guessing the code for a phone that allows 3 attempts to

guess a 4-digit PIN code, then 10 attempts to guess an 8-digit PUK code? In what respects is PBKDF2 better for password hashing than crypt(3)? Why may mandatory password changes increase security? What is the optimal

interval? How to limit the number of login attempts without creating a DoS vulnerability? Learn about graphical passwords and compare their entropy to different length

passwords and PIN codes. Learn about HTTP Digest Authentication [RFC2617] and MS-Chap-V2 [RFC2759].

Explain how to perform an offline password guessing attack after sniffing a login. In a social network, could authentication be based on who you know (or who knows

you), or where you are? What advantages and disadvantages might a fingerprint reader have in a car lock?