User authentication

Tuomas AuraCSE-C3400 Information security

Aalto University, autumn 2014

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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 methods (and from the lack of cryptographic computing capability)

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 typed on a computer– Used to be a problem in public-access

computers e.g. at libraries and cafes– Now can be malware on any computer– 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 Recovery mechanisms often enable new attacks 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 memory stick with a password recovery file

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

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

that one compromised system or account can lead to compromise of the other accounts

Administrative countermeasures:– Passwords chosen by the service, not set by users– Exotic password format requirements– Single sign-on to enable just one password

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

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

password (e.g. F-Secure Key)7

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 input?

Increasingly, showing the characters is a UI option

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Password guessing Dictionary attack and other intelligent

guessing vs. brute-force trials Countermeasures against guessing– 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)

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Online vs. offline guessing attacks Offline attack: cracking the password from a know hash (or other

value computed from) of the password– E.g. MS-CHAPv2, Kerberos, HTTP digest authentication without SSL– Unlimited number of guesses attacker can perform an exhaustive

brute-force search Online guessing: attacker tries to login many times

– E.g. PIN code entry on a phone– E.g. network login to an authenticated server over SSH or SSL– System can limit the number or rate of guesses

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

≈ number of allowed guesses / number of possible passwords– Offline attack requires cryptographic strength from the password, e.g.

128-bit entropy, to prevent exhaustive search Authentication protocols that are vulnerable to network sniffing

and offline guessing are simply outdated

Measuring password strength Many possible metrics:

– Number of possible passwords– Entropy = amount of missing information– Average/median time to crack a specific password– Average/median time to crack any one password– Probability of success as a function of time or number of trials– etc.

When the user is allowed to choose the password, measuring its strength accurately is impossible

Metrics are important to consider when designing new types of passwords– Graphical passwords– Password complexity requirements

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Password entropy Entropy = the amount of missing information

Entropy H = - ∑ x passwords∈ (P(x) ⋅ log2 P(x))

≤ log2(number of possible passwords)

Examples:– Random 8-character alphanumeric passwords have

H = 8 log⋅ 2(26+26+10) = 47.6 bits– Random 4-digit PIN codes have about H = 13.3 bits of entropy

One-bit increase in entropy approximately doubles the cost of guessing attacks (exactly so for even probability distribution)

Human-selected passwords have less entropy than random ones because some are chosen more often than other Should banks allow the customer to choose the PIN? Do password quality checks increase entropy?

Passwords rely on human memory password entropy cannot grow over time human memory cannot compete with computer speed

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Botnets and parallel online guessing 10 banks, each with 106 customer accounts

– Public or easy-to-guess user ID– 4-digit PIN or one-time code required to log in– Client IP address blocked after 3 failed logins per day

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, randomly selected, and

different from account numbers– 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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Storing passwords on server It is prudent to assume that your password database is public

– Unix /etc/password is traditionally world readable– Attackers often manage to read files or database tables on a web

server e.g. with 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 standard 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, pre-computation attacks, and equality comparison between passwords

!

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Password hashing Password-based key derivation function PBKDF2

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

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

– Historical function for hashing passwords stored in /etc/passwd

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

– Password = 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– Too short salt enables e.g. rainbow table attacks– Replaced by more modern hash functions and encrypted, read-

protected shadow passwords (why?)

PBKDF2 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 produced18

Function for slow hashing of passwords

Many iterations to make the computation slower

Used in WPA2-Personal for deriving keys from password (makes offline cracking more difficult)

Could also be used for hashing stored passwords on a server

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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 asks user to enter H99. Next, stores H99 and ask for H98, and so on

Unix S/KEY or OTP [RFC1760,RFC1938] 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,i) = HMAC-SHA-1(K,i) mod 10D

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

– Many commercial products such as RSA SecurID Which attacks do one-time passwords prevent and which not?

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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 (with the TCB)– Crtl+Alt+Del in Windows opens a security screen that

is difficult to spoof– Web browser shows the URL in the address bar in a

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

hard to know what is real22

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) Some other attacks against PINs

and passwords:– Phishing and social engineering– User mistakes: using wrong password– Camera to record key presses– Heat camera to detect pressed keys– Acoustic emanations from the keyboard– etc.

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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, bank card Other security token implementations: smart

button, USB dongle, mobile phone Two-method authentication: require both

physical token and a PIN – Attacker needs to both steal the physical device and

learn the PIN clear qualitative increase in security

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

to all systems– E.g. how to use a physical token if the application

requires cached credentials (password) on the client or on a proxy server

Process needed for recovering from the loss of tokens

Are smart card + PIN really two factors?26

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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

(less informative)

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FAR FRR50%

EER

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Issues with biometrics Biometrics require enrollment and readers Typically not usable for online users (over the Internet) Big difference in the security of unsupervised vs.

supervised readers – E.g. fingerprint reader on computer vs. iris scanner at

immigration Suitability for security architectures:

– Are biometric characteristics secrets? – Can they be copied? E.g. fingerprints on iPhone– 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; 3rd ed. chapter 4 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,

at least 2 digits, at least 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)? How many hash values van a brute-force attacker test in second with a new GPU? Hint:

Check the BitCoin mining speeds. How do mandatory periodical 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?