Tuomas AuraT-110.4206 Information security technology

User authentication

Outline1. Passwords2. Physical security tokens and

two-method authentication3. Biometrics

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

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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?

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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?

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Password recovery Humans are prone to forget things need a

process for recovering from password loss What 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 customer support– Yellow sticky on the back of the keyboard– USB key or CD with a password recovery file

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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 accountcan lead to compromise of the other accounts

Adminsitrative 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 that helps the memory, encrypted with a

master password

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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 vs. brute-force guessing

– dictionary attack

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

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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Password entropy Entropy = the amount of information the attacker

is missing about the passwordEntropy ≤ log2(number of possible passwords)

Examples:– Random 8-character 7-bit passwords have 56 bits of

entropy– 8-character alphanumeric passwords have at most

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

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

– Eight 7-bit characters = 56-bit DES key, e.g. ½=(– Encrypt a zero block 25 times with modified DES– 12-bit salt used to make slight modifications to DES– Stored value includes the salt and encryption result– Very slowly replaced with more modern hash functions

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

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Online and offline guessing attacks In an online attack, target system can limit the number of

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

In an offline attack, the attacker can perform an exhaustive brute-force search – 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 Big difference in the required password entropy:

– Online attack success probability ≈ number of allowed guesses / number of possible passwords

– Offline attack requires cryptographic password strength, e.g. 128-bit entropy

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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 [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,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

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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 window 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 real

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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 Christinson) Examples of unexpected attacks:

– 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 form factors: button, USB stick, mobile

phone

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Issues with security tokens Physical tokes require distribution and recovery

process when tokens are lost Computers (or doors etc.) must have readers Some systems and protocols not compatible with

cryptographic tokens– E.g. applications that depend on a cached password

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, post

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BIOMETRICS

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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

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FAR FRR50%

EER

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

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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? Wouldn’t just

one secret identifier (password) be sufficient for logging in? What effect do strict guidelines for password format (e.g. eight

characters, use at least two capitals, at least two digits, at least one special symbol) have on the password entropy?

What is the success probability of guessing a 4-digit PIN code on a phone that locks up after three failed login attempts?

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.

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?

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