Authentication and Authorization

Authentication and Authorization • Tjaden, Fundamentals of Secure Computer Systems, Franklin, Beedle & Associates, 2004, Chapters 6, 7 and 9. • Smith & Marchesini, The Craft of System Security, Addison-Wesley, 2008, Chapter 9 Fundamentals of Secure Computer Systems

Overview • User authentication – determine the identity of an individual accessing the system • Mechanisms, attacks, defenses • Authorization Fundamentals of Secure Computer Systems

User Authentication • Three basic approaches: • Knowledge-based – usersprove their identity through something that they know • Example: passwords • Token-based – usersprove their identity through something they possess (something they have) • Example: passport • Biometric – users prove their identity through a unique physiological characteristic (something they are) • Example: fingerprint Fundamentals of Secure Computer Systems

Multi-Factor Authentication • Multi-factor authentication – uses a combination of approaches • Example: ATM card + pin • More expensive • More secure • Tradeoffs – cost, usability, security Fundamentals of Secure Computer Systems

Passwords • Passwords are widely-used for user authentication • Advantages: • Easy to use, understood by most users • Require no special equipment • Offer an adequate degree of security in many environments • Disadvantages: • Users tend to choose passwords that are easy to guess • Many password-cracking tools are available • Users often reuse passwords Fundamentals of Secure Computer Systems

Using Passwords • User enters username and password • The operating system consults its table of passwords: • Match = user is assigned the corresponding uid • Problem: the table of passwords must be protected Fundamentals of Secure Computer Systems

Using Passwords and One-Way Functions • User’s password is not stored in the table • A one-way hash of the password, h(password), is stored in the table • h(dumptruck) = JFNXPEMD • h(baseball) = WSAWFFVI Fundamentals of Secure Computer Systems

Using Passwords and One-Way Functions (cont) • User enters username and password • The operating system hashes the password • The operating system compares the result to the entry in the table • Match = user is assigned the corresponding uid • Advantage: password table does not have to be protected • Disadvantage: dictionary attack Fundamentals of Secure Computer Systems

A Dictionary Attack • An attacker can compile a dictionary of several thousand common words and compute the hash for each one: • Look for matches between the dictionary and the password table • Example: WSAWFFVI tells us Bob’s password is baseball Fundamentals of Secure Computer Systems

Dictionary Attacks (cont) • Dictionary attacks are a serious problem: • Costs an intruder very little to send tens of thousands of common words through the one-way function and check for matches • Between 20 and 40 percent of the passwords on a typical system can be cracked in this way • Solution #1: don’t allow users to select their own passwords • System generates a random password for each user • Drawback: • Many people find system-assigned passwords hard to remember and write them down • Example: L8f#n!.5rH’ Fundamentals of Secure Computer Systems

Combating Dictionary Attacks • Solution #2: password checking • Allow users to choose their own passwords • Do not allow them to use passwords that are in a common dictionary • Solution #3: salt the password table • A salt is a random string that is concatenated with a password before sending it through the one-way hash function • Random salt value chosen by system • Example: plre • Password chosen by user • Example: baseball Fundamentals of Secure Computer Systems

Salting the Password Table • Password table contains: • Salt value = plre • h(password+salt) = h(baseballplre) = FSXMXFNB Fundamentals of Secure Computer Systems

Salting the Password Table (cont) • User enters username and password • The operating system combines the password and the salt and hashes the result • The operating system compares the result to the entry in the table • Match = user is assigned the corresponding uid • Advantages: • Password table does not have to be protected • Dictionary attacks are much harder Fundamentals of Secure Computer Systems

A Dictionary Attack • Attacker must now expand the dictionary to contain every possible salt with each possible password: • baseballaaaa • baseballaaab • baseballaaac …. • baseballaaaz • baseballaaba • baseballaabb …. • 264 (about half a million) times more work to check each word in the dictionary (for 4-letter salts) Fundamentals of Secure Computer Systems

Attacks and Countermeasures • Online attack – manual attempt to break in • Countermeasures • exponential backoff – wait longer and longer after each attempt • restricted list – stop accepting input after N guesses, which can lead to DoS Fundamentals of Secure Computer Systems

Attacks (continued) • Offline attack • Attacker captures some information (password file?) and takes it offline for analysis • Dictionary attack • Brute force methods Fundamentals of Secure Computer Systems

Defenses • CAPTCHA - Completely Automated Public Turing test to tell Computers and Humans Apart • Special case of knowledge-based authentication • Differentiates between humans and automated users

CAPTCHA • Something easy for a human and difficult for a computer • Turing test – AI has been achieved when a human communicating with a human and a computer cannot tell the difference Fundamentals of Secure Computer Systems

Human Factors • “A good password should be too complex to remember.” • “You should never write a password down.” • Bob Blakely Fundamentals of Secure Computer Systems

Human Factors (continued) • Strong passwords • Humans are not good at spontaneous, context-free recall • Credential-recovery problem – often knowledge-based • The ‘spouse’ effect • Other reasons for credential transfer Fundamentals of Secure Computer Systems

Biometrics • Measure a physical aspect • Compare it to a stored template • Fingerprints, hand geometry, ear prints, iris scans, DNA, face recognition • Readers are not accurate • Physical attributes vary from day to day Fundamentals of Secure Computer Systems

ROC Curve • Sensitivity of biometric systems is tunable • False positives – imposter it authenticated • False negatives – legitimate user is rejected • Tradeoffs shown be receiver operations characteristics curve Fundamentals of Secure Computer Systems

Receiver Operations Characteristics (ROC) Curve • False positives – imposter is authenticated • False negatives – legitimate user is rejected

Biometrics • Invasive • Threatening Fundamentals of Secure Computer Systems

Token-based Authentication • Something the user has • ATM card • Token • a small computational device which generates one-time passwords based on the real-time clock • the authenticating computer generates matching tokens using its own clock • susceptible to clock drift Fundamentals of Secure Computer Systems

Attacks • Man-in-the-middle, bucket brigade or chess grandmaster attack • Adversary takes over user interface and collects user name and password • Adversary logs in for the user • Session hijacking – examples of TOCTOU • Social engineering Fundamentals of Secure Computer Systems

Cryptographic Protocols • A protocol is an agreed-upon sequence of actions performed by two or more principals • Cryptographic protocols make use of cryptography to accomplish some task, such as authentication, securely Fundamentals of Secure Computer Systems

Authentication • Authentication is the process of proving your identity to someone else • One-way • Two-way • Authentication protocols are often designed using a challenge and response mechanism • Authenticator creates a random challenge • Authenticatee proves identity by replying with the appropriate response Fundamentals of Secure Computer Systems

Using Nonces to Establish Freshness • A nonce is a randomly-generated value that: • Is never reused • Can be used to prove the freshness of a message Fundamentals of Secure Computer Systems

One-way Authentication Using Symmetric-Key Cryptography • Assume that Alice and Bob share a secret symmetric key, KAB • One-way authentication protocol: • Alice creates a nonce, NA, and sends it to Bob as a challenge • Bob encrypts Alice’s nonce with their secret key and returns the result, Encrypt(NA, KAB), to Alice • Alice can decrypt Bob’s response and verify that the result is her nonce A: => B(NA); B: => A(Encrypt(NA, KAB)); • A decrypts her own nonce and authenticates Bob Fundamentals of Secure Computer Systems

Two-way Authentication A: => B(NA); B: => A(NB, Encrypt(NA, KAB)); A: => B(Encrypt (NB, KAB)); Fundamentals of Secure Computer Systems

One-way Authentication Using Symmetric-Key Cryptography • Problem: an adversary, Mallory, might be able to impersonate Bob to Alice: • Alice sends challenge to Bob (intercepted by Mallory) • Mallory does not know KAB and thus cannot create the appropriate response • Mallory may be able to trick Bob (or Alice) into creating the appropriate response for her: A: => M(NA); M: => B(NA); B: => M(Encrypt(NA, KAB)); M: => A(Encrypt(NA, KAB)); Fundamentals of Secure Computer Systems

One-way Authentication Using Public-Key Cryptography • Alice sends a nonce to Bob as a challenge • Bob replies by encrypting the nonce with his private key • Alice decrypts the response using Bob’s public key and verify that the result is her nonce A: => B(NA); B: => A(Encrypt(NA, BPrivate)); • Encrypting just any message that someone sends as an authentication challenge might not be a good idea Fundamentals of Secure Computer Systems

Authentication and Key-Exchange Protocols • Combine authentication and key-exchange • Two parties are on opposite ends of a network and want to talk securely • Want to agree on a new session key securely • Want to each be sure that they are talking to the other and not an intruder • Wide Mouth Frog • Yaholom • Denning and Sacco Fundamentals of Secure Computer Systems

Single Sign-on (SSO) • Multiple applications, each requires login • Provide users with the ability to log in only once for usability • Automatically propagate login to all applications

Advantages and Disadvantages of SSO • Advantages: • Unified mechanism • One login/password to remember • One login/password for staff to set up • New applications reuse code • Disadvantages: • Cost of retrofitting old applications is high • Can weaken security

Access Control Policies • Once a user has logged in the system must decide which actions she can and cannot perform • Examples: • Bob may be allowed to read files that Alice cannot • Alice may be permitted to use a printer that Bob cannot • In general, we view the system as a collection of: • Subjects (users) • Objects (resources) • An access control policy specifies how each subject can use each object Fundamentals of Secure Computer Systems

Authorization • Authorization entails determining whether or not the protection policy permits a given user to perform a given action • Example: • Badges at a military installation • Many operating systems base authorization decisions on a user’s unique user identifier (or uid): • User is authenticated during log on and given an appropriate uid • Must enter valid username and password • The uid is used to determine which actions are authorized Fundamentals of Secure Computer Systems

Summary • Important components of computer security: • User authentication – determine the identity of an individual accessing the system • Knowledge-based (knows), token-based (has), and biometrics (is) • Authorization - access control policies stipulate what actions a given user is allowed to perform on the system Fundamentals of Secure Computer Systems

Authentication and Authorization