authentication.ppt
- 1. Outline • User authentication – Password authentication, salt – Challenge-response authentication protocols – Biometrics – Token-based authentication • Authentication in distributed systems (multi service providers/domains) – Single sign-on, Microsoft Passport – Trusted Intermediaries (KPC and CA)
- 2. Objectives • Understand the four major individual authentication mechanisms and their comparison • Understand the basic mechanisms of trusted intermediaries for distributed authentication using different crypto methods – Symmetric key: KDC (the key concept of ticket) – Asymmetric key: CA • Know the practical protocols of distributed authentication – Symmetric key: Kerberos – Asymmetric key: X.509
- 3. Password authentication • Basic idea – User has a secret password – System checks password to authenticate user • Issues – How is password stored? – How does system check password? – How easy is it to guess a password? • Difficult to keep password file secret, so best if it is hard to guess password even if you have the password file
- 4. Basic password scheme Password file User exrygbzyf kgnosfix ggjoklbsz … … kiwifruit hash function
- 5. Basic password scheme • Hash function h : strings strings – Given h(password), hard to find password – No known algorithm better than trial and error • User password stored as h(password) • When user enters password – System computes h(password) – Compares with entry in password file • No passwords stored on disk
- 6. Unix password system • Hash function is 25xDES – 25 rounds of DES-variant encryptions • Any user can try “dictionary attack” R.H. Morris and K. Thompson, Password security: a case history, Communications of the ACM, November 1979
- 7. UNIX Password System • Password line walt:fURfuu4.4hY0U:129:129:Belgers:/home/walt:/bin/csh 25x DES Input Salt Key Constant, A 64-bit block of 0 Plaintext Ciphertext Compare
- 8. Dictionary Attack – some numbers • Typical password dictionary – 1,000,000 entries of common passwords • people's names, common pet names, and ordinary words. – Suppose you generate and analyze 10 guesses per second • This may be reasonable for a web site; offline is much faster – Dictionary attack in at most 100,000 seconds = 28 hours, or 14 hours on average • If passwords were random – Assume six-character password • Upper- and lowercase letters, digits, 32 punctuation characters • 689,869,781,056 password combinations. • Exhaustive search requires 1,093 years on average
- 9. Outline • User authentication – Password authentication, salt – Challenge-response authentication protocols – Biometrics – Token-based authentication • Authentication in distributed systems (multi service providers/domains) – Single sign-on, Microsoft Passport – Trusted Intermediaries
- 10. Challenge-response Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” Failure scenario?? “I am Alice”
- 11. Authentication Goal: Bob wants Alice to “prove” her identity to him Protocol ap1.0: Alice says “I am Alice” in a network, Bob can not “see” Alice, so Trudy simply declares herself to be Alice “I am Alice”
- 12. Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Failure scenario?? “I am Alice” Alice’s IP address
- 13. Authentication: another try Protocol ap2.0: Alice says “I am Alice” in an IP packet containing her source IP address Trudy can create a packet “spoofing” Alice’s address “I am Alice” Alice’s IP address
- 14. Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr
- 15. Authentication: another try Protocol ap3.0: Alice says “I am Alice” and sends her secret password to “prove” it. playback attack: Trudy records Alice’s packet and later plays it back to Bob “I’m Alice” Alice’s IP addr Alice’s password OK Alice’s IP addr “I’m Alice” Alice’s IP addr Alice’s password
- 16. Authentication: yet another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. Failure scenario?? “I’m Alice” Alice’s IP addr encrypted password OK Alice’s IP addr
- 17. Authentication: another try Protocol ap3.1: Alice says “I am Alice” and sends her encrypted secret password to “prove” it. record and playback still works! “I’m Alice” Alice’s IP addr encryppted password OK Alice’s IP addr “I’m Alice” Alice’s IP addr encrypted password
- 18. Authentication: yet another try Goal: avoid playback attack Failures, drawbacks? Nonce: number (R) used only once –in-a-lifetime ap4.0: to prove Alice “live”, Bob sends Alice nonce, R. Alice must return R, encrypted with shared secret key “I am Alice” R K (R) A-B Alice is live, and only Alice knows key to encrypt nonce, so it must be Alice!
- 19. Authentication: ap5.0 ap4.0 doesn’t protect against server database reading • can we authenticate using public key techniques? ap5.0: use nonce, public key cryptography “I am Alice” R Bob computes K (R) A - (K (R)) = R A - KA + and knows only Alice could have the private key, that encrypted R such that (K (R)) = R A - K A +
- 20. Outline • User authentication – Password authentication, salt – Challenge-response authentication protocols – Biometrics – Token-based authentication • Authentication in distributed systems (multi service providers/domains) – Single sign-on, Microsoft Passport – Trusted Intermediaries
- 21. Biometrics • Use a person’s physical characteristics – fingerprint, voice, face, keyboard timing, … • Advantages – Cannot be disclosed, lost, forgotten • Disadvantages – Cost, installation, maintenance – Reliability of comparison algorithms • False positive: Allow access to unauthorized person • False negative: Disallow access to authorized person – Privacy? – If forged, how do you revoke?
- 22. Biometrics • Common uses – Specialized situations, physical security – Combine • Multiple biometrics • Biometric and PIN • Biometric and token
- 23. Token-based Authentication Smart Card • With embedded CPU and memory – Carries conversation w/ a small card reader • Various forms – PIN protected memory card • Enter PIN to get the password – PIN and smart phone based token • Google authentication – Cryptographic challenge/response cards • Computer create a random challenge • Enter PIN to encrypt/decrypt the challenge w/ the card
- 24. Smart Card Example • Some complications – Initial data (PIN) shared with server • Need to set this up securely • Shared database for many sites – Clock skew Challenge Time function Time Initial data (PIN)
- 25. Outline • User authentication – Password authentication, salt – Challenge-Response – Biometrics – Token-based authentication • Authentication in distributed systems – Single sign-on, Microsoft Passport – Trusted Intermediaries
- 26. Single sign-on systems Rules Authentication Application Database Server LAN user name, password, other auth • Advantages – User signs on once – No need for authentication at multiple sites, applications – Can set central authorization policy for the enterprise
- 27. Microsoft Passport • Launched 1999 – Claim > 200 million accounts in 2002 – Over 3.5 billion authentications each month • Log in to many websites using one account – Used by MS services Hotmail, MSN Messenger or MSN subscriptions; also Radio Shack, etc. – Hotmail or MSN users automatically have Microsoft Passport accounts set up
- 28. Passport log-in
- 29. Trusted Intermediaries Symmetric key problem: • How do two entities establish shared secret key over network? Solution: • trusted key distribution center (KDC) acting as intermediary between entities Public key problem: • When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: • trusted certification authority (CA)
- 30. Key Distribution Center (KDC) • Alice, Bob need shared symmetric key. • KDC: server shares different secret key with each registered user (many users) • Alice, Bob know own symmetric keys, KA-KDC KB-KDC , for communicating with KDC. KB-KDC KX-KDC KY-KDC KZ-KDC KP-KDC KB-KDC KA-KDC KA-KDC KP-KDC KDC
- 31. Ticket and Standard Using KDC • Ticket – In KA-KDC(R1, KB-KDC(A,R1) ), the KB-KDC(A,R1) is also known as a ticket – Comes with expiration time • KDC used in Kerberos: standard for shared key based authentication – Users register passwords – Shared key derived from the password
- 32. Kerberos • Trusted key server system from MIT – one of the best known and most widely implemented trusted third party key distribution systems. • Provides centralised private-key third-party authentication in a distributed network – allows users access to services distributed through network – without needing to trust all workstations – rather all trust a central authentication server • Two versions in use: 4 & 5 • Widely used – Red Hat 7.2 and Windows Server 2003 or higher
- 33. slide 33 Two-Step Authentication Encrypted TGS ticket Joe the User Key distribution center (KDC) USER=Joe; service=TGS Prove identity once to obtain special TGS ticket Use TGS to get tickets for any network service File server, printer, other network services Encrypted service ticket Ticket granting service (TGS) TGS ticket Encrypted service ticket
- 34. slide 34 Still Not Good Enough • Ticket hijacking – Malicious user may steal the service ticket of another user on the same workstation and use it • IP address verification does not help – Servers must verify that the user who is presenting the ticket is the same user to whom the ticket was issued
- 35. slide 35 Symmetric Keys in Kerberos • Kc is long-term key of client C – Derived from user’s password – Known to client and key distribution center (KDC) • KTGS is long-term key of TGS – Known to KDC and ticket granting service (TGS) • Kv is long-term key of network service V – Known to V and TGS; separate key for each service • Kc,TGS is short-term key between C and TGS – Created by KDC, known to C and TGS • Kc,v is short-term key betwen C and V – Created by TGS, known to C and V
- 36. slide 36 “Single Logon” Authentication User • Client only needs to obtain TGS ticket once (say, every morning) – Ticket is encrypted; client cannot forge it or tamper with it kinit program (client) Key Distribution Center (KDC) password IDc , IDTGS , timec EncryptKc (Kc,TGS , IDTGS , timeKDC , lifetime , ticketTGS) Kc Convert into client master key Key = Kc Key = KTGS TGS … All users must pre-register their passwords with KDC Fresh key to be used between client and TGS Decrypts with Kc and obtains Kc,TGS and ticketTGS EncryptKTGS (Kc,TGS , IDc , Addrc , IDTGS , timeKDC , lifetime) Client will use this unforgeable ticket to get other tickets without re-authenticating
- 37. slide 37 Obtaining a Service Ticket User • Client uses TGS ticket to obtain a service ticket and a short-term key for each network service – One encrypted, unforgeable ticket per service (printer, email, etc.) Client Ticket Granting Service (TGS) usually lives inside KDC System command, e.g. “lpr –Pprint” IDv , ticketTGS , authC EncryptKc,TGS (Kc,v , IDv , timeTGS , ticketv) Fresh key to be used between client and service Knows Kc,TGS and ticketTGS EncryptKc,TGS (IDc , Addrc , timec) Proves that client knows key Kc,TGS contained in encrypted TGS ticket EncryptKv (Kc,v , IDc , Addrc , IDv , timeTGS , lifetime) Client will use this unforgeable ticket to get access to service V Knows key Kv for each service
- 38. slide 38 Obtaining Service User • For each service request, client uses the short-term key for that service and the ticket he received from TGS Client Server V System command, e.g. “lpr –Pprint” ticketv , authC EncryptKc,v (timec+1) Knows Kc,v and ticketv EncryptKc,v (IDc , Addrc , timec) Proves that client knows key Kc,v contained in encrypted ticket Authenticates server to client Reasoning: Server can produce this message only if he knows key Kc,v. Server can learn key Kc,v only if he can decrypt service ticket. Server can decrypt service ticket only if he knows correct key Kv. If server knows correct key Kv, then he is the right server.
- 40. slide 40 Important Ideas in Kerberos • Short-term session keys – Long-term secrets used only to derive short-term keys – Separate session key for each user-server pair • … but multiple user-server sessions re-use the same key • Proofs of identity are based on authenticators – Client encrypts his identity, address and current time using a short-term session key • Also prevents replays (if clocks are globally synchronized) – Server learns this key separately (via encrypted ticket that client can’t decrypt) and verifies user’s identity • Symmetric cryptography only
- 41. Practical Uses of Kerberos • Email, FTP, network file systems and many other applications have been kerberized – Use of Kerberos is transparent for the end user – Transparency is important for usability! • Local authentication – login and su in OpenBSD • Authentication for network protocols – rlogin, rsh, telnet
- 42. When NOT Use Kerberos • No quick solution exists for migrating user passwords from a standard UNIX password database to a Kerberos password database – such as /etc/passwd or /etc/shadow • For an application to use Kerberos, its source must be modified to make the appropriate calls into the Kerberos libraries • All-or-nothing proposition – If any services that transmit plaintext passwords remain in use, passwords can still be compromised
- 43. Trusted Intermediaries Symmetric key problem: • How do two entities establish shared secret key over network? Solution: • trusted key distribution center (KDC) acting as intermediary between entities Public key problem: • When Alice obtains Bob’s public key (from web site, e-mail, diskette), how does she know it is Bob’s public key, not Trudy’s? Solution: • trusted certification authority (CA)
- 44. Certification Authorities • Certification authority (CA): binds public key to particular entity, E. • E (person, router) registers its public key with CA. – E provides “proof of identity” to CA. – CA creates certificate binding E to its public key. – Certificate containing E’s public key digitally signed by CA – CA says “this is E’s public key” Bob’s public key KB + Bob’s identifying information digital signature (encrypt) CA private key KCA - KB + certificate for Bob’s public key, signed by CA
- 45. Certification Authorities • When Alice wants Bob’s public key: – gets Bob’s certificate (Bob or elsewhere). – apply CA’s public key to Bob’s certificate, get Bob’s public key • CA is heart of the X.509 standard used extensively in – SSL (Secure Socket Layer)/TLS: deployed in every Web browser – S/MIME (Secure/Multiple Purpose Internet Mail Extension), and IP Sec, etc. Bob’s public key KB + digital signature (decrypt) CA public key KCA + KB +
- 46. General Process of SSL C Version, Crypto choice, nonce Version, Choice, nonce, signed certificate containing server’s public key Ks S Secret key K encrypted with server’s key Ks hash of sequence of messages hash of sequence of messages switch to negotiated cipher
- 47. Authentication in SSL/HTTPS • Company asks CA (e.g., Verisign) for a certificate • CA creates certificates and signs it • Certificate installed in server (e.g., web server) • Browser issued with root certificates – Windows Root Certificates List • http://social.technet.microsoft.com/wiki/contents/articles/2 592.aspx • Browser verify certificates and trust correctly signed ones
- 48. Single KDC/CA • Problems – Single administration trusted by all principals – Single point of failure – Scalability • Solutions: break into multiple domains – Each domain has a trusted administration
- 49. Multiple KDC/CA Domains Secret keys: • KDCs share pairwise key • topology of KDC: tree with shortcuts Public keys: • cross-certification of CAs • example: Alice with CAA, Boris with CAB – Alice gets CAB’s certificate (public key p1), signed by CAA – Alice gets Boris’ certificate (its public key p2), signed by CAB (p1)
- 50. Key Distribution Center (KDC) Alice knows R1 Bob knows to use R1 to communicate with Alice Alice and Bob communicate: using R1 as session key for shared symmetric encryption Q: How does KDC allow Bob, Alice to determine shared symmetric secret key to communicate with each other? KDC generates R1 KB-KDC(A,R1) KA-KDC(A,B) KA-KDC(R1, KB-KDC(A,R1) )
- 51. Group Quiz Consider the KDC and CA servers. Suppose a KDC goes down. What is the impact on the ability of parties to communicate securely; that is, who can and cannot communicate? Justify your answer. Suppose now a CA goes down. What is the impact of this failure?
- 52. Backup Slides
- 53. Advantages of salt • Without salt – Same hash functions on all machines • Compute hash of all common strings once • Compare hash file with all known password files • With salt – One password hashed 212 different ways • Precompute hash file? – Need much larger file to cover all common strings • Dictionary attack on known password file – For each salt found in file, try all common strings
- 54. Four parts of Passport account • Passport Unique Identifier (PUID) – Assigned to the user when he or she sets up the account • User profile, required to set up account – Phone number or Hotmail or MSN.com e-mail address – Also name, ZIP code, state, or country, … • Credential information – Minimum six-character password or PIN – Four-digit security key, used for a second level of authentication on sites requiring stronger sign-in credentials • Wallet – Passport-based application at passport.com domain – E-commerce sites with Express Purchase function use wallet information rather than prompt the user to type in data
- 55. slide 55 Kerberos in Large Networks • One KDC isn’t enough for large networks (why?) • Network is divided into realms – KDCs in different realms have different key databases • To access a service in another realm, users must… – Get ticket for home-realm TGS from home-realm KDC – Get ticket for remote-realm TGS from home-realm TGS • As if remote-realm TGS were just another network service – Get ticket for remote service from that realm’s TGS – Use remote-realm ticket to access service – N(N-1)/2 key exchanges for full N-realm interoperation