CS255: Dan Boneh. Iden+fica+on Protocols. Authen+ca+ng users. Dan Boneh
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1 CS255: Iden+fica+on Protocols Authen+ca+ng users
2 The Setup sk Alg. G vk vk either public or secret User P (prover) Server V (verifier) no key exchange yes/no
3 Applica+ons Physical locks: (friend- or- foe) Wireless car entry system (e.g. KeeLoq) Opening an office door or a garage door Login at a bank ATM or a desktop computer Login to a remote web site once key- exchange with one- sided authen+ca+on completes (e.g. SSL)
4 ID Protocols: how not to use ID protocol do not establish a secure session between Alice and Bob!! Not even when combined with anonymous key exch. Vulnerable to man in to the middle awacks sk Prover k anon. key exchange ID protocol Insecure! Verifier k vk Alice
5 ID Protocols: how not to use ID protocol do not set up a secure session between Alice and Bob!! Not even when combined with anonymous key exch. Vulnerable to man in to the middle awack sk vk Prover k a key exch. k a k b proxy ID protocol key exch. Verifier k b Alice
6 ID Protocols: Security Models 1. Direct A5acker: impersonates prover with no addi+onal informa+on (other than vk) Door lock 2. Eavesdropping a5acker: impersonates prover axer eavesdropping on a few conversa+ons between prover and verifier Wireless car entry system 3. Ac;ve a5acker: interrogates prover and then awempts to impersonate prover Fake ATM in shopping mall
7 Iden+fica+on Protocols Security against direct awacks (password systems)
8 Basic Password Protocol (incorrect version) PWD: finite set of passwords Algorithm G (setup): choose pw PWD. output sk = vk = pw. User P (prover) sk sk vk Server V (verifier) yes iff sk=vk
9 Basic Password Protocol (incorrect version) Problem: VK must be kept secret Compromise of server exposes all passwords Never store passwords in the clear! password file on server Alice Bob pw alice pw bob
10 A (small) sample of server- side password breaches 2012: Linked- in: 6 million passwords (hashed, unsalted) 2013: Twi5er: 250,000 passwords (hashed, salted) LivingSocial: 50 million records Names, s, DOB, passwords (hashed, salted) Evernote: 50 million records usernames, s, hashed passwords Adobe: 38 million records ( hwp://xkcd.com/1286/ ) addrs., password hints, and encrypted passwords
11 Basic Password Protocol: version 1 H: one- way hash func+on from PWD to X Given H(x) it is difficult to find y such that H(y)=H(x) example: SHA- 256, PBKDF2 User P (prover) sk Server V (verifier) password file on server Alice H(pw A ) sk vk = H(sk) Bob H(pw B ) yes iff H(sk)=vk
12 Weak password choice Users frequently choose weak passwords: (adobe list, 2013) Password: password adobe qwerty Frac+on of users: 5% 1.1% 0.9% 0.5% 0.5% 0.5% 0.3% A common occurrence Total: 8.8% Example: the Rockyou password list, 2009 (6 most common pwds) , 12345, Password, iloveyou, princess, abc123 Dic+onary of 360,000,000 words covers about 25% of user passwords
13 Offline Dic+onary AWacks Suppose awacker obtains a single vk = H(pw) from server Offline awack: hash all words in Dict un+l a word w is found such that H(w) = vk Time O( Dict ) per password Off the shelf tools (e.g. John the ripper): Scan through all 7- lewer passwords in a few minutes Scan through 360,000,000 guesses in few seconds will recover 23% of passwords
14 Batch Offline Dic+onary AWacks Suppose awacker steals en;re pwd file F Obtains hashed pwds for all users Example (2012): Linkedin (6M: SHA1(pwd) ) Alice H(pw A ) Bob H(pw B ) Batch dict. awack: For each w Dict: test if H(w) appears in F (using fast look- up) Total +me: O( Dict + F ) [Linkedin: 6 days, 90% of pwds. recovered] Much bewer than a dic+onary awack on each password!
15 Preven+ng Batch Dic+onary AWacks Public salt: When seqng password, pick a random n- bit salt S When verifying pw for A, test if H(pw, S A ) = h A Recommended salt length, n = 64 bits AWacker must re- hash dic+onary for each user Alice S A H(pw A, S A ) Bob S B H(pw B, S B ) id password database salt hash (PBKDF2) Batch awack +me is now: O( Dict F )
16 Further Slowing Down Dic+onary AWacks Slow hash func;on H: (say 0.1 sec. to hash pw) Example: H(pw) = SHA1(SHA1( SHA1(pw, S A ) )) Unno+ceable to user, but makes offline dic+onary awack harder Use PBKDF2: tunable # itera+ons Alice S A H(pw A, S A, r A ) Secret salts: When seqng pwd choose Bob S B H(pw B, S B, r B ) short random r (8 bits) When verifying pw for A, try all values of r A mes slow down on average mes slow down for awacker
17 Strengthening User Authen+ca+on One op+on: biometrics: Fingerprints, re+na, facial recogni+on, Benefit: hard to forget Problems: Biometrics are not generally secret Cannot be changed, unlike passwords note: CCC 13 Should primarily be used as a second factor authen+ca+on
18 The Common Password Problem Users tend to use the same password at many sites Password at a high security site can be exposed by a break- in at a low security site Standard defense: (PwdHash) Client side soxware that converts a common password pw into a unique site password pw is sent to server pw H( pw, user- id, server- id )
19 Iden+fica+on Protocols Security against eavesdropping awacks (one- +me password systems)
20 Eavesdropping Security Model Adversary is given: Server s vk, and the transcript of several interac+ons between honest prover and verifier. (example: remote car unlock) adv. goal is to impersonate prover to verifier A protocol is secure against eavesdropping if no efficient adversary can win this game The password protocol is clearly insecure!
21 oxen, +me- based updates: r F(k, +me) [stateless] 2 nd factor OTP authen+ca+on (secret vk, stateful) Setup (algorithm G): Choose random key k Output sk = (k,0) ; vk = (k,0) Iden+fica+on: prover sk = (k,0) sk = (k,1) r 0 F(k,0) r 1 F(k,1) 6 digits server vk = (k,0) vk = (k,1) Yes iff r = F(k,0)
22 Google authen+cator 6- digit +med one- +me passwords (TOTP) based on [RFC 6238] Wide web- site adop+on: Evernote, Dropbox, WordPress, outlook.com, To enable TOTP for a user: web site presents QR code with embedded data: otpauth://totp/example:alice@dropbox.com? secret=jbswy3dpehpk3pxp & issuer=example (Subsequent user logins require user to present TOTP) Danger: password reset upon user lockout
23
24 Server compromise exposes secrets March 2011: RSA announced servers awacked, secret keys stolen enabled SecurID user impersona+on Is there an ID protocol where server key VK is not- secret?
25 The S/Key system (public vk, stateful) Nota+on: H (n) (x) = H(H( H(x) )) Algorithm G: (setup) n +mes Choose random key k K Output sk = (k,n) ; vk = H (n+1) (k) Iden+fica+on: k H(k) H (n- 2) (k) H (n- 1) (k) H (n) (k) H (n+1) (k) pwd #4 pwd #3 pwd #2 pwd #1 vk
26 The S/Key system (public vk, stateful) Iden+fica+on (in detail): Prover (sk=(k,i)): send t H (i) (k) ; set sk (k,i- 1) Verifier( vk=h (i+1) (k) ): if H(t)=vk then vk t, output yes Notes: vk can be made public; but need to generate new sk axer n logins (n 10 6 ) Thm : S/Key n is secure against eavesdropping (public vk) provided H is one- way on n- iterates
27 TOTP vs. S/Key S/Key: public vk, limited number of authen+ca+ons Long one- +me passwords (128 bits) TOTP: secret vk, unlimited number of authen+ca+ons Short one- +me passwords (6 digits, i.e. 20 bits)
28 Iden+fica+on Protocols Security against ac+ve awacks (challenge- response protocols)
29 Ac+ve AWacks User P (prover) probe #1 probe #q vk Server V (verifier) sk impersonate vk Offline fake ATM: interacts with user; later tries to impersonate to legit. ATM Offline phishing: phishing site interacts with user; later authen+cates to real site Protocols so far are vulnerable
30 MAC- based Challenge Response (secret vk) User P (prover) sk sk = k k K random m M t S MAC (k, m) vk = k vk Server V (verifier) V MAC (k, m, t) Thm : protocol is secure against ac+ve awacks (secret vk), provided (S MAC, V MAC ) is a secure MAC
31 MAC- based Challenge Response Problems: vk must be kept secret on server dic+onary awack when k is a human pwd: Given [ m, S MAC (pw, m) ] eavesdropper can try all pw Dict to recover pw Main benefit: Both m and t can be short CryptoCard: 8 chars each
32 Sig- based Challenge Response (public vk) Replace MAC with a digital signature: User P (prover) sk sk (sk, vk) G SIG random m M t Sign(k, m) vk vk Server V (verifier) Verify(k, m, t) Thm : Protocol is secure against ac+ve awacks (public vk), provided (G SIG,Sign,Verify) is a secure digital sig. but t is long ( 20 bytes)
33 Summary ID protocols: useful in seqngs where adversary cannot interact with prover during impersona+on awempt Three security models: Direct: passwords (properly salted and hashed) Eavesdropping a5acks: One +me passwords SecurID: secret vk, unbounded logins S/Key: public vk, bounded logins Ac;ve a5acks: challenge- response
34 THE END
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