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P1363.2 submission: Password authentication using m ultiple servers

P1363.2 submission: Password authentication using m ultiple servers. David Jablon March 13, 2002. Password authentication using multiple servers [Jab2001]. Author: David Jablon Presented at April 2001 RSA conference Published paper (Springer LNCS) Extends work of Ford & Kaliski 2000.

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P1363.2 submission: Password authentication using m ultiple servers

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  1. P1363.2 submission:Password authenticationusing multiple servers David Jablon March 13, 2002

  2. Password authentication using multiple servers [Jab2001] • Author: David Jablon • Presented at April 2001 RSA conference • Published paper (Springer LNCS) • Extends work of Ford & Kaliski 2000

  3. Multi-server systems • Ford & Kaliski, WETICE, June 2001 • Multiple servers share responsibility to defend against password database cracking • Ford & Kaliski, proceedings, Sep. 2001 • Prior server-authenticated channel not needed for password security

  4. A neat trick Alice “small” P Bob big y • QA = g2 RA  • K1 = QB2 RA • K2 = QBP • K = h( K1, K2 ) •  (P x) y • P x  • K = (P x y) (1/ x) • K = P y converts low-entropy secret P into big secret K uses prime order group (e.g. mod p)

  5. Do it twice [Ford & Kaliski 2000] Alice P Bob1y1 • P x  • K1 = P y1 • K2 = P y2 • Km = h(K1 || K2) •  (P x) y1 •  (P x) y2 Bob2y2

  6. Benefits of multiple servers • Alice uses Km as a master key to encrypt all kinds of stuff, with less fear of her stuff being cracked. • the password “database” is split. • all Bobs must collude to get a chance to crack it.

  7. Main points of [Jab2001] paper • Alice tests Km before using it in public • Alice signs Px to prove she’s real • no server pre-auth (as in [FK2001b]) • Alice can use P = g1  g2hash(Password) • to sleep better when o(x) << p • forgiveness protocol • better handling of errors in password entry

  8. Test Km before using Alice P Bobs y1 y2 • P x  • Km = h(P y1 || P y2) • if owf(Km) V,abort(don’t reveal f(Km)) •  (P x) y1 •  (P x) y2 • V= owf(Km)

  9. Sign {P x} Alice P Bobs y1 y2 • P x  • Km = h(P y1 || P y2) • verify Km == V • PrivAlice{ P x }  •  (P x) y1 •  (P x) y2 • V = owf(Km) • If no valid signature • in time, log failure

  10. Compound base (1) • use group G of order q  2160, p  21000 • g1 & g2 not related by known exponents try g1=hash(“1”), g2=hash(“2”) • P = g1  g2hash(Password) • x, y in range [0, q] • uses smaller group in case short exponents don’t work out so well for the group of order ~21000. (open question)

  11. Compound base (2) • Since x, y are uniformly chosen random values in [1, o(G)], each value Px, PY individually reveals zero information • Would be nice to have a proof that this construction doesn’t introduce other new problems

  12. Password-in-exponent problem revisited • (g1  g2hash(Password))x • (g1  g2hash(Password))y

  13. Forgiveness protocol Scene: Alice mistypes a few passwordsP1, P2, ..., Pn, but finally gets P right. • Alice signs & sends prior mistaken valuesPrivAlice { P1x1, P2x2, ..., Pnxn } to each Bobn. • Each Bobn forgives Alice for a few mistakes,if she proves P in time. • Mistakes not counted towards illegal login threshholds.

  14. Relevance to 1363.2 • Variation of public-key retrieval scheme • Composite P used in {DL,EC}REDP-2 • Appears potentially useful for PKA Schemes • Forgiveness protocol • Fodder for an annex?

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