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Key Management

Key Management. Shared Key Exchange Problem. How do Alice and Bob exchange a shared secret? Offline Doesn’t scale Using public key cryptography (possible) Using specially crafted messages ( Diffie Hellman) Using a trusted third party (KDC) Secrets should never be sent in clear

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Key Management

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  1. Key Management

  2. Shared Key Exchange Problem • How do Alice and Bob exchange a shared secret? • Offline • Doesn’t scale • Using public key cryptography (possible) • Using specially crafted messages (Diffie Hellman) • Using a trusted third party (KDC) • Secrets should never be sent in clear • We should prevent replay attacks • We should prevent reuse of old keys

  3. Diffie Hellman Key Exchange • Exchange a secret with someone you never met while shouting in a room full of people • Alice and Bob agree on g and large n • Alice chooses random a, sends • Bob chooses random b, sends • Alice takes Bob’s message and calculates • Bob does the same; now they both know shared secret

  4. KDC Based Key Distribution • Building up to Needham Schroeder/Kerberos • User sends req. to KDC (key distrib. center) • KDC generates a shared key: Kc,s • Keys KKDC,C andKKDC,S are preconfigured • No keys ever traverse net in the clear • Why are identities in tickets? • 1. C, S • 3. EKKDC,S{C, Kc,s} C KDC S ticket • 2. EKKDC,C{S, Kc,s}

  5. KDC Based Key Distribution • KDC does not have to talk both to C and S • Messages 2 or 3 can be replayed by M • Force C and S to use same secret for a long time • Cause S to have an old ticket, break comm. w C • ticketS = EKKDC,S{C, Kc,s} • 1. C, S C KDC S • 2. EKKDC,C{S, Kc,s}, ticketS • 3. ticketS

  6. Needham-Shroeder Key Exchange • Use nonces to prevent replay attacks • ticketS = EKKDC,S{C, Kc,s} • 1. N1, C, S C S KDC • 2. EKKDC,C{N1, S, Kc,s,ticketS} • 3. EKC,S{N2}, ticketS • 4. EKC,S{N2-1, N3} • 5. EKC,S{N3-1}

  7. Whys … • Why N1? • Why N2? • Why N3? • Why encrypt ticketS

  8. Problem • What happens if attacker gets session key? • Can reuse old session key to answer challenge-response, generate new requests, etc • Need timestamps to ensure freshness = tickets expire after some time

  9. Solution • Introduce Ticket Granting Server (TGS) • Daily ticket plus session keys • Authentication server (AS) authenticates users • TGS+AS = KDC • This is modified Needham-Schroeder • Basis for Kerberos

  10. TGS 3. TGSReq 4. TGSRep C S 5. SReq Kerberos Third-party authentication service • Distributes session keys for authentication, confidentiality, and integrity AS 2. ASRep 1. ASReq

  11. Kerberos KC, AS = f(passuser) • ASReq= username, TGS, timestamp1 • TTGS = EKAS,TGS(C, KTGS,C, timestamp2, lifetime2) • ASRep = EKC,AS(KTGS,C, TGS, timestamp2, lifetime2), TTGS • TGSReq = TTGS, S, EKTGS,C(C, timestamp3) • TS = EKS,TGS(C, KC,S, timestamp4, lifetime4) • TGSRep = TS, EKC,TGS(KC,S, S, timestamp4, lifetime4) • SReq = EKC,S(C, timestamp5), TS

  12. Public Key Exchange Problem • How do we verify an identity: • Alice sends to Bob her public key Pub(A) • Bob sends to Alice his public key Pub(B) • How do we ensure that those keys really belong to Alice and Bob?Need a trusted third party

  13. Man-in-the-Middle Attack On Key Exchange • Alice sends to Bob her public key Pub(A) • Mallory captures this and sends to Bob Pub(M) • Bob sends to Alice his public key Pub(B) • Mallory captures this and sends to Alice Pub(M) • Now Alice and Bob correspond through Mallory who can read/change all their messages

  14. Public Key Exchange • Public key is public but … • How does either side know who and what the key is for? • Does this solve key distribution problem? • No – while confidentiality is not required, integrity is • Still need trusted third party • Digital certificates – certificate authority (CA) signs identity+public key tuple with its private key • Problem is finding a CA that both client and server trust

  15. Digital Certificates • Everyone has Trent’s public key • Trent signs both Alice’s and Bob’s public keys – he generates public-key certificate • When they receive keys, verify the signature • Mallory cannot impersonate Alice or Bob because her key is signed as Mallory’s • Certificate usually contains more than the public key • Name, network address, organization • Trent is known as Certificate Authority (CA)

  16. Certificate-Based Key Exchange • Authentication steps • Alice provides nonce, or a timestamp is used instead, along with her certificate. • Bob selects session key and sends it to Alice with nonce, encrypted with Alice’s public key, and signed with Bob’s private key. He sends his certificate too • Alice validates certificate – it is really Bob’s key inside • Alice checks signature and nonce – Bob really generated the message and it is fresh

  17. PGP • Pretty Good Privacy • “Web of Trust” • Public key, identity association is signed by many entities • Receiver hopefully can locate several signatures that he can trust • Like an endorsement scheme

  18. SSH • User keys installed on server out of band • User logs in with a password • Copies her public key onto server • Weak assurance of server keys • User machine remembers server keys on first contact • Checks if this is still the same host on subsequent contact • But no check on first contact

  19. Recovery From Stolen Private Keys • Revocation lists (CRL’s) • Long lists • Hard to propagate • Lifetime / Expiration • Short life allows assurance of validity at time of issue • Real time validation • Receiver of a certificate asks the CA who signed it if corresponding private key was compromised • Can cache replies

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