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CNS2010lecture 6 :: key management1 ELEC5616 computer and network security matt barrie mattb@ee.usyd.edu.au

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CNS2010lecture 6 :: key management2 key management Suppose we have a symmetric key network: Alice, Bob, Carol and Dave want to talk to each other For secure communication, for n parties, we require n n(n-1) 2 2 Key distribution and management becomes a major issue! Alice Carol Bob Dave k ab k ac k bd k cd k ad k bc ( ) = keys

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CNS2010lecture 6 :: key management3 definitions Key establishment is any process whereby a shared secret becomes available to two or more parties, for subsequent cryptographic use. Key management is the set of processes and mechanisms which support key establishment and the maintenance of ongoing keying relationships between parties, including replacing older keys with newer ones: –key agreement –key transport

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CNS2010lecture 6 :: key management4 key distribution centre Naïve solution: Protocol: (1) Alice → KDC: “want to talk with Bob” (2) KDC → Alice: KDC picks random key k ab, sends Ek a [k ab ], Ek b [k ab, “ticket a-b”] (3) Alice → Bob: Alice decrypts Ek a [k ab ], sends ticket to Bob (4) Bob: Bob decrypts ticket Alice and Bob now share secret key k ab. Alice Carol Bob Dave KDC kaka kbkb kckc kdkd All parties share a key with the KDC

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CNS2010lecture 6 :: key management5 problems with naïve approach Naïve solution: Problems: –Single point of failure, the KDC (a juicy target to attack) –No authentication –Poor scalability –Slow Alice Carol Bob Dave KDC kaka kbkb kckc kdkd All parties share a key with the KDC

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CNS2010lecture 6 :: key management6 merkle’s puzzles Ralph Merkle (Stanford, 1974) Merkle’s puzzles are a way of doing key exchange between Alice and Bob without the need for a KDC Protocol: (1)Alice creates lots of puzzles P i = E pi [“This is puzzle #X i ”, k i ] where i = 1.. 2 20, |p i | = 20 bits (weak), |k i | = 128 bits (strong) X i, p i and k i are chosen randomly and different for each i. (2)Alice sends all puzzles P i to Bob. (3)Bob picks a random puzzle j є {1 … 2 20 } and solves P j by brute force (i.e. search on key p j ). This recovers X j and k j from the puzzle. (4)Bob sends X j to Alice in the clear. (5) Alice looks up the index j of X j (from a table) to get k j. => Alice and Bob now both share a secret key k j.

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CNS2010lecture 6 :: key management7 merkle’s puzzles AliceBob P1P1 P6P6 P3P3 P4P4 P 12 P7P7 P9P9 P2P2 P5P5 P 14 P 11 P 13 Makes 2 20 puzzles Alice looks up X j Shared secret is k j Picks a random puzzle P j and breaks it. Sends X j back to Alice Shared secret is k j P i = Ep i [“This is puzzle #X i ”, k i ] Eve ??? Only knows the puzzles and X j

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CNS2010lecture 6 :: key management8 attack on merkle’s puzzles Eve must break on average half the puzzles to find X j (hence k j ) –Time required to do so for 2 20 puzzles = 2 19 x 2 19 = 2 38 If Alice and Bob can try 10,000 keys/second : –It will take a minute for each of them to perform their steps (2 19 for Bob) –Plus another minute to communicate the puzzles on a 1.544MB (T1) link With comparable resources, it will take Eve about a year to break the system. Note: Merkle’s Puzzles uses a lot of bandwidth (impractical!)

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CNS2010lecture 6 :: key management9 diffie-hellman key exchange Diffie-Hellman (Stanford, 1976) Worldwide standard used in smart cards, etc. Protocol: Consider the finite field Z p = where p is prime (p is about 300 digits long) Let g є Z p (the generator) (1)Alice: Alice chooses a random large integer a є Z p (2)Bob: Bob choses a random large integer b є Z p (3)Alice → Bob: Alice sends Bob g a (mod p) (4)Bob → Alice: Bob sends Alice g b (mod p) (5)Alice and Bob: compute g ab : Alice computes (g b ) a = g ab (mod p) : Bob computes (g a ) b = g ab (mod p) => Alice and Bob now share secret g ab

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CNS2010lecture 6 :: key management10 diffie-hellman key exchange AliceBob p, g, g a (mod p) g b (mod p) Computes g ab (mod p) Eve ??? Only knows p, g, g a, g b

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CNS2010lecture 6 :: key management11 strength of diffie-hellman The strength of Diffie-Hellman is based upon two issues: –given p, g, g a, it is difficult to calculate a (the discrete logarithm problem) –given p, g, g a, g b it is difficult for Eve to calculate g ab (the Diffie-Hellman problem) –we know that DL DH but it is not known if DH DL. Essentially, the strength of the system is based on the difficulty of factoring numbers the same size as p. The generator, g, can be small Do not use the secret g ab directly as a session key –it is better to either hash it or use it as a seed for a PRNG – not all bits of the secret have a flat distribution

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CNS2010lecture 6 :: key management12 references Handbook of Applied Cryptography –read §1, §2-2.4.4, §2.5 - 2.5.3 Stallings (3 rd Ed) –6.3 – 6.4

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