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Modelling and Analysing of Security Protocol: Lecture 3 Protocol Goals

Modelling and Analysing of Security Protocol: Lecture 3 Protocol Goals. Tom Chothia CWI. Last Lecture. Part 1: Simple notion for protocols Modelling “rules” Needham-Schroeder and Kerberos protocols Part 2: A high level overview the to cryptography

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Modelling and Analysing of Security Protocol: Lecture 3 Protocol Goals

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  1. Modelling and Analysing of Security Protocol: Lecture 3Protocol Goals Tom Chothia CWI

  2. Last Lecture • Part 1: • Simple notion for protocols • Modelling “rules” • Needham-Schroeder and Kerberos protocols • Part 2: • A high level overview the to cryptography • Symmetric key Encryption, public key encryptions and signing • Abstract equation for modelling encryption

  3. The Needham-Schroeder Protocol An (in) famous authentication protocol 1. A  B : EB( Na, A ) 2. B  A : EA( Na, Nb ) 3. A  B : EB( Nb ) Na and Nb can then be used to generate a symmetric key

  4. An Attack Against the Needham-Schroeder Protocol The attack acts as a man in the middle: 1. A  C : EC( Na, A ) 1`. C(A)  B : EA( Na, A ) 2`. B  C(A) : EA( Na, Nb ) 2. C  A : EA( Na, Nb ) 3. A  C : EC( Nb ) 3`. C(A)  B : EB( Nb )

  5. The Corrected Version A very simple fix: 1. A  B : EB( Na, A ) 2. B  A : EA( Na, Nb ) 3. A  B : EB( Nb )

  6. The Corrected Version A very simple fix: 1. A  B : EB( Na, A ) 2. B  A : EA( Na, Nb, B) 3. A  B : EB( Nb )

  7. Today: • First Lecture: Goals for security protocol • To know if a protocol is secure you must know what is aims to achieve. • Example: Diffie-Hellman & STS Protocol. • Second Part: Attacks and Principles • Common types of attacks on protocols • Good design principles for protocol.

  8. Is This An Attack? The Needham-Schroeder key establishment protocol is the bases for Kerberos • A and B use the trusted 3rd party S to establish a key Kab: • A  S : A, B, Na • S  A : { Na , B, Kab, {Kab , A}Kbs }Kas • A  B : { Kab , A}Kbs • B  A : { Nb }Kab • A  B : { Nb + 1 }Kab

  9. Is This An Attack? A forces B to use an old key: -999. A  S : A, B, Na -998. S  A : { Na , B, Kab, {oldKab , A}Kbs }Kas ..... ..... 3. A  B : { oldKab , A}Kbs 4. B  A : { Nb }oldKab 5. A  B : { Nb + 1 }oldKab

  10. What is a Protocol Trying to Achieve? We MUST be clear about what we are trying to do with a protocol: • Security • If we can securely establish a fresh key then secure transfer of data is easy. • Key establishment • Yes, but we saw the problem in the last example • Authentication • Again we have to be exact.

  11. Some Key Establishment Goals • Key Freshness: the key established is new • either from some trusted 3rd party • or because it uses a new nonce. • Key Exclusivity: the key is only known to the principals in the protocol. • Good Key: the key is both fresh and exclusive

  12. A Hierarchy of Goals Good Key Fresh Key Key Exclusivity

  13. A Good Key is Not Enough • A “Good Key” provides a secure communication channel.... ... but we have no idea who is on the other end of the channel. • We need authentication as well.

  14. Authentication Goals The most simple: • Far-end Operative: A knows that “B” is currently active: • For instance B might have signed a nonce generated by A e.g. • A  B : Na • B  A : SignB ( Na )

  15. Authentication Goals and • Once Authentication: A knows that B wishes to communicate with A • For instance, B might have the name A in a message e.g. • B  A : SignB ( A)

  16. Entity Authentication Both of these together give: • Entity Authentication: A knows that B is currently active and wants to communicate with A. e.g. • A  B : Na • B  A : SignB ( A, Na )

  17. A Hierarchy of Goals Entity Authentication Good Key Fresh Key Key Exclusivity Far-end Operative Once Authenticated

  18. Key Confirmation A common goal is: • Key Confirmation of A to B is provided if • B can be sure that “K” is a good key to communicate with A • and that A knows “K”.

  19. A Hierarchy of Goals Key Confirmation Entity Authentication Good Key Fresh Key Key Exclusivity Far-end Operative Once Authenticated

  20. The Highest Goal • Mutual Belief in Key: is provided for B only if • “K” is a good key with A • A believes that “K” is a good key for “B” • and A wishes to communicate with B using “K”

  21. A Hierarchy of Goals Mutual Belief in Key Key Confirmation Entity Authentication Good Key Fresh Key Key Exclusivity Far-end Operative Once Authenticated

  22. Example: Diffie-Hellman • The Diffie-Hellman is a widely used key agreement protocol. • It relies on some number theory: • a mod b = n where m s.t. a = m.b + n • The protocol uses two public parameters • generator “g” (often 160 bits long) • prime “p” (often 1024 bits long)

  23. Diffie-Hellman • A and B pick random numbers rA and rB and calculate “tA = grA mod p” and “tB = grB mod p” • The protocol just exchanges these numbers: • A  B : tA • B  A : tB • A calculates “tBrA mod p” and B “tArB mod p” this is the key: • K = grArB mod p

  24. Diffie-Hellman • A and B pick random numbers rA and rB and calculate “tA = grA mod p” and “tB = grB mod p” • The protocol just exchanges these numbers: • A  B : p, g, tA • B  A : tB • A calculates “tArA mod p” and B “tArB mod p” this is the key: • K = grArB mod p

  25. Diffie-Hellman • An observer cannot work out rA and rB from tA and tB therefore the attacker cannot calculate the key • The values of “g” and “p” must be big enough to make it intractable to try all possible combinations. • So we have a “Good Key” but know nothing about the participants. • We did not need to share any keys at the start, therefore this is a very powerful protocol.

  26. Station-to-Station Protocol • The Station-to-Station (STS) protocol adds authentication: • A  B : tA • B  A : tB , { SignB(tA, tB ) }Kab • A  B : { SignA(tA, tB ) }Kab

  27. Station-to-Station Protocol • A  B : A, B, tA • B  A : B, A, tB , { SignB(tA, tB ) }Kab • A  B : A, B, { SignA(tA, tB ) }Kab • Good Key: as before Key • Key Confirmation: A knows that B knows the Kab.

  28. Attack on Mutual Belief • A  E(B) : A, B, tA 1’. E  B : E, B, tA 2’. B  E : B,E,tB,{SignB(tA,tB)}Kab 2. E(B) A : B,A,tB,{SignB(tA,tB)}Kab 3. A  E(B) : A, B, { SignA(tA, tB ) }Kab

  29. What does STS provide? • Attacker E does NOT learn the key. • B does not accept the key. • But A does accept the key. This can be fixed by changing line 2 to: 2. B  A : tB , { A, SignB(tA, tB ) }Kab This is not done because this attack does not pose a real risk. In this case Key Confirmation is enough.

  30. Very Important Homework: • Whenever you do something secure over the Internet (i.e., logon remotely, check your bank balance, pay a bill), think about the protocol used. Is the communication secure? • Have you established a key? Is it fresh? Think about who has been authenticated: • Have you been authenticated? • Has the computer you’re talking to?

  31. Today: • First Lecture: Goals for security protocol • To know if a protocol is secure you must know what is aims to achieve. • Example: Diffie-Hellman & STS Protocol. • Second Part: Attacks and Principles • Common types of attacks on protocols • Good design principles for protocol.

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