Security Services in Information Systems

1. Security Services in Information Systems

2. Basic Concepts

3. Machine A MachineB Key exchange: Diffie-Hellman protocol 1. Picks a  GF(p)at random 2. Computes TA = ga mod p 3. Sends TA 4. Receives TB 5. Computes KA = TBamod p 1. Picks b  GF(p) at random 2. Computes TB = gb mod p 3. Receives TA 4. Sends TB 5. Computes KB = TAbmod p Where K = KA =KB, Because: TBa= (gb)a= gba= gab= (ga)b= TAb mod p

4. Man-in-the-Middle Attack • Consider the following scenario: AnitaMiddlepersonBetito ga = 8389 gx = 5876 gb = 9267 8389 5876 5876 9267 Shared key KAX: Shared key KBX 5876a = 8389x 9267x = 5876b • After this exchange, the middle-person attacker simply decrypts any • messages sent out by A or B, and then reads any possibly modifies • them before re-encrypting with the appropriate key and transmitting • them to the correct party. • Middle-person attack is possible due to the fact that DHC does not • authenticate the participants. Possible solutions are digital signatures • and other protocol variants.

5. Mensaje cifrado Mensaje en claro Clave de sesión cifrada Clave de sesión Clave pública del destinatario Cifrado con clave pública de destino [SItema16] El documento se comprime antes con el algoritmo ZIP Necesitamos una clave de sesión... Se busca en el anillo de claves públicas del emisor

6. Mensaje cifrado Mensaje en claro Clave de sesión cifrada Clave de sesión Clave privada descifrada Clave privada destino cifrada Descifrado con la clave privada de destino [SItema16] Se busca en el anillo de claves privadas del receptor CONTRASEÑA

7. Key Distribution Problem[proposed solutions]

8. Key Distribution/Management and Authentication two closely related subjects why?

9. Key Distribution • symmetric schemes require both parties to share a common secret key • issue is how to securely distribute this key without revealing that key to an adversary • many attacks are based on poor key management and distribution • rather than breaking the codes • This is, actually, the most difficult problem in developing secure systems

10. Key Distribution various key distribution alternatives for parties A and B : • A can select key and physically deliver to B • does not scale for a large and distributed group • how many keys do we need for N users? • third party can select & deliver key to A & B • similar comment as 1 • sometimes finding a “trusted” third party is another problem • if A & B have communicated previously can use previous key to encrypt a new key • good option but initially several keys to be distributed • if A & B have secure communications with a third party C, C can relay key between A & B • only N master keys are enough

11. Session Key / Master Key • The idea is having a master encryption key (master key) to generate random and temporary session keys • can be implemented in several ways • Basic D-H is such an example • public/private keys are master keys, exchanged key is a session key • Kerberos is another example • SSL uses three layers • D-H for master key, master key for the session key

12. Session Key / Master Key • Session key lifetime is a trade-off • if small, • more secure since attacker can obtain less ciphertext to analyze • But this creates more delay • If large • less secure, but less delay

13. Key Distribution Facts • Conservation of trust principle • a secure communication cannot be based on nothing; either there should be an initial direct contact or an indirect one • Either physical delivery or a trusted third party • physical delivery is the only option to avoid a third party • most basic system is PIN entry • the case in Bluetooth • otherwise regardless of symmetric or asymmetric encryption, you need a trusted third party • even D-H does not work without a third party, why?

14. A Key Distribution Example • Symmetric crypto based • Each user shares a symmetric master key with the KDC (Key Distribution Center) • e.g. Ka, Kb, Kc, … • possibly physically distributed • Basic idea: • whenever a user A wants to communicate with B, it requests a session key (Ks) from KDC • Protocol is in the next slide

15. Nonce Definition • Nonce: The present or particular occasion. • Nonce word: A word occurring, invented, or used just for a particular occasion.

16. A Key Distribution Example

17. An Alternative • In the previous figure, can KDC send message 3 directly to B? • If not, why? • If so, what are pros and cons?

18. Hierarchies of KDCs • we may have several KDCs connected to each other in a tree topology • each leaf KDC serves to a local community • intra-domain communication passes thru only the local KDC, however inter-domain communication requires several KDC-KDC interaction • master key delivery is only in local domains

19. Decentralized Key Control • We may avoid using KDC in a small group by having a master key for each pair

20. Key Management in PKC • In other words • distribution of public keys • use of PKC to distribute secret keys • public/private key as a master key

21. Distribution of the Public Keys • the most important barrier against the deployment of PKC in applications • Basic question? • how can I make sure about the legitimacy of a public key? • how can I make sure that Bob’s public key really belongs to Bob, not to Charlie? • Why this is so important? • Name spoofing attacks become possible • remember the man-in-the-middle attacks in anonymous Diffie-Hellman

22. Distribution of the Public Keys • Some methods • Public Announcement • Publicly available databases/directories • Centralized distribution • Certificates

23. Public Announcement • Broadcast your public key to the public • via newsgroups, mailing lists, from personal website, etc. • major weakness is anyone can easily pretend as yourself • so attacks are possible

24. Publicly available directories/databases • There exists a directory/database for {name, public key} pairs • write controlled • a trusted administrator • if administered thoroughly, good • but a proper administration is difficult • need secure mechanisms for registration, update, delete.

25. Centralized Distribution - Public-Key Authority • Similar to directory/database approach, but access to the directory is automated via a secure protocol • users interact with directory to obtain any desired public key securely • requires real-time access to directory when keys are needed • users should know public key for the directory • the directory/database contains {name, public key} pairs • write permit is restricted

26. Centralized Distribution - Public-Key Authority PROTOCOL

27. Centralized Distribution - Public-Key Authority • Disadvantages • authority is an active entity and may create a performance bottleneck • database should be kept secure to prevent unauthorized modification

28. Public-Key Certificates • certificates allow key exchange without real-time access to public-key authority • a certificate binds identity to public key • usually with other info such as period of validity, rights of use, issuer’s info, etc • all contents signed by a trusted Certification Authority (CA) • can be verified by anyone who knows the CA public-key • CA must make sure about the identity of the cert owner

29. Public-Key Certificates

30. Public-Key Certificates • Certificates are widely used in practice • previous slides only show the idea • need lots of polishing for practical use • is a single CA sufficient? • what happens if the CA’s public key is not known? • how to distribute CA public keys? • what happens if a certificate is revoked? • We will discuss the use of certificates later

31. What can you do with securely distributed public keys? • Digital signatures • talked about them • confidentiality • theoretically possible but slow • instead session keys can be distributed • those session keys are used for symmetric encryption

32. Distribution of Secret Keys using PKC • Several methods exist • Diffie-Hellman is one way • we will see some alternatives

33. Simple Secret Key Distribution • proposed by Merkle in 1979 • A generates a new temporary public key pair • A sends B its public key and identity • B generates a session key and sends it to A encrypted using the supplied public key • A decrypts the session key and both use it

34. Simple Secret Key Distribution • problem is that an opponent can intercept and impersonate both halves of protocol • man-in-the-middle attack • result: A, B think that they exchanged Ks securely but C also knows Ks and use it to eavesdrop the communication passively KUc C EKUa[Ks] EKUc[Ks]

35. Public-Key Distribution of Secret Keys • assumption: public-keys are securely exchanged a priori • First three steps are for authentication purposes • Last step provides both the secrecy and authenticity of the session key

36. In practice • Most systems offer a three-level approach • use of PKC to exchange master-key • use of master-key to exchange session keys • most important advantage is at performance

37. A closer look to authentication • making sure of peer entity’s identity • mutual or one-way • non-repudiation is not an aim here • generally implemented as a protocol • basic idea: making sure that other party knows a common secret • also used for session key distribution • two types • message authentication • mostly one-way • peer entity authentication • connection oriented approach

38. Two key issues • Protection of any secret information • Timeliness • to prevent replay attacks • a valid message is copied and later resent • Why replays are bad? • at minimum, disrupt proper operation by presenting messages that appear genuine but actually are not • may also cause impersonation and key compromise

39. Countermeasures - 1 • Sequence numbers • not a practical method • parties should keep track of the sequence numbers • and should take care of message losses, duplications in a secure manner • complicates stuff

40. Countermeasures - 2 • Timestamps • message contains a timestamp • accepts only fresh messages based on this timestamp • sometimes used but some practical problems • clocks must be synchronized in a secure manner (some attacks are merely based on failure on clock synchronization) • tolerance to network delays

41. Countermeasures - 3 • Challenge/response • Initiator sends a nonce (one-time challenge phrase or number) and expects that nonce in the response • easier to implement • but may require extra message transfer

42. Authentication using Symmetric Encryption • We start with well-known Needham-Schroeder protocol • actually have seen it as a key distribution protocol • There exists a Key Distribution Center (KDC) • each party shares own master key with KDC • KDC generates session keys used for connections between parties • master keys used to distribute these keys to them

43. Needham-Schroeder Protocol • original three-party key distribution protocol • for session between A and B mediated by a trusted KDC • KDC should be trusted since it knows the session key • protocol overview 1. A→KDC: IDA|| IDB|| N1 2. KDC→A: EKa[Ks|| IDB|| N1 || EKb[Ks||IDA] ] 3. A→B: EKb[Ks||IDA] 4. B→A: EKs[N2] 5. A→B: EKs[f(N2)] • 4 and 5 are to prevent a kind of a replay attack • against replay of message 3 by an attacker

44. Needham-Schroeder (NS) Protocol • but still vulnerable to a replay attack • if an old session key has been compromised, then message 3 can be resent to B by an attacker X impersonating A • after that X intercepts message 4 and send a message 5 to B as if it is A • now X can impersonate A for the future communications with the session key • unlikely but a vulnerability • modifications to address this problem • timestamps (Denning 81), B contacted at the beginning (Needham Schroeder 87) • see http://www.lsv.ens-cachan.fr/spore/index.html for a repository of security protocols

45. NS Protocol with timestamps • proposed by Dorothy Denning • A and B can understand replays by checking the timestamp in the message • even if attacker knows Ks, he cannot generate message 3 with a fresh timestamp since he does not know Kb • open question: why do we need messages 4 and 5?

46. Needham – Schroeder Protocol Revisited • Amended by the creators themselves in 1987 by putting B in the loop early in the protocol 1. A -> B : A 2. B -> A : EKb[A, Nb] 3. A -> KDC : A, B, Na, EKb[A, Nb] 4. KDC -> A : EKa [Na, B, Ks, EKb [Ks, Nb, A]] 5. A -> B : EKb [Ks, Nb, A] 6. B -> A : Ks[Nb] 7. A -> B : Ks[f(Nb)]

47. Synchronization problem • Proper use of timestamps requires synchronized clocks • e.g. when sender’s clock is ahead of recipients clock • attacker can intercept the message and replay later • Neuman proposed use of nonces with timely session keys to constitute tickets in 93 • nonces are used to avoid replays • timestamps are expiration dates of the session key established • attacker only has a limited time to break the session key • in original NS, attacker has unlimited time to break the session key

48. Neuman Protocol • EKb[IDA || Ks || Tb] is a ticket that can be used later within the limit of Tb

49. Authentication using Public-Key Encryption • We have given an example method that assumes public keys are known • There exists protocols that also distributes public keys • using a central Authentication Server (AS) or KDC • using timestamps or nonces

50. One-Way Authentication • required when sender & receiver are not online at same time • e-mail is a typical application • protocol should not rely on the processing of B • A symmetric encryption approach 1. A→KDC: IDA|| IDB|| N1 2. KDC→A: EKa[Ks|| IDB|| N1 || EKb[Ks||IDA] ] 3. A→B: EKb[Ks||IDA] || EKs[M] • Provides authentication that the sender is A • does not protect against replays (of msg. 3) • could rely on timestamp in message, but delays in email make this problematic