Exploring Cryptography and Key Management Essentials

1. COEN 351 E-Commerce Security Essentials of Cryptography

2. Cryptography • Scrambles a plain-text into crypto-text. • Enables to descramble plain text.

3. Symmetric Cryptography • Uses the same key for encryption, decryption

4. Asymmetric Cryptography • Uses different key for encryption, decryption

5. Message Authentication Codes • Condenses message into a short hash • SHA1, … MD5, … are appropriate cryptographically secure hash functions • For example, encrypt only the MAC with a key known to sender and receiver.

6. Message Authentication Code • Alternatively, use a secret key. • This also provides authentication.

7. Use of Asymmetric Cryptography • Generic idea: Make one key public. • How? • Website • Website can be spoofed. • On your business card • Works for individuals, requires recipient to type in several lines of gibberish correctly. • From a trusted source • Going back and back: Where does the trust stem from?

8. Use of Asymmetric Cryptography • Notations: • E – public key, D – secret key • EC (M) – encryption of M using key C. • DC(M) – decryption of M using key C. • Asymmetric cryptography key identities • DEED(M) = M • DDEE(M) = M

9. Use of Asymmetric Cryptography • Secret Transmission of messages • Alice uses public key of Bob to encrypt her messages to him: EE(Bob)(M). • Bob uses his private key to decrypt the message: DD(Bob)EE(Bob)(M).

10. Use of Asymmetric Cryptography • Signing a message I: • Alice encrypts the message with her private key: ED(Alice)(M). • Bob decrypts with her public key and obtains M = DE(Alice) ED(Alice)(M). • If M makes sense, Bob knows that someone with Alice secret key send the message.

11. Use of Asymmetric Cryptography • Signing a message II • This method avoids encryption of the whole message. • Asymmetric cryptography is very compute intensive. • Alice uses a MAC of her message: MAC(M). • She sends Bob M and ED(Alice)(MAC(M)). • Bob calculates • MAC(M) = DE(Alice) (ED(Alice)(MAC(M))). • Bob verifies that this is the correct MAC. • Bob concludes that the message was sent by someone knowing Alice’s private key.

12. Key Management • Generic Rules: • Use symmetric cryptography as much as possible for performance. • Never use keys more than once. • Key Management becomes an issue.

13. Key Management • Keys have limited lifetimes: • Cryptanalysis is easier with more material. • Breaking WEP involves harvesting a large number of packets. • Once found, a compromised key continues to do damage.

14. Key Management • Key Management Life Cycles: • Key establishment • Key generation • Key distribution • Key backup / recovery, key escrow • Key replacement / update (rekeying) • Key revocation • Key expiration / Key termination / Key destruction

15. Key Management • Key generation • Uses random number generation • Pseudo-random generation derived from a seed • WEP: seed based on user key word. Not as random as appeared. • Hardware random number generation • Combined methods

16. Key Management • Key distribution • Has issues of authentication and confidentiality. • Diffie-Hellman protocol solves confidentiality: • Allows two parties to agree on a common secret. • Subject to the man-in-the-middle attack • Alice thinks that she shares a secret with Bob. • In reality, she communicates with M, and shares the secret with him. • M shares another secret with Bob.

17. Key Management • Key backup / recovery • Accidental loss of key • hardware failure, forgotten password … • Control of encrypted information • Employer cannot entrust enterprise-critical data to complete control of a single / group of employees. • Key escrow • To preserve possibility of access by law enforcement agencies. • In the UK, it is a crime to withhold a key to encrypted data under subpoena. • In the US, such a law is seen to contradict 5th amendment protection.

18. Key Management • Key destruction • Secure key destruction is far easier than secure file erasure. • Key destruction destroys accessibility to encrypted data. • Key archiving • Necessary for validation of old signatures, of integrity of old messages, …

19. Key Management • Symmetric key transport: • Send symmetric key along, protected by public key of recipient. • Saves on processing time

20. Diffie-Hellman • Uses calculation modulo p, p a large prime. • Chooses generatorg. • Ideally, gx, x = 0, …, p -2 runs through all numbers 1, … p -1. • Uses the fact that calculating powers gx is computationally feasible. • But discrete logarithm (given gx find x) is not.

21. Diffie Hellman • Alice generates random number a mod p. • Bob generates random number b mod p. • Alice sends Bob gamod p. • Bob sends Alice gbmod p. • Alice calculates (gb)a mod p. • Bob calculates (ga)b mod p. • These numbers are identical and the shared key.

22. Diffie Hellman • Man in the middle attack Bob Man in the Middle Alice

23. Diffie Hellman • Alice sends Bob gamod p. • But message goes to alien. Alien sends Bob gc mod p. • Bob sends Alice gbmod p. • But message goes to alien. Alien sends Alice gd mod p. • Alice calculates (gd)a mod p. • Bob calculates (gc)b mod p. • These set up a secure communication channel between the alien and Bob and one between the alien and Alice.

24. Diffie Hellman • Secure against eavesdroppers. • Can be secured against man-in-the-middle by using authenticated gbmod p or by using a published value gbmod p.

25. Diffie Hellman and all other schemes • The problem is one of authentication and trust.