Security

1. Security

2. Security Needs • Computers and data are used by the authorized persons • Computers and their accessories, data, and information are available to the genuine users Security policy is to ensure that

3. Security Services • Authentication • Access control • Data confidentiality • Data integrity • Non-repudiation

4. Security Services Authentication • A user proves its identity to another party • A data sender proves that the data is actually sent by him/her

5. Security Services Access control • Guard against unauthorized use of resources

6. Security Services Data confidentially • Data and its meanings are only available to those who are the genuine receivers • For other parties, the data would appear to be “rubbish”

7. Security Services Data integrity • Guards against active attack – modification, insertion, deletion, replay • If a piece of data is changed, such a change can be detected

8. Security Services Non-repudiation • When a party sends a piece of information, it can be proved that the sender is actually that party • The sender cannot subsequently deny the act of having sent a piece of information

9. Security Mechanisms To provide security services, some specific security mechanisms may be implemented: • Encipherment • Digital signature • Access control

10. DES • The Data Encryption Standard (DES) is a private key encryption system developed by the U.S. government in the 1970s • It was based on a previous IBM encryption system called “Lucifer” • It was adopted as a U.S. federal standard in 1976, and then as an international standard

11. Encryption 64 bit message 56 bit key DES Overview • Plaintext size : 64 bits • Key size : 64 bits input, only 56 bits are used • Ciphertext size : 64 bits 64 bit ciphertext

12. Strength of DES • DES has been cryptanalyzed for many years by many people, no serious flaws have been revealed up to now • The 56-bit key size : there are 256=7.2x1016 different possible keys • May not be sufficient to resist brute-force key search attack

13. Strength of DES • If it takes 1 sec to test 1 key then 228 million years are needed to test all keys • If it takes 1 μsec to test 1 key then 2,280 years to test all keys • If there are 1 million machines working in parallel then the key can be found in a day!

14. k2 k1 k1 DES Encrypt DES Decrypt DES Encrypt ciphertext plaintext Triple DES • Triple DES employs the Encrypt-Decrypt-Encrypt (EDE) mode of operation with two different keys – equivalent to a key of 112 bits

15. k2 k1 k1 DES Decrypt DES Encrypt DES Decrypt plaintext ciphertext Triple DES • The decryption process is:

16. Triple DES • Triple DES can use the existing DES block • When K2=K1, the triple DES system “falls back” to the single DES system • It is “backward compatible” with single key DES

17. AES • AES stands for “Advanced Encryption System” • NIST (National Institute of Standards and Technology) of USA announced AES in 1997, and then called for algorithms from the public on 12 Sept 1997

18. AES • Researchers from 12 different countries submitted 15 algorithms for the AES • As at Aug 1999, 5 algorithms have been chosen by NIST for further consideration • On 3-Oct-2000, the proposal by Rijdael [pro. Rhine doll] – Joan Daemen and Vincent Rijmen of Belgium was selected

19. Public Key Encryption

20. Public Key Encryption • Each user will have a pair of keys K1 & K2 • Use keys K1 to encrypt and K2 to decrypt • Keep K1 private and top secret • Gives out K2 to anybody who needs it • K1 is called the private key • K2 is called the public key

21. Plaintext Encryption Decryption Key K1 Key K2 Two Keys • In a public key encryption system, the encryption key and the decryption key are different

22. English Message Encryption Decryption English Message Alice’s Private Key K1 Alice’s Public Key K2 Bob Alice Proof of Identity • Alice sends a message to Bob • Bob can prove that the message could only have been created by Alice

23. English Message Encryption Encryption Encrypted Message Alice’s Private Key Bob’s Public Key Alice Confidentiality + Identity • Alice sends an encrypted message to Bob so that only Bob can decrypt the message and Bob can later prove that the creator was Alice

24. RSA Algorithm • The most widely used public key algorithm • Proposed by Rivest, Shamir, and Adleman • Security is based on the difficulty in factorizing a large integer that is the product of two large prime numbers • E.g. 437 = ? x ? • 437 = 19 x 23 • Reference web page: http://www.rsa.comhttp://www.orst.edu/dept/honors/makmur/

25. Input = x (variable Length) Hash Function Output = y (fixed length) Hash Function • A Hash Functionis a one-way function y=H(x), designed to produced a fixed length “message digest” or a “fingerprint” of a variable-length message

26. MD5 • MD5 – Message Digest 5 • Designed by Prof. R. Rivest of MIT • Internet standard – RFC1321 • Thought to be a strong hash function • The message digest is 128 bits • Message is processed in 512-bit blocks

27. Secure Hash Algorithm (SHA) • SHA was FIPS PUB 180-1, designed by the U.S. National Security Agency (NSA) • To be used in the Digital Signature Algorithm (DSA) – part of the Digital Signature Standard (DSS) • Input data length is less than 264 bits • Message digest is 160 bits

28. Digital Signature • A digital signature has functions similar to those of conventional signature • Support authentic messages: • Signer of document can be confirmed • Contents of a signed document can be verified

29. ….. …… ….. ….. ….. …… ….. ….. Alice’s Private key DS Hash Encrypt Alice Digital Signature Generation • A widely adopted scheme is based on hash function and public key encryption

30. ….. …… ….. ….. Hash Alice’s Public key DS Compare Decrypt Equal => authentic message Not equal => non-authentic Bob Digital Signature Verification

31. Public Key Infrastructure • How to give your public key to your friend? • How can you be sure that the public key you obtain is indeed your friend’s public key? • For a small number of mutually trusted users, a “web of trust” system is O.K.

32. Bob Public key Alice Public key Eve Public key David Public key Web of Trust

33. Certification Authority • For a large population of users, a central trusted party can act as a Certification Authority (CA) • Users may deposit their public keys in a CA who they trust • The CA may pass out the public keys to any user who need them in certificates

34. CA d c a b A CA Supporting Many Users

35. Certificate • A certificate for a user (also called a subscriber) contains the user’s particulars and the user’s public key • The certificate is an electronic document signed by the CA who issue it

36. CA Alice’s certificate Other certificates to other users Cert. I.D.:123716 Name:Alice Public key:001010… Valid date:xx to yy …… …… Sign:________ Signed by CA Certificate

37. Revocation • A user may revoke the validity of his/her certificate before the actual expiry date • Revocation information about a CA’s subscribers are published in a Certificate Revocation List (CRL)

38. Public Key Infrastructure • When there are many CA’s and many subscribers, a hierarchy can be formed linking all the CA’s and the subscribers • This form a public key infrastructure • The subscribers can communicate securely by using digital signature techniques

39. Public Key Infrastructure CA 3 CA 2 CA 4 CA 1 user 6 user 1 user2 user 3 user 4 user 5