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Lecture 2: Private Key Cryptography II

Lecture 2: Private Key Cryptography II. CS 436/636/736 Spring 2014 Nitesh Saxena. Today’s fun/informative bit – The Smudge Attack. See: http://www.usenix.org/event/woot10/tech/full_papers/Aviv.pdf. Course Administration. Everything seems to be under control  Any questions?

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Lecture 2: Private Key Cryptography II

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  1. Lecture 2: Private Key Cryptography II CS 436/636/736 Spring 2014 Nitesh Saxena

  2. Today’s fun/informative bit – The Smudge Attack • See: http://www.usenix.org/event/woot10/tech/full_papers/Aviv.pdf Lecture 2.2 - Private Key Cryptography II

  3. Course Administration • Everything seems to be under control  • Any questions? • HW1 heads up • To be posted by this weekend • Covers lecture 1, 2 • 10-12 days for you to work on it Lecture 2.2 - Private Key Cryptography II

  4. Outline of today’s lecture • Block Ciphers • Data Encryption Standard (DES) • DES Security and Extensions • Block Cipher Modes of Encryption • AES (at home reading assignment) Lecture 2.2 - Private Key Cryptography II

  5. Block Ciphers and Stream Ciphers • Block ciphers partition plaintext into blocks and encrypt each block independently (with the same key) to produce ciphertext blocks. • A stream cipher generates a keystream and encrypts by combining the keystream with the plaintext, usually with the bitwise XOR operation. • We will focus mostly on Block Ciphers Lecture 2.2 - Private Key Cryptography II

  6. DES – Data Encryption Standard • Encrypts by series of substitution and transpositions. • Based on Feistel Structure • Worldwide standard for more than 25 years. • Designed by IBM (Lucifer) with later help from NSA. • No longer considered secure for highly sensitive applications. • Replacement standard AES (advanced encryption standard) recently completed. Lecture 2.2 - Private Key Cryptography II

  7. DES – Overview (Block Operation) Lecture 2.2 - Private Key Cryptography II

  8. DES – Each Round

  9. DES – Function F Lecture 2.2 - Private Key Cryptography II

  10. DES – Key Schedule (KS) Lecture 2.2 - Private Key Cryptography II

  11. Operation Tables of DES: Key Schedule, PC-1, PC-2 Lecture 2.2 - Private Key Cryptography II

  12. Operation Tables (IP, IP-1, E and P) Lecture 2.2 - Private Key Cryptography II

  13. S-boxes: S1 (as an example) Is the table entry from Lecture 2.2 - Private Key Cryptography II

  14. DES Decryption • Same as the encryption algorithm with the “reversed” key schedule – NEXT! Lecture 2.2 - Private Key Cryptography II

  15. Plain text Initial permutation (IP) Round-1 (key K1) Rounds 2-15 Round-16 (key K16) swap IP inverse Cipher text

  16. IP Round-1 (K16) = Since encrypt IP inverse Cipher text decrypt Lecture 2.2 - Private Key Cryptography II

  17. DES Example We choose a random plaintext block and a random key, and determine what the ciphertext block would be (all in hexadecimal): Lecture 2.2 - Private Key Cryptography II

  18. Example (contd) -- encryption Lecture 2.2 - Private Key Cryptography II

  19. Example (contd) -- decryption Let us see how Bob, at the destination, can decipher the ciphertext received from Alice using the same key. Table 6.16 shows some interesting points. Lecture 2.2 - Private Key Cryptography II

  20. DES Security: Avalanche Effect Lecture 2.2 - Private Key Cryptography II

  21. Further Reading • Chapter 7.4 of HAC • Chapter 3 of Stallings Lecture 2.2 - Private Key Cryptography II

  22. DES Security • S-Box design not well understood • Has survived some recent sophisticated attacks (differential cryptanalysis) • Key is too short. Hence is vulnerable to brute force attack. • 1998 distributed attack took 3 months. • $1,000,000 machine will crack DES in 35 minutes – 1997 estimate. $10,000 – 2.5 days. Lecture 2.2 - Private Key Cryptography II

  23. DES Cracking machine Lecture 2.2 - Private Key Cryptography II

  24. Super-encryption. • If key length is a concern, then instead of encrypting once, encrypt twice!! C = EK2(EK1(P)) P = DK1(DK2(C)) • Does this result in a larger key space? • Encrypting with multiple keys is known as super-encryption. • May not always be a good idea Lecture 2.2 - Private Key Cryptography II

  25. Double DES • Double DES is almost as easy to break as single DES (Needs more memory though)! Lecture 2.2 - Private Key Cryptography II

  26. Double DES – Meet-in-the-middle Attack (due to Diffie-Hellman) • Based on the observation that, if C = EK2(EK1(P)) Then X = EK1(P) = DK2(C). • Given a known (P, C) pair, encrypt P with all possible values of K and store result in table T. • Next, decrypt C with all possible keys K and check result. If match occurs then check key pair with new known (P, C) pair. If match occurs, you have found the keys. Else continue as before. • Process will terminate successfully. Lecture 2.2 - Private Key Cryptography II

  27. Meet-in-the-middle Explanation • The first match does not say anything as we have 264 ciphertexts and 2112 keys. • On the average 2112 / 264 = 248 keys will produce same ciphertext. • So there could be 248 possible candidates • We can use a second pair (P’,C’) • So, probability that false alarm will survive two known (P, C) pairs is 248 / 264 = 2-16. • One can always check a third pair to further reduce the chance of a false alarm. Lecture 2.2 - Private Key Cryptography II

  28. Triple DES • Triple DES (2 keys) requires 2112 search. Is reasonably secure. • Triple DES (3 keys) requires 2112 as well • Which one is better?

  29. Block Cipher Encryption modes • Electronic Code Book (ECB) • Cipher Block Chain (CBC) • Most popular one • Cipher Feed Back (CFB) • Output Feed Back (OFB) Lecture 2.3 - Private Key Cryptography III

  30. Analysis We will analyze each of the modes in terms of: • Security • Computational Efficiency (parallelizing encryption/decryption) • Transmission Errors • Integrity Protection Lecture 2.3 - Private Key Cryptography III

  31. Electronic Code Book (ECB) Mode • Although DES encrypts 64 bits (a block) at a time, it can encrypt a long message (file) in Electronic Code Book (ECB) mode. • Deterministic -- If same key is used then identical plaintext blocks map to identical ciphertext Lecture 2.3 - Private Key Cryptography III

  32. Example – why ECB is bad? Tux encrypted with AES in ECB mode Tux Lecture 2.3 - Private Key Cryptography III

  33. Cipher Block Chain (CBC) Mode encryption decryption Lecture 2.3 - Private Key Cryptography III

  34. CBC Traits • Randomized encryption • IV – Initialization vector serves as the randomness for first block computation; the ciphertext of the previous block serves as the randomness for the current block computation • IV is a random value • IV is no secret; it is sent along with the ciphertext blocks (it is part of the ciphertext) Lecture 2.3 - Private Key Cryptography III

  35. Example – why CBC is good? Tux encrypted with AES in CBC mode Tux Lecture 2.3 - Private Key Cryptography III

  36. CBC – More Properties • What happens if k-th cipher block CK gets corrupted in transmission. • With ECB – Only decrypted PK is affected. • With CBC? • Only blocks PK and PK+1 are affected!! • What if one plaintext block PK is changed? • With ECB only CK affected. • With CBC all subsequent ciphertext blocks will be affected. • “Avalanche effect” • This leads to an effective integrity protection mechanism (or message authentication code (MAC)) Lecture 2.3 - Private Key Cryptography III

  37. Cipher Feedback Mode (CFB)

  38. CFB Properties • Randomized encryption – good for security (Tux won’t be visible after encryption!) • Change in one plaintext bit is going to affect all subsequent ciphertext bits. So can be used for MAC. • Change in ciphertext bit results in? Lecture 2.3 - Private Key Cryptography III

  39. Output Feedback Mode (OFB)

  40. OFB Properties • Randomized encryption – good for security (Tux won’t be visible after encryption!) • Bit errors in transmission do not propagate (except for the IV) • Not good for authentication – no avalanche effect Lecture 2.3 - Private Key Cryptography III

  41. Security of Block Cipher Modes • ECB is not even secure against eavesdroppers (ciphertext only and known plaintext attacks) • CBC, CFB and OFB are secure against CPA attacks (assuming 3-DES or AES is used in each block computation); automatically secure against eavesdropping attacks • However, none is secure against CCA. Why? • Intuitively, this is because the ciphertext can be “massaged” in a meaningful way -- see whiteboard (please take notes)

  42. Summary of CCA Attacks • Assume adversary has eavesdropped upon a ciphertext – (C0, C1, C2) -- corresponding to a plaintext (M1, M2). C0 is IV. • Adversary is not allowed to query for (C0, C1, C2) itself • With CBC, adversary queries for (C0’, C1, C2) and obtains (M1’, M2) • With CFB, he queries for (C0, C1, C2’) and obtains (M1, M2’) • With OFB, he queries for (C0, C1’,C2)/(C0,C1, C2’)/(C0, C1’,C2’) and obtains (M1’,M2)/(M1,M2’)/(M1’,M2’), respectively

  43. How to achieve CCA security? • Prevent any massaging of the ciphertext • Intuitively, this can be achieved by using integrity protection mechanisms (such as MACs), which we will study later • The ciphertext is generated using CBC/CFB/OFB and a MAC is generated on this ciphertext • Both ciphertext and the MAC is sent off • The other party decrypts only if MAC is valid Lecture 2.3 - Private Key Cryptography III

  44. Advanced Encryption Standard (AES) • National Institute of Science and Technology • DES is an aging standard that no longer addresses today’s needs for strong encryption • Triple-DES: Endorsed by NIST as today’s defacto standard • AES: The Advanced Encryption Standard • Finalized in 2001 • Goal – To define Federal Information Processing Standard (FIPS) by selecting a new powerful encryption algorithm suitable for encrypting government documents • AES candidate algorithms were required to be: • Symmetric-key, supporting 128, 192, and 256 bit keys • Royalty-Free • Unclassified (i.e. public domain) • Available for worldwide export Lecture 2.3 - Private Key Cryptography III

  45. AES • AES Round-3 Finalist Algorithms: • MARS • Candidate offering from IBM • RC6 • Developed by Ron Rivest of RSA Labs, creator of the widely used RC4 algorithm • Twofish • From Counterpane Internet Security, Inc. • Serpent • Designed by Ross Anderson, Eli Biham and Lars Knudsen • Rijndael: the winner! • Designed by Joan Daemen and Vincent Rijmen Lecture 2.3 - Private Key Cryptography III

  46. Other Symmetric Ciphers and their applications • IDEA (used in PGP) • Blowfish (password hashing in OpenBSD) • RC4 (used in WEP), RC5 • SAFER (used in Bluetooth) Lecture 2.3 - Private Key Cryptography III

  47. Some Questions • Double encryption in DES increases the key space size from 2^56 to 2^112 – true or false? • Is known-plaintext an active or a passive attack? • Is chosen-ciphertext attack an active or a passive attack? • Reverse Engineering is applied to what design of systems – open or closed? • Alice needs to send a 64-bit long top-secret letter to Bob. Which of the ciphers that we studied today should she use? Lecture 2.2 - Private Key Cryptography II

  48. Some Questions • C=DES(K,P); where (P, C are 64-bit long blocks). What would be DES(K,”PPPP”) in ECB mode? What it would be in CBC mode? • ECB is secure for sending just one block of data: true or false? • Is it okay to re-use IV in CBC? Why/why not? • Alice needs to send a *long* top-secret message to Bob. Which of the ciphers that we studied today can she use? • Is ECB secure against CPA? • Is CBC secure against CPA? • Is CBC secure against CCA? • Is OFB secure against CCA? Lecture 2.3 - Private Key Cryptography III

  49. AES: Rinjdael At home reading assignment! Lecture 2.3 - Private Key Cryptography III

  50. Rijndael • Joan Daemen (of Proton World International) and Vincent Rijmen (of Katholieke Universiteit Leuven). • (pronounced “Rhine-doll”) • Allows only 128, 192, and 256-bit key sizes (unlike the other candidates) • Variable block length of 128, 192, or 256 bits. All nine combinations of key/block length possible. • A block is the smallest data size the algorithm will encrypt • Vast speed improvement over DES in both hardware and software implementations • 8416 bytes/sec on a 20MHz 8051 (@ 12 CPI) • 8.8 Mbytes/sec on a 200MHz Pentium Pro Lecture 2.3 - Private Key Cryptography III

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