1. Politiche delle Reti e Sicurezza 2008 UNICAM. M.L.Maggiulli ©2004-2008 1 Maria Laura Maggiulli marialaura.maggiulli@unicam.it Dipartimento di Informatica Facoltà di Scienze e Tecnologie Università di Camerino (AN) AA. 2007-2008 Politiche delle Reti e Sicurezza Approfondimenti sulla Crittograf ia simmetrica

2. Chapter 6 – Contemporary Symmetric Ciphers "I am fairly familiar with all the forms of secret writings, and am myself the author of a trifling monograph upon the subject, in which I analyze one hundred and sixty separate ciphers," said Holmes. —The Adventure of the Dancing Men, Sir Arthur Conan Doyle

3. Multiple Encryption & DES clear a replacement for DES was needed theoretical attacks that can break it demonstrated exhaustive key search attacks AES is a new cipher alternative prior to this alternative was to use multiple encryption with DES implementations Triple-DES is the chosen form

4. Double-DES? could use 2 DES encrypts on each block C = E K2 (E K1 (P)) issue of reduction to single stage and have “meet-in-the-middle” attack works whenever use a cipher twice since X = E K1 (P) = D K2 (C) attack by encrypting P with all keys and store then decrypt C with keys and match X value can show takes O(2 56 ) steps

5. Triple-DES with Two-Keys hence must use 3 encryptions would seem to need 3 distinct keys but can use 2 keys with E-D-E sequence C = E K1 (D K2 (E K1 (P))) nb encrypt & decrypt equivalent in security if K1=K2 then can work with single DES standardized in ANSI X9.17 & ISO8732 no current known practical attacks

6. Triple-DES with Three-Keys although are no practical attacks on two-key Triple-DES have some indications can use Triple-DES with Three-Keys to avoid even these C = E K3 (D K2 (E K1 (P))) has been adopted by some Internet applications, eg PGP, S/MIME

7. Modes of Operation block ciphers encrypt fixed size blocks eg. DES encrypts 64-bit blocks with 56-bit key need some way to en/decrypt arbitrary amounts of data in practise ANSI X3.106-1983 Modes of Use (now FIPS 81) defines 4 possible modes subsequently 5 defined for AES & DES have block and stream modes

8. Electronic Codebook Book (ECB) message is broken into independent blocks which are encrypted each block is a value which is substituted, like a codebook, hence name each block is encoded independently of the other blocks C i = DES K1 (P i ) uses: secure transmission of single values

9. Electronic Codebook Book (ECB)

10. Advantages and Limitations of ECB message repetitions may show in ciphertext if aligned with message block particularly with data such graphics or with messages that change very little, which become a code-book analysis problem weakness is due to the encrypted message blocks being independent main use is sending a few blocks of data

11. Cipher Block Chaining (CBC) message is broken into blocks linked together in encryption operation each previous cipher blocks is chained with current plaintext block, hence name use Initial Vector (IV) to start process C i = DES K1 (P i XOR C i-1 ) C -1 = IV uses: bulk data encryption, authentication

12. Cipher Block Chaining (CBC)

13. Message Padding at end of message must handle a possible last short block which is not as large as blocksize of cipher pad either with known non-data value (eg nulls) or pad last block along with count of pad size eg. [ b1 b2 b3 0 0 0 0 5] means have 3 data bytes, then 5 bytes pad+count this may require an extra entire block over those in message there are other, more esoteric modes, which avoid the need for an extra block

14. Advantages and Limitations of CBC a ciphertext block depends on all blocks before it any change to a block affects all following ciphertext blocks need Initialization Vector (IV) which must be known to sender & receiver if sent in clear, attacker can change bits of first block, and change IV to compensate hence IV must either be a fixed value (as in EFTPOS) or must be sent encrypted in ECB mode before rest of message

15. Cipher FeedBack (CFB) message is treated as a stream of bits added to the output of the block cipher result is feed back for next stage (hence name) standard allows any number of bit (1,8, 64 or 128 etc) to be feed back denoted CFB-1, CFB-8, CFB-64, CFB-128 etc most efficient to use all bits in block (64 or 128) C i = P i XOR DES K1 (C i-1 ) C -1 = IV uses: stream data encryption, authentication

16. Cipher FeedBack (CFB)

17. Advantages and Limitations of CFB appropriate when data arrives in bits/bytes most common stream mode limitation is need to stall while do block encryption after every n-bits note that the block cipher is used in encryption mode at both ends errors propogate for several blocks after the error

18. Output FeedBack (OFB) message is treated as a stream of bits output of cipher is added to message output is then feed back (hence name) feedback is independent of message can be computed in advance C i = P i XOR O i O i = DES K1 (O i-1 ) O -1 = IV uses: stream encryption on noisy channels

19. Output FeedBack (OFB)

20. Advantages and Limitations of OFB bit errors do not propagate more vulnerable to message stream modification a variation of a Vernam cipher hence must never reuse the same sequence (key+IV) sender & receiver must remain in sync originally specified with m-bit feedback subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used

21. Counter (CTR) a “new” mode, though proposed early on similar to OFB but encrypts counter value rather than any feedback value must have a different key & counter value for every plaintext block (never reused) C i = P i XOR O i O i = DES K1 (i) uses: high-speed network encryptions

22. Counter (CTR)

23. Advantages and Limitations of CTR efficiency can do parallel encryptions in h/w or s/w can preprocess in advance of need good for bursty high speed links random access to encrypted data blocks provable security (good as other modes) but must ensure never reuse key/counter values, otherwise could break (cf OFB)

24. Stream Ciphers process message bit by bit (as a stream) have a pseudo random keystream combined (XOR) with plaintext bit by bit randomness of stream key completely destroys statistically properties in message C i = M i XOR StreamKey i but must never reuse stream key otherwise can recover messages (cf book cipher)

25. Stream Cipher Structure

26. Stream Cipher Properties some design considerations are: long period with no repetitions statistically random depends on large enough key large linear complexity properly designed, can be as secure as a block cipher with same size key but usually simpler & faster

27. RC4 a proprietary cipher owned by RSA DSI another Ron Rivest design, simple but effective variable key size, byte-oriented stream cipher widely used (web SSL/TLS, wireless WEP) key forms random permutation of all 8-bit values uses that permutation to scramble input info processed a byte at a time

28. RC4 Key Schedule starts with an array S of numbers: 0..255 use key to well and truly shuffle S forms internal state of the cipher for i = 0 to 255 do S[i] = i T[i] = K[i mod keylen]) j = 0 for i = 0 to 255 do j = (j + S[i] + T[i]) (mod 256) swap (S[i], S[j])

29. RC4 Encryption encryption continues shuffling array values sum of shuffled pair selects "stream key" value from permutation XOR S[t] with next byte of message to en/decrypt i = j = 0 for each message byte M i i = (i + 1) (mod 256) j = (j + S[i]) (mod 256) swap(S[i], S[j]) t = (S[i] + S[j]) (mod 256) C i = M i XOR S[t]

30. RC4 Overview

31. Chapter 7 – Confidentiality Using Symmetric Encryption John wrote the letters of the alphabet under the letters in its first lines and tried it against the message. Immediately he knew that once more he had broken the code. It was extraordinary the feeling of triumph he had. He felt on top of the world. For not only had he done it, had he broken the July code, but he now had the key to every future coded message, since instructions as to the source of the next one must of necessity appear in the current one at the end of each month. —Talking to Strange Men, Ruth Rendell

32. Confidentiality using Symmetric Encryption traditionally symmetric encryption is used to provide message confidentiality

33. Placement of Encryption have two major placement alternatives link encryption encryption occurs independently on every link implies must decrypt traffic between links requires many devices, but paired keys end-to-end encryption encryption occurs between original source and final destination need devices at each end with shared keys

34. Placement of Encryption

35. when using end-to-end encryption must leave headers in clear so network can correctly route information hence although contents protected, traffic pattern flows are not ideally want both at once end-to-end protects data contents over entire path and provides authentication link protects traffic flows from monitoring

36. Placement of Encryption can place encryption function at various layers in OSI Reference Model link encryption occurs at layers 1 or 2 end-to-end can occur at layers 3, 4, 6, 7 as move higher less information is encrypted but it is more secure though more complex with more entities and keys

37. Encryption vs Protocol Level

38. Traffic Analysis is monitoring of communications flows between parties useful both in military & commercial spheres can also be used to create a covert channel link encryption obscures header details but overall traffic volumes in networks and at end-points is still visible traffic padding can further obscure flows but at cost of continuous traffic

39. Key Distribution symmetric schemes require both parties to share a common secret key issue is how to securely distribute this key often secure system failure due to a break in the key distribution scheme

40. Key Distribution given parties A and B have various key distribution alternatives: 1. A can select key and physically deliver to B 2. third party can select & deliver key to A & B 3. if A & B have communicated previously can use previous key to encrypt a new key 4. if A & B have secure communications with a third party C, C can relay key between A & B

41. Key Hierarchy typically have a hierarchy of keys session key temporary key used for encryption of data between users for one logical session then discarded master key used to encrypt session keys shared by user & key distribution center

42. Key Distribution Scenario

43. Key Distribution Issues hierarchies of KDC’s required for large networks, but must trust each other session key lifetimes should be limited for greater security use of automatic key distribution on behalf of users, but must trust system use of decentralized key distribution controlling key usage

44. Random Numbers many uses of random numbers in cryptography nonces in authentication protocols to prevent replay session keys public key generation keystream for a one-time pad in all cases its critical that these values be statistically random, uniform distribution, independent unpredictability of future values from previous values

45. Pseudorandom Number Generators (PRNGs) often use deterministic algorithmic techniques to create “random numbers” although are not truly random can pass many tests of “randomness” known as “pseudorandom numbers” created by “Pseudorandom Number Generators (PRNGs)”

46. Linear Congruential Generator common iterative technique using: X n+1 = (aX n + c) mod m given suitable values of parameters can produce a long random-like sequence suitable criteria to have are: function generates a full-period generated sequence should appear random efficient implementation with 32-bit arithmetic note that an attacker can reconstruct sequence given a small number of values have possibilities for making this harder

47. Using Block Ciphers as PRNGs for cryptographic applications, can use a block cipher to generate random numbers often for creating session keys from master key Counter Mode X i = E Km [i] Output Feedback Mode X i = E Km [X i-1 ]

48. ANSI X9.17 PRG

49. Blum Blum Shub Generator based on public key algorithms use least significant bit from iterative equation: x i = x i-1 2 mod n where n=p.q, and primes p,q=3 mod 4 unpredictable, passes next-bit test security rests on difficulty of factoring N is unpredictable given any run of bits slow, since very large numbers must be used too slow for cipher use, good for key generation

50. Natural Random Noise best source is natural randomness in real world find a regular but random event and monitor do generally need special h/w to do this eg. radiation counters, radio noise, audio noise, thermal noise in diodes, leaky capacitors, mercury discharge tubes etc starting to see such h/w in new CPU's problems of bias or uneven distribution in signal have to compensate for this when sample and use best to only use a few noisiest bits from each sample

51. Published Sources a few published collections of random numbers Rand Co, in 1955, published 1 million numbers generated using an electronic roulette wheel has been used in some cipher designs cf Khafre earlier Tippett in 1927 published a collection issues are that: these are limited too well-known for most uses

52. Summary have considered: use and placement of symmetric encryption to protect confidentiality need for good key distribution use of trusted third party KDC’s random number generation issues