Cryptography and Network Security Chapter 6

1. Cryptography and Network SecurityChapter 6 Fifth Edition by William Stallings Lecture slides by Lawrie Brown

2. Chapter 6 – Block Cipher Operation Many savages at the present day regard their names as vital parts of themselves, and therefore take great pains to conceal their real names, lest these should give to evil-disposed persons a handle by which to injure their owners. — The Golden Bough, Sir James George Frazer

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 = EK2(EK1(P)) issue of reduction to single stage and have “meet-in-the-middle” attack works whenever use a cipher twice since X = EK1(P) = DK2(C) attack by encrypting P with all keys and store then decrypt C with keys and match X value can show takes O(256) steps

8. 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 = EK1(DK2(EK1(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 several proposed impractical attacks might become basis of future attacks

9. 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 = EK3(DK2(EK1(P))) has been adopted by some Internet applications, eg PGP, S/MIME

10. 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 NIST SP 800-38A defines 5 modes have block and stream modes to cover a wide variety of applications can be used with any block cipher

11. To apply a block cipher in a variety of applications, five "modes of operation" have been defined by NIST (SP 800-38A). • In essence, a mode of operation is a technique for enhancing the effect of a cryptographic algorithm • or adapting the algorithm for an application, such as • applying a block cipher to a sequence of data blocks or a data stream.

12. The five modes are intended to cover a wide variety of applications of encryption for which a block cipher could be used. • These modes are intended for use with any symmetric block cipher, including triple DES and AES. .

13. 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 Ci = EK(Pi) uses: secure transmission of single values

14. Electronic Codebook Book (ECB)

15. 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

16. For lengthy messages, the ECB mode may not be secure. • If the message is highly structured, it may be possible for a cryptanalyst to exploit these regularities. • If the message has repetitive elements, with a period of repetition a multiple of b bits, then these elements can be identified by the analyst. .

17. This may help in the analysis or may provide an opportunity for substituting or rearranging blocks. • Hence ECB is not appropriate for any quantity of data, since repetitions can be seen, esp. with graphics, • and because the blocks can be shuffled/inserted without affecting the en/decryption of each block. • Its main use is to send one or a very few blocks, eg a session encryption key

21. 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 Ci = EK(Pi XOR Ci-1) C-1 = IV uses: bulk data encryption, authentication

23. 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

24. 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

25. Stream Modes of Operation • block modes encrypt entire block • may need to operate on smaller units • real time data • convert block cipher into stream cipher • cipher feedback (CFB) mode • output feedback (OFB) mode • counter (CTR) mode • use block cipher as some form of pseudo-random number generator

26. 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) Ci = Pi XOR EK(Ci-1) C-1 = IV uses: stream data encryption, authentication

29. 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

30. A possible problem is that if its used over a "noisy" link, then any corrupted bit will destroy values in the current and next blocks (since the current block feeds as input to create the random bits for the next). • So either must use over a reliable network transport layer (typical) or use OFB/CTR.

31. 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 Oi = EK(Oi-1) Ci = Pi XOR Oi O-1 = IV uses: stream encryption on noisy channels

34. Advantages and Limitations of OFB needs an IV which is unique for each use if ever reuse attacker can recover outputs bit errors do not propagate disadvantage :- more vulnerable to message stream modification sender & receiver must remain in sync ,or all data is lost. only use with full block feedback subsequent research has shown that only full block feedback (ie CFB-64 or CFB-128) should ever be used eg satellite TV transmissions etc

35. Counter (CTR) a “new” mode, though proposed early on similar to OFB but encrypts counter value rather than any feedback value must have a same key & differentcounter value for every plaintext block (never reused) Oi = EK(i) Ci = Pi XOR Oi uses: high-speed network encryptions,ATM(asynchronous transfer mode) n/w security and IP security

37. 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)

39. XTS-AES Mode new mode, for block oriented storage use in IEEE Std 1619-2007 concept of tweakable block cipher different requirements to transmitted data uses AES twice for each block Tj = EK2(i) XOR αj Cj = EK1(Pj XOR Tj) XOR Tj where i is tweak & j is sector no each sector may have multiple blocks

40. Storage Encryption Requirements • The requirements for encrypting stored data, also referred to as “data at rest” differsomewhat from those for transmitted data. • The P1619 standard was designed tohave the following characteristics: • 1. The ciphertext is freely available for an attacker. Among the circumstances • that lead to this situation:

41. a. A group of users has authorized access to a database. Some of the records in the database are encrypted so that only specific users can successfullyread/write them. • Other users can retrieve an encrypted record but areunable to read it without the key. • b. An unauthorized user manages to gain access to encrypted records.

42. c. A data disk or laptop is stolen, giving the adversary access to the encrypteddata. 2. The data layout is not changed on the storage medium and in transit. The encrypted data must be the same size as the plaintext data. 3. Data are accessed in fixed sized blocks, independently from each other.That is, anauthorized user may access one or more blocks in any order.

43. 4. Encryption is performed in 16-byte blocks, independently from other blocks (except the last two plaintext blocks of a sector, if its size is not a multiple of 16 bytes). • only exception occurs when the last block has less than 128 bits. In that case, the last two blocks are encrypted/decrypted using a ciphertext- stealing technique instead of padding • 5. There are no other metadata used, except the location of the data blocks withinthe whole data set.

44. 6. The same plaintext is encrypted to different ciphertexts at different locations, butalways to the same ciphertext when written to the same location again. • 7. A standard conformant device can be constructed for decryption of data encrypted by another standard conformant device

46. Key :- The 256 or 512 bit XTS-AES key; this is parsed as a concatenation of twofields of equal size called and , such that Key = Key1 ||Key2 • Pj:- The jth block of plaintext. All blocks except possibly the final blockhave a length of 128 bits.A plaintext data unit, typically a disk sector,consists of a sequence of plaintext blocks P1, P2, ..Pm.

47. Cj:- The th block of ciphertext. All blocks except possibly the final blockhave a length of 128 bits. • j :- The sequential number of the 128-bit block inside the data unit.

48. i :-The value of the 128-bit tweak. Each data unit (sector) is assigned atweak value that is a nonnegative integer.The tweak values areassigned consecutively, starting from an arbitrary nonnegative integer. • α:- A primitive element of GF(2128) that corresponds to polynomial x • (i.e., 0000 ... 0102). • 2128x

49. αj:- a multiplied by itself j times, in GF(2128). • Bitwise XOR. • Modular multiplication of two polynomials with binary coefficientsmodulo x128 + x7 + x2 + x + 1.Thus, this is multiplication in GF(2128).

50. Encryption of block j is function of: • 128 bit keys K1and K2 • “Tweak” value i • Each sector assigned different tweak value consecutively (like counter in CTR mode) • Multiplier αj • α = 000…00010 (that is, x in GF(2128 )) • αj= αmultiplied by itself j times mod x128+x7+x2+x+1 • Different for each block j in sector i