Cryptography and Network Security Block Cipher

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1 Cryptography and Network Security Block Cipher Xiang-Yang Li

2 Modern Private Key Ciphers Stream ciphers The most famous: Vernam cipher Invented by Vernam, ( AT&T, in 1917) Process the message bit by bit (as a stream) (Also known as the one-time pad) Simply add bits of message to random key bits

3 Cont. Key Key Plaintext Ciphertext Ciphertext Plaintext

4 Pros and Cons Drawbacks Need as many key bits as message, difficult in practice (ie distribute on a mag-tape or CDROM) Strength Is unconditionally secure provided key is truly random

5 Key Generation Why not to generate keystream from a smaller (base) key? Use some pseudo-random function to do this Although this looks very attractive, it proves to be very very difficult in practice to find a good pseudo-random function that is cryptographically strong This is still an area of much research

6 Block Ciphers The message is broken into blocks, Each of which is then encrypted (Like a substitution on very big characters bits or more)

7 Substitution and Permutation In his 1949 paper Shannon also introduced the idea of substitution-permutation (S-P) networks, which now form the basis of modern block ciphers An S-P network is the modern form of a substitution-transposition product cipher S-P networks are based on the two primitive cryptographic operations we have seen before

8 Substitution A binary word is replaced by some other binary word The whole substitution function forms the key If use n bit words, The key space is 2 n! Can also think of this as a large lookup table, with n address lines (hence 2 n addresses), each n bits wide being the output value Will call them s-boxes

9 Cont.

10 Permutation A binary word has its bits reordered (permuted) The re-ordering forms the key If use n bit words, The key space is n! (Less secure than substitution) This is equivalent to a wire-crossing in practice (Though is much harder to do in software) Will call these p-boxes

11 Cont.

12 Substitution-permutation Network Shannon combined these two primitives He called these mixing transformations A special form of product ciphers where S-boxes Provide confusion of input bits P-boxes Provide diffusion across s-box inputs

13 Confusion and Diffusion Confusion A technique that seeks to make the relationship between the statistics of the ciphertext and the value of the encryption keys as complex as possible. Cipher uses key and plaintext. Diffusion A technique that seeks to obscure the statistical structure of the plaintext by spreading out the influence of each individual plaintext digit over many ciphertext digits.

14 Desired Effect Avalanche effect A characteristic of an encryption algorithm in which a small change in the plaintext gives rise to a large change in the ciphertext Best: changing one input bit results in changes of approx half the output bits Completeness effect where each output bit is a complex function of all the input bits

15 Practical Substitutionpermutation Networks In practice we need to be able to decrypt messages, as well as to encrypt them, hence either: Have to define inverses for each of our S & P- boxes, but this doubles the code/hardware needed, or Define a structure that is easy to reverse, so can use basically the same code or hardware for both encryption and decryption

16 Feistel Cipher Invented by Horst Feistel, working at IBM Thomas J Watson research labs in early 70's, The idea is to partition the input block into two halves, l(i-1) and r(i-1), use only r(i-1) in each round i (part) of the cipher The function g incorporates one stage of the S-P network, controlled by part of the key k(i) known as the ith subkey

17 Cont.

18 Cont. This can be described functionally as: L(i) = R(i-1) R(i) = L(i-1) g(k(i), R(i-1)) This can easily be reversed as seen in the above diagram, working backwards through the rounds In practice link a number of these stages together (typically 16 rounds) to form the full cipher

19 Data Encryption Standard Adopted in 1977 by the National Bureau of Standards, now the National Institute of Standards and Technology Data are encrypted in 64-bit blocks using a 56-bit key The same algorithm is used for decryption. Subject to much controversy

20 History IBM LUCIFER 60 s Uses 128 bits key Proposal for NBS, 1973 Adopted by NBS, 1977 Uses only 56 bits key Possible brute force attack Design of S-boxes was classified Hidden weak points in in S-Boxes? Wiener (93) claim to be able to build a machine at \$100,00 and break DES in 1.5 days

21 DES DES encrypts 64-bit blocks of data, using a 56-bit key the basic process consists of: an initial permutation (IP) 16 rounds of a complex key dependent calculation f a final permutation, being the inverse of IP Function f can be described as L(i) = R(i-1) R(i) = L(i-1) P(S( E(R(i-1)) K(i) ))

22 DES

23 Initial and Final Permutations Inverse Permutations

24 Function f

25 Expansion Table Expands the 32 bit data to 48 bits Result(i)=input( array(i))

26 S-Boxes S-Box is a fixed 4 by 16 array Given 6-bits B=b 1 b 2 b 3 b 4 b 5 b 6, Row r=b 1 b 6 Column c=b 2 b 3 b 4 b 5 S(B)=S(r,c) written in binary of length 4

27 Example S-Box S

28 Permutation Table The permutation after each round

29 Subkey Generation Given a 64 bits key (with parity-check bit) Discard the parity-check bits Permute the remaining bits using fixed table P1 Let C 0 D 0 be the result (total 56 bits) Let C i =Shift i (C i-1 ); D i =Shift i (D i-1 ) and K i be another permutation P2 of C i D i (total 56 bits) Where cyclic shift one position left if i=1,2,9,16 Else cyclic shift two positions left

30 Permutation Tables Permutation table P1 Permutation table P2

31 DES in Practice DEC (Digital Equipment Corp. 1992) built a chip with 50k transistors Encrypt at the rate of 1G/second Clock rate 250 Mhz Cost about \$300 Applications ATM transactions (encrypting PIN and so on)

32 Model Mode of use The way we use a block cipher Four have been defined for the DES by ANSI in the standard: ANSI X modes of use) Block modes Splits messages in blocks (ECB, CBC) Stream modes On bit stream messages (CFB, OFB)

33 Block Modes Electronic Codebook Book (ECB) where the message is broken into independent 64-bit blocks which are encrypted C i = DES K1 (P i ) Cipher Block Chaining (CBC) again the message is broken into 64-bit blocks, but they are linked together in the encryption operation with an IV C i = DES K1 (P i C i-1 ) C -1 =IV (initial value)

34 Stream Model Cipher FeedBack (CFB) where the message is treated as a stream of bits, added to the output of the DES, with the result being feed back for the next stage C i = P i DES K1 (C i-1 ) C -1 =IV (initial value)

35 Cont. Output FeedBack (OFB) where the message is treated as a stream of bits, added to the message, but with the feedback being independent of the message C i = P i O i O i = DES K1 (O i-1 ) O -1 =IV (initial value)

36 DES Weak Keys With many block ciphers there are some keys that should be avoided, because of reduced cipher complexity These keys are such that the same sub-key is generated in more than one round, and they include:

37 Cont. Weak keys The same sub-key is generated for every round DES has 4 weak keys Semi-weak keys Only two sub-keys are generated on alternate rounds DES has 12 of these (in 6 pairs) Demi-semi weak keys Have four sub-keys generated

38 Cont. None of these causes a problem since they are a tiny fraction of all available keys However they MUST be avoided by any key generation program

39 Possible Techniques for Improving DES Multiple enciphering with DES Extending DES to 128-bit data paths and 112-bit keys Extending the key expansion calculation

40 Double DES? Using two encryption stages and two keys C=E k2 (E k1 (P)) P=D k1 (D k2 (C)) It is proved that there is no key k3 such that C=E k2 (E k1 (P))=E k3 (P) But Meet-in-the-middle attack

41 Meet-in-the-Middle Attack Assume C=E k2 (E k1 (P)) Given the plaintext P and ciphertext C Encrypt P using all possible keys k 1 Decrypt C using all possible keys k 2 Check the result with the encrypted plaintext lists If found match, they test the found keys again for another plaintext and ciphertext pair If it turns correct, then find the keys Otherwise keep decrypting C

42 Triple DES DES variant Standardized in ANSI X9.17 & ISO 8732 and in PEM for key management Proposed for general EFT standard by ANSI X9 Backwards compatible with many DES schemes Uses 2 or 3 keys

43 Cont. No known practical attacks Brute force search impossible (very hard) Meet-in-the-middle attacks need 2 56 Plaintext-Ciphertext pairs per key Popular current alternative

44 IDEA: Developed by James Massey & Xuejia Lai at ETH originally in Zurich in 1990, then called IPES: X Lai, J L Massey, "A Proposal for a New Block Encryption Standard" in Advances in Cryptology - Eurocrypt '90, Lecture Notes in Computer Science, vol 473, pp , X Lai, J L Massey, S Murphy, "Markov Ciphers and Differential Cryptanalysis" in Advances in Cryptology - Eurocrypt '91, Lecture Notes in Computer Science, vol 547, pp 17-38, name changed to IDEA in 1992

45 Basic Features Encrypts 64-bit blocks using a 128-bit key Based on mixing operations from different (incompatible) algebraic groups XOR, + mod 2^(16), X mod 2^(16) +1) On 16-bit sub-blocks, with no permutations used IDEA is patented in Europe & US, however noncommercial use is freely permitted used in the public domain PGP (with agreement) currently no attack against IDEA is known Seem secure against differential cryptanalysis, brute force

46 Operations Operations XOR, Addition mod 2 16, multiplication mod Why these special mod for addition, multiplication They do not satisfy the distributive law They do not satisfy the associative law

47 MA: multiplication/addition Multiplication/addition Basic block to provide diffusion Input of MA Two sub-blocks derived from 4 input sub-blocks, 4 sub-keys Two other sub-keys Output Two sub-blocks Needs four operations Four operations are the minimum to provide full diffusion

48 Overview

49 Cont. IDEA encryption works as follows: Use 8-rounds The 64-bit data is divided into: X 1, X 2, X 3, X 4 Each round The sub-blocks are added (2,3), multiplied (1,4) with sub-keys The results are XORed [1,3] and [2,4] to 2 sub-blocks The XOR results set as input of MA structure, o o It outputs two subblocks Results are then XORed with 2,4 and 1,3 subblocks respectively The second and third sub-blocks are swapped Finally new sub-keys are combined with the sub-blocks

50 Sub-Keys Total need 52=6*8+4 sub-keys First are directly from key in order Left shift of 25 bits, and then next 8 sub-keys Each sub-key is a sub-block of the original key Decryption Much more complicated It needs the inverse of the encryption key For addition, multiplication

51 Decryption The process of decryption is essentially the same as encryption But with different selection of sub-keys Basic Operations K1.1^(-1 ) is the multiplicative inverse mod 2^(16) +1 -K1.2 is the additive inverse mod 2^(16) The original operations are: o o (+) bit-by-bit XOR + additional mod 2^(16) of 16-bit integers o * multiplication mod 2^(16) +1 (where 0 means 2^(16) )

52 Decryption Sub-Keys Round Encryption Keys Decryption Keys 1 K1.1 K1.2 K1.3 K1.4 K1.5 K1.6 K K9.2 -K9.3 K9.4-1 K8.5 K8.6 2 K2.1 K2.2 K2.3 K2.4 K2.5 K2.6 K K8.3 -K8.2 K8.4-1 K7.5 K7.6 3 K3.1 K3.2 K3.3 K3.4 K3.5 K3.6 K K7.3 -K7.2 K7.4-1 K6.5 K6.6 4 K4.1 K4.2 K4.3 K4.4 K4.5 K4.6 K K6.3 -K6.2 K6.4-1 K5.5 K5.6 5 K5.1 K5.2 K5.3 K5.4 K5.5 K5.6 K K5.3 -K5.2 K5.4-1 K4.5 K4.6 6 K6.1 K6.2 K6.3 K6.4 K6.5 K6.6 K K4.3 -K4.2 K4.4-1 K3.5 K3.6 7 K7.1 K7.2 K7.3 K7.4 K7.5 K7.6 K K3.3 -K3.2 K3.4-1 K2.5 K2.6 8 K8.1 K8.2 K8.3 K8.4 K8.5 K8.6 K K2.3 -K2.2 K2.4-1 K1.5 K1.6 Output K9.1 K9.2 K9.3 K9.4 K K1.2 -K1.3 K1.4-1

53 Important Feature The size of the sub-block Need be prime number To compute the inverse for each possible subkey So sub-block size 8 is also possible =257 is prime number

54 CAST-128 By Carlisle Adams, Stafford Tavares Defined in RFC 2144 Use key size varying from 40 to 128 bits Structure of Feistel network 16 rounds on 64-bits data block Four primitive operations Addition, substration (mod 2 32 ) Bitwise exclusive-or Left-circular rotation

55 Skipjack and Clipper Skipjack used in Clipper escrowed encryption scheme(us govt) Skipjack is a block cipher, 64-bit data hardware only implementation 80-bit key (escrowed in 2 halves) 32 round all design details and descriptions are classified has been very considerable debate over its use attack by Matt Blaze (ATT) on the LEAF component of the Clipper protocol for secure phone communications

56 Blowfish Scheme Developed by Bruce Schneier Fast, compact, simple and variably secure Two basic operations: addition, XOR Key ranges from 32 bits to 448 bits Similar to Feistel scheme The sub-key and s-boxes are complicated So not suitable when key changes often Function g is very simple, unlike DES

57 RC5 Developed by R. Rivest Suitable for hardware or software Fast, simple, low memory, data-dependent rotations Adaptable to processors of different word length A family of algorithms determined by word length, number of rounds, size of secret key Decryption and encryption are not the same With little variations Primitive operations Addition, XOR, left circular rotation

58 Characteristics Key features of advanced sym block cipher Variable key length Mixed operators Data dependent rotation Key dependent rotation Key dependent S-boxes Lengthy key schedule algorithm Variable function F Variable of number of rounds Operation on both halved data each round

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