Modern Symmetric Block cipher

Transcription

1 Modern Symmetric Block cipher 81

2 Shannon's Guide to Good Ciphers Amount of secrecy should determine amount of labour appropriate for encryption and decryption The set of keys and enciphering algorithm should be free from complexity The implementation should be as simple as possible Errors in ciphering should not propagate Size of the enciphered text should be no larger than the original 82

3 Confusion and Diffusion Confusion Interceptor should not be able to predict how cipher text will change by changing one character Diffusion Cipher should spread information from plaintext over cipher text See avalanche effect 83

4 Avalanche Effect Key desirable property of an encryption algorithm Where a change of one input or key bit results in changing approx. half of the output bits If the change were small, this might provide a way to reduce the size of the key space to be searched 84

5 Modern Symmetric Block cipher In a block cipher the message is broken into blocks each of which is then encrypted Like a substitution on very big characters - 64-bits or more modern ciphers (developed from product ciphers) include DES, Blowfish, IDEA, LOKI, RC5, Rijndael (AES) and others 85

6 Modern Symmetric cipher (Stream Vs. Block) 86

7 Modern Symmetric Block cipher in 1949 Shannon introduced the idea of substitutionpermutation (S-P) networks which form the basis of modern block ciphers S-P network is the modern form of a substitutiontransposition product cipher 87

8 The Substitution Operation S-boxes The Permutation Operation P-boxes 88

9 The Mixing transformations 89

10 Feistel Ciphers the idea is to partition the input block into two halves, L(i- 1) and R(i-1), and use only R(i-1) in the ith round (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 can be described functionally as:l(i) = R(i-1) R(i) = L(i-1) XOR g(k(i), R(i-1)) 90

11 DES Data Encryption Standard 91

12 DES DES is a 64 bit block cipher with a 56 bit key. It selects a 64 bit block and modifies it depending on the key. the basic process in enciphering: an initial permutation (IP) 16 rounds of a complex key dependent calculation f a final permutation, the inverse of IP Encrypt Single round Decrypt 92

13 DES DES is a 64 bit block cipher with a 56 bit key. It selects a 64 bit block and modifies it depending on the key. the basic process in enciphering: an initial permutation (IP) 16 Rounds of a complex key dependent calculation f The final permutation, the inverse of IP 93

14 Permutation (IP/ FP) Input Only bit 25 and bit 64 are 1s; the other bits are 0s. In the final permutation, bit 25 becomes bit 64 and bit 63 becomes bit 15. Bit 15 in the input becomes bit 63 in the output. Bit 64 in the input becomes bit 25 in the output. So the output has only two 1s, bit 25 and bit 63. The result in hexadecimal 94

15 DES Single round of 16 Rounds 95

16 Using mixers and swappers, we can create the cipher and reverse cipher, each having 16 rounds. The last round (round 16) different from the others; it has only a mixer and no swapper. 96

17 The function f consists of: Expand R to 48 bits. Apply xor with 48 bits from the key, which are selected depending on the current iteration number. Divide the 48 bit result into eight different parts of 6 bits each, and through the use of the eight S-boxes, project each part into a four bits part Permute the resulting 32 bits. 6bit s 4bit s 97

18 Column= 0101 in binary = 5 decimal For example, S 1 (101010) = 6 = Row= 10 in binary = 2 decimal 98

19 Parity-bit drop table (the 8 th bits from every byte) 64 bits = 8 bytes Removed 8 bits from 64 bits then the rest 56 bits Number of bits shifts Key generation Key-compression table 99

20 DES Single round 100

21 Modes of operation 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) 101

22 Block Modes Electronic Codebook Book (ECB) where the message is broken into independent 64-bit blocks which are encrypted Ci = DESK1 (Pi) 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 Ci = DESK1 (Pi Ci-1) C-1=IV (initial value) 102

23 Stream Mode 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 Ci = Pi DESK1 (Ci-1) C-1=IV (initial value) 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. Ci = Pi Oi Oi = DESK1 (Oi-1) O-1=IV (initial value) 103

24 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: 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 104