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Cryptography -- Block Ciphers Anita Jones CS451 Information Security Copyright(C) Anita Jones Overview terms and principles Claude Shannon Feistel cipher DES September, 2006 A few terms block cipher block of plaintext is treated as a whole & used to produce a ciphertext block of equal length typical size: 64 bits most modern ciphers are block ciphers stream cipher digital data is encrypted one bit (or one unit) at a time In both cases, plaintext is transformed incrementally September, 2006 Symmetric ciphers Symmetric implies ONE key Secret key shared by sender & receiver Background ideally want one extremely large substitution not practical since would need a table with 264 entries in it for a 64-bit block so approximate the ideal by constructing from smaller building blocks September, 2006 Basis of modern ciphers Claude Shannon („45) - information theory product cipher perform two or more ciphers in sequence so that result (product) is cryptographically stronger than any component cipher alternate confusion & diffusion virtually all significant symmetric block ciphers currently in use are of this type September, 2006 Shannon‟s strategy thwart cryptanalysis that is based on statistical analysis hacker has some knowledge of statistical characteristic of plaintext if statistics are reflected in ciphertext, then analyst may be able to deduce encryption key, or part of it in Shannon‟s ideal cipher, statistics of ciphertext are independent of plaintext September, 2006 Shannon‟s building blocks confusion make relation between statistics of ciphertext and the value of the encryption key as complex as possible diffusion diffuse statistical property of plaintext digit across a range of ciphertext digits i.e. each plaintext digits affects value of many ciphertext digits September, 2006 Shannon‟s building blocks Shannon proposed product ciphers with two components S-Boxes -- substitution providing confusion of input bits P-Boxes -- permutation providing diffusion across S-box inputs n rounds of S-P boxes September, 2006 S-box (substitution) 3 bit 3 bit input output 0 0 0 1 1 1 2 2 3 3 1 1 4 4 5 5 0 6 6 0 7 7 Word size of 3 bits => mapping of 23 = 8 values Note: mapping can be reversed September, 2006 P-box (permutation) 4 bit input 1 1 1 1 1 0 1 0 1 1 1 1 0 1 0 1 Example 1 Example 2 - swap two Note: reversible halves of input September, 2006 S-P networks alternate S and P boxes BUT, in practice we must decrypt as well as encrypt so define the sequence of boxes so that precisely the same system will decrypt as well as encrypt just run it backwards September, 2006 Feistel cipher input plaintext of 2w bits key K = n sub-keys: K1, K2, …, Kn sequence of n “rounds” each using Ki substitution followed by a permutation apply function F(Ki) to right half of data, then exclusive-OR it to left half of data permutation: interchange two result halves of data DES is essentially a Feistel cipher September, 2006 Feistel cipher Multiple rounds round i input is Li-1, Ri-1 Li = Ri-1 Ri = (Li-1 XOR F(Ri-1 , Ki)) L – left portion of intermediate data R – right ….. September, 2006 plaintext (2w bits) L0 w bits w bits R0 K1 + Round 1 F L1 R1 . . . . . . Kn + Round n F Ln Rn Ln+1 Rn+1 ciphertext (2w bits) Feistel cipher dependencies block size – increasing size increases security – 64 bits common key size – increasing size improves security, – 128 bits common number of rounds – 16 is typical subkey generation – complex generation makes cryptanalysis harder round function – complex function is stronger … but all increases slow the implementation September, 2006 Feistel decryption same as encryption, except ciphertext is input use keys in reverse order at each round the output is equal to the corresponding value of the encryption process with the two halves of the value swapped final permutation (swap) realigns 2 halves September, 2006 History of DES DES – Data Encryption Standard Horst Feistel at IBM developed LUCIFER about 1971, sold to Lloyds of London Nat‟l Bureau of Standards issued request for national cipher standard IBM submitted (refined) LUCIFER NSA worked with IBM to refine cipher adopted in 1977 by Nat‟l Bureau of Stds. September, 2006 DES Characteristics Plaintext is 64 bits long 16 rounds Key length is 56 bits 16 sub-keys generated, one used in each round DES algorithm is a variant of the Feistel algorithm September, 2006 plaintext (64 bits) 56 bit key init permutation permute K1 round 1 perm left circ shift K2 round 2 perm left circ shift . . . . . . Kn round n perm left circ shift 32 bit swap inverse permutation ciphertext (64 bits) DES cipher round i input is Li-1, Ri-1 Li = Ri-1 Ri = (Li-1 XOR F(Ri-1 ,Ki)) September, 2006 One DES Round <----32 bits------> <----32 bits------> Li-1 Ri-1 exp/perm to 48 --- 48 bits x Ki --- 48 bits S-box --- 32 bits permutation --- 32 bits x Li Ri Key property avalanche small change in plaintext or in key produces significant change in ciphertext test for avalanche encrypt two plaintext blocks that differ only in one bit about half the (ciphertext) bits will differ September, 2006 DES controversy DES choice was intensely criticized: original LUCIFER key length was 128 bits, and DES used 56 bit key (to fit on chip, they said) critics feared brute force attacks design criteria for the S-boxes was classified, so users not sure that internal structure was free of hidden weak points that might let NSA break cipher September, 2006 DES status no weak points have surfaced DES is widely used 1994, NIST reaffirmed DES for federal use NIST recommends DES use for all except classified information generally considered a sound standard Need more security: use Triple DES Future: Adv.d Encryption Standard (AES) September, 2006 Cryptanalysis of DES increased computing speed has made a 56 bit key susceptible to exhaustive key search demonstrated breaks: 1997 – taking a few months, a large network of computers broke DES 1998 – Electronic Frontier Foundation broke DES in a few days on dedicated hardware 1999 – break accomplished in 22 hours in practice DES is used, and works September, 2006 1997 break RSA issued reward of $10,000 for finding a DES key, given ciphertext for known and unknown plaintext solution found in 96 days – involved 70,000 computers on the Internet an embarrassingly parallel problem – just divide the key space being searched (brute force) each time a new computer joins in found the key after searching 1/4 key space September, 2006 So, how does the Prez talk? STU-III: http://webhome.idirect.com/~jproc/crypto/stuiii.html http://www.tscm.com/stu.html “A STU-III operates by taking an audio signal and digitizing it into a serial data stream (usually 8,000 bits per second). This is then mixed with a "keying stream" of data created by an internal ciphering algorithm. This mixed data is them passed though an internal CODEC to convert it back to audio so it can be passed over the phone lines. STU-III's also allow a serial data stream to pass though the phone and into the ciphering engine to allow its usage as an encrypted modem when not being "keying stream" is a polymorphic used for voice. The regenerating mathematic algorithm which takes a initialization key and mathematically morphs it into a bit stream pattern. The "keying stream" is created by the "Key Generator" and is the actual heart of the STU. A portion of the "keying stream" are then mixed back into to the original key, and process repeated. The results is a pseudo-random bit stream that if properly implemented is extremely difficult (but not impossible) to decrypt.” Source: http://www.tscm.com/stu.html September, 2006 Model for cryptography-revisit Trusted 3rd Party Principal (arbitrates, distributes Principal secret information) Message Message Info channel Secret Secret Information Information Security Security Transform Transform Opponent September, 2006

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block ciphers, block cipher, public key, block size, the block, stream cipher, stream ciphers, secret key, private key, key size, bit key, hash function, digital signatures, plaintext block, 128 bits

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posted: | 5/26/2010 |

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