Provides Confidentiality DES Algorithm Provides Confidentiality

Conventional Encryption Algorithms Data Encryption Standard (DES) The most widely used encryption scheme The algorithm is reffered to the Data Encryption Algorithm (DEA) DES is a block cipher The plaintext is processed in 64-bit blocks The key is 56-bits in length

Data Encryption Standard (DES) The algorithm has 16 rounds. Each round has the following architecture: Li and Ri are each 32-bit long strings

DES Concerns about: The overall processing at each iteration: Li = Ri-1 Ri = Li-1 F(Ri-1, Ki) Concerns about: The algorithm and the key length (56-bits)

X=IP(M) Upper Lower 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

DES Before any rounds, the plaintext bits are permuted using an initial permutation. Hence, at the end of the 16 rounds the inverse permutation is applied. The initial permutation is public knowledge

DES The key (J) length is 48 bits. The 32-bit text argument (A) needs to be expanded to 48 bits.

DES: Expansion Function Added The 32-bits of R_i are permuted and 16 of them are repeated twice to obtain a 48 bit string.

DES Round Structure column Stallings Fig 3.9

DES: S Blocks. S blocks takes in as input 6-bit arguments and outputs four bits. This is the substitution part of the cipher. Each S block has a different functionality as defined by the corresponding tables.

DES After substitution, the function output is now 32 bits and it goes through a fixed permutation. Thus we perform “confusion” and “diffusion” steps in each round.

DES: Key generation for each round The parity bits are stripped away. The bits are permuted by PC-1 LS_i represents cyclic shift by one position if i=1, 2, 9, or 16; otherwise shift by 2

Key Generation (Discard each 8th bit) Discard these 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

DES PC-2 converts 56 bits into 48 bits Permuted Choice Two (PC-21) Permuted Choice One (PC-1)

DES The overall effect is to pick at each round 48 of the 56 bits and permute the selected bits. Thus for each round one can perform a table look up to calculate K_i.

DES Decryption decrypt must unwind steps of data computation with Feistel design, do encryption steps again using subkeys in reverse order (SK16 … SK1) note that IP undoes final FP step of encryption 1st round with SK16 undoes 16th encrypt round …. 16th round with SK1 undoes 1st encrypt round then final FP undoes initial encryption IP thus recovering original data value

Time to break a code (106 decryptions/µs)

Triple DEA Use three keys and three executions of the DES algorithm (encrypt-decrypt-encrypt) C = ciphertext P = Plaintext EK[X] = encryption of X using key K DK[Y] = decryption of Y using key K Effective key length of 168 bits C = EK3[DK2[EK1[P]]]

Triple DEA

Other Symmetric Block Ciphers International Data Encryption Algorithm (IDEA) 128-bit key Used in PGP Blowfish Easy to implement High execution speed Run in less than 5K of memory

Other Symmetric Block Ciphers RC5 Suitable for hardware and software Fast, simple Adaptable to processors of different word lengths Variable number of rounds Variable-length key Low memory requirement High security Data-dependent rotations Cast-128 Key size from 40 to 128 bits The round function differs from round to round

Cipher Block Modes of Operation Cipher Block Chaining Mode (CBC) The input to the encryption algorithm is the XOR of the current plaintext block and the preceding ciphertext block. Repeating pattern of 64-bits are not exposed