Information Security Lab. Dept. of Computer Engineering 122/151 PART I Symmetric Ciphers CHAPTER 5 Advanced Encryption Standard 5.1 Evaluation Criteria For AES 5.2 The AES Cipher

Information Security Lab. Dept. of Computer Engineering 123/151 KEY POINTS  AES is a block cipher intended to replace DES for commercial applications. It uses a 128-bit block size and a key size of 128, 192, or 256 bits.  AES does not use a Feistel structure. Instead, each full round consists of four separate functions: byte substitution, permutation, arithmetic operations over a finite field, and XOR with a key.

Information Security Lab. Dept. of Computer Engineering 124/ Evaluation Criteria for AES The Origins of AES  A replacement for DES was needed have theoretical attacks that can break it have demonstrated exhaustive key search attacks  Can use Triple-DES – but slow, has small blocks  US NIST issued call for ciphers in Sep. 12, 1997; (block length:128bits, key length: 128, 192, 256bits, royalty-free basis, stronger & faster than Triple-DES)  Submission were due on June 15, 1998, Of 21submitted cryptosystems, 15 met all the necessary criteria. AES candidates  “First AES Candidate Conference” on Aug. 20, 98. “Second AES Candidate Conference” on Mar., 99.

Information Security Lab. Dept. of Computer Engineering 125/151 The Origins of AES  Aug. 99, five of the candidates were chosen by NIST as finalist: MARS, RC6, Rijndael, Serpent, Twofish  Apr., 2000, Third AES candidates Conference Oct. 2, 2000, Rijndael was selected to be the AES Belgian researcher, Joan Daemen, Vincent Rijmen  Feb. 28, 2001, (NIST) AES was available for public review and comment. Nov. 26, 2001, Rijndael was adopted as a standard (AES). Dec. 4, 2001, published as FIPS 197.  Rijndael was selected because its combination of security, performance, efficiency, implementability and flexibility 5.1 Evaluation Criteria for AES

Information Security Lab. Dept. of Computer Engineering 126/151 AES Evaluation  initial criteria: security – effort for practical cryptanalysis cost – in terms of computational efficiency algorithm & implementation characteristics  final criteria (Oct. 2, 2000) general security ease of software & hardware implementation implementation attacks flexibility (in en/decrypt, keying, other factors) 5.1 Evaluation Criteria for AES

Information Security Lab. Dept. of Computer Engineering 127/ The AES Cipher  Designed by Rijmen-Daemen in Belgium  Block & Key size : 128/192/256 bit keys Table 5.3 AES Parameters  an iterative rather than feistel cipher processes data as block of 4 columns of 4 bytes operates on entire data block in every round  Designed to be: resistant against known attacks speed and code compactness on many CPUs design simplicity

Information Security Lab. Dept. of Computer Engineering 128/ The AES Cipher  Variable block length : 128, 192, 256 bits;  State : Intermediate cipher result Nb : the # of 4-bytes(word; 32 bits) of a block Nb = 4, 6, 8 if the block length is 128, 192, 256 bits One dimensional array of a byte within a block; x 0, x 1, x 2, …, x 15,…, x 23,…, x 31 Rectangular(2D) array with four rows Index of One dimensional array: n 0  n  15(Nb = 4); 0  n  23(Nb = 6); 0  n  31(Nb = 8 ) Index of Rectangular(2D): (i, j) i = n mod 4, j =  n / 4 , n = i + 4 * j 0  j  3(Nb = 4); 0  j  5(Nb = 6); 0  n  7(Nb = 8)

Information Security Lab. Dept. of Computer Engineering 129/ The AES Cipher  State : Intermediate cipher result  Variable key length : 128, 192, 256 bits Nk : the # of 4-bytes(word; 32 bits) of a key Nk = 4, 6, 8 if the key length is 128, 192, 256 bits x 0 x 4 x 8 x 12 x 1 x 5 x 9 x 13 x 2 x 6 x 10 x 14 x 3 x 7 x 11 x 15 s 0,0 s 0,1 s 0,2 s 0,3 s 1,0 s 1,1 s 1,2 s 1,3 s 2,0 s 2,1 s 2,2 s 2,3 s 3,0 s 3,1 s 3,2 s 3,3 State (Nb=4) Nb=6 Nb=8 Plaintext block (Nb=4) 

Information Security Lab. Dept. of Computer Engineering 130/ The AES Cipher  Variable key length : 128, 192, 256 bits Nk : the # of 4-bytes(word; 32 bits) of a key Nk = 4, 6, 8 if the key length is 128, 192, 256 bits k 0 k 4 k 8 k 12 k 1 k 5 k 9 k 13 k 2 k 6 k 10 k 14 k 3 k 7 k 11 k 15 w0 w0 w1w1 w2 w2 w3w3  w 42 w 43 Key and expanded key Nr = 10 Key size (Nk = 4) 

Information Security Lab. Dept. of Computer Engineering 131/ The AES Cipher  The AES is an iterated cipher; the # of rounds; Nr  Nr depends on the block length and key length. NrNb = 4Nb = 6Nb = 8 Nk = Nk = Nk = 814 Nr =The number of rounds

Information Security Lab. Dept. of Computer Engineering 132/151 Fig. 5.1 AES Encryption/ Decryption Nb = 4 Nk = 4 Nr = 10

Information Security Lab. Dept. of Computer Engineering 133/ The AES Cipher Substitute Bytes Transformation (SubBytes)  Forward and Inverse substitute byte transformation S 1,1 = {95} 9 5 S 1,1 = {2A}  S-box constructed using defined transformation of values in GF(2 8 )  designed to be resistant to all known attacks

Information Security Lab. Dept. of Computer Engineering 134/ The AES Cipher Substitute Bytes Transformation (SubBytes) Table 5.4 AES S-box

Information Security Lab. Dept. of Computer Engineering 135/ The AES Cipher Substitute Bytes Transformation (SubBytes)  The S-box is constructed in the following fashion: The value of the byte at row x, column y is { xy }. Map each byte { xy } in the S-Box to its multiplicative inverse in the finite field GF(2 8 ) =F[x]/(x 8 +x 4 +x 3 +x+1) Let { xy }  1 = b = (b 7 b 6 b 5 b 4 b 3 b 2 b 1 b 0 ). Apply the following transformation to each bit of b: b i = b i  b (i+4) mod 8  b (i+5) mod 8  b (i+6) mod 8  b (i+7) mod 8  c i where c i such that (c 7 c 6 c 5 c 4 c 3 c 2 c 1 c 0 )=( )={63} 16 b  Affine transformation  b

Information Security Lab. Dept. of Computer Engineering 136/ The AES Cipher Substitute Bytes Transformation (SubBytes)  The affine transformation of the S-box in field GF(2 8 ).

Information Security Lab. Dept. of Computer Engineering 137/ The AES Cipher Substitute Bytes Transformation (SubBytes)  Example : {xy}={95}= ( ): A(x) = x 7 + x 4 + x 2 +1 A(x)  1 = x 7 + x 3 + x  b= ( )={8A} b = ( ) = {2A}

Information Security Lab. Dept. of Computer Engineering 138/ The AES Cipher Inverse SubBytes Transformation Table 5.4 AES Inverse S-box

Information Security Lab. Dept. of Computer Engineering 139/ The AES Cipher Inverse SubBytes Transformation  The inverse affine transformation: b = {2A}  {8A}  {8A}  1 = {95} : inverse in the field GF(2 8 )

Information Security Lab. Dept. of Computer Engineering 140/ The AES Cipher Forward ShiftRows Transformation  A circular byte shift in each row 87F24D97 EC6E4C90 4AC346E7 8CD895A6 87F24D97 6E4C90EC 46E74AC3 A68CD895 no left shift 1 left shift 2 left shifts 3 left shifts  NbRow 1Row 2Row 3Row  Shift offsets for different block lengths

Information Security Lab. Dept. of Computer Engineering 141/ The AES Cipher Forward ShiftRows Transformation Inverse ShiftRows Transformation  Decrypt inverts using shifts to right

Information Security Lab. Dept. of Computer Engineering 142/ The AES Cipher Forward MixColumn Transformation  Each column is processed separately.  Each byte is replaced by a value dependent on all 4 bytes in the column

Information Security Lab. Dept. of Computer Engineering 143/ The AES Cipher Forward MixColumn Transformation  effectively a matrix multiplication in GF(2 8 ) using prime poly. m(x) = x 8 + x 4 + x 3 + x +1

Information Security Lab. Dept. of Computer Engineering 144/ The AES Cipher Forward MixColumn Transformation  Example : 4740A34C 37D4709F 94E43A42 EDA5A6BC 87F24D97 6E4C90EC 46E74AC3 A68CD895  Inverse MixColumn Transformation  decryption requires use of inverse matrix

Information Security Lab. Dept. of Computer Engineering 145/ The AES Cipher Forward AddRoundKey Transformation  XOR state with 128-bits of the round key Inverse AddRoundKey Transformation  inverse for decryption identical; since XOR own inverse, with reversed keys

Information Security Lab. Dept. of Computer Engineering 146/ The AES Cipher AES Key Expansion  Takes 128-bit (16-byte) key and expands into array of Nk = 44/52/60 32-bit words The function g : 1: RotWord : one-byte circular left shift w i = [b 0 b 1 b 2 b 3 ]  [b 1 b 2 b 3 b 0 ] 2: SubWord : SubBytes transformaton 3: The result of 1 & 2  Rcon[j] Rcon[j] = 1, Rcon[j] = 2  Rcons[j  1] over GF(2 8 ) Rcon[2] = 02 Rcon[3] = 04 Rcon[4] = 08 Rcon[5] = 10 Rcon[6] = 20 Rcon[7] = 40 Rcon[8] = 80 Rcon[9] = 1B Rcon[10] = 36

Information Security Lab. Dept. of Computer Engineering 147/ The AES Cipher AES Key Expansion Rationale  designed to resist known attacks  design criteria included knowing part key insufficient to find many more invertible transformation fast on wide range of CPU ’ s use round constants to break symmetry diffuse key bits into round keys enough non-linearity to hinder analysis simplicity of description

Information Security Lab. Dept. of Computer Engineering 148/ The AES Cipher Equivalent Inverse Cipher  AES decryption is not identical to encryption since steps done in reverse; but can define an equivalent inverse cipher with steps as for encryption using inverses of each step with a different key schedule  Interchangeing InvShiftRows and InvSubBytes InvShiftRows[InvSubBytes(S i )] = InvSubBytes[InvShiftRows (S i )]  Interchanging AddRoundKey and InvMixColumns InvMixColumns(S i  w j ) = InvMixColumns(S i )  InvMixColumns(w j )

Information Security Lab. Dept. of Computer Engineering 149/ The AES Cipher

Information Security Lab. Dept. of Computer Engineering 150/ The AES Cipher Implementation Aspects   can efficiently implement on 8-bit CPU byte substitution; shift rows is byte shift; add round key works on byte XOR’s mix columns requires matrix multiply in GF(2 8 )   can efficiently implement on 32-bit CPU redefine steps to use 32-bit words can precompute 4 tables of 256-words then each column in each round can be computed using 4 table lookups + 4 XORs at a cost of 4Kb to store tables   Very efficient implementation was a key factor in its selection as the AES cipher

Information Security Lab. Dept. of Computer Engineering 151/151 Summary  have considered: the AES selection process the details of Rijndael – the AES cipher looked at the steps in each round the key expansion implementation aspects