傳統的加解密法aesorigins.pptVIP

Origins clear 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 1997 15 candidates accepted in Jun 98 5 were shortlisted in Aug-99 Rijndael was selected as the AES in Oct-2000 issued as FIPS PUB 197 standard in Nov-2001 The AES Cipher - Rijndael designed by Rijmen-Daemen in Belgium has 128/192/256 bit keys, 128 bit data an iterative 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 Rijndael data block of 4 columns of 4 bytes is state key is expanded to array of words has 9/11/13 rounds in which state undergoes: byte substitution (1 S-box used on every byte) shift rows (permute bytes between groups/columns) mix columns (subs using matrix multipy of groups) add round key (XOR state with key material) view as alternating XOR key scramble data bytes initial XOR key material incomplete last round with fast XOR table lookup implementation Rijndael Byte Substitution a simple substitution of each byte uses one table of 16x16 bytes containing a permutation of all 256 8-bit values each byte of state is replaced by byte indexed by row (left 4-bits) column (right 4-bits) eg. byte {95} is replaced by byte in row 9 column 5 which has value {2A} S-box constructed using defined transformation of values in GF(28) designed to be resistant to all known attacks Byte Substitution Shift Rows a circular byte shift in each each 1st row is unchanged 2nd row does 1 byte circular shift to left 3rd row does 2 byte circular shift to left 4th row does 3 byte circular shift to left decrypt inverts using shifts to right since state is processed by columns, this step permutes bytes between the columns Shift Rows Mix Columns each column is processed separately each byte is re