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Unformatted text preview: sible key until plaintext is recovered rapidly becomes infeasible as key size increases 56-bit key is not secure brute force Block Ciphers Block most common symmetric algorithms process plain text in fixed block sizes process producing block of cipher text of equal size producing most important current block ciphers: Data Encryption Standard (DES) Data (DES) Advanced Encryption Standard Data Encryption Standard Data US standard 64 bit plain text blocks 56 bit key broken in 1998 by Electronic Frontier broken Foundation Foundation special purpose US$250,000 machine with detailed published description less than three days DES now worthless Triple DEA Triple ANSI X9.17 (1985) incorporated in DEA standard 1999 uses 2 or 3 keys uses 3 executions of DEA algorithm effective key length effective slow block size (64 bit) now too small 112 or 168 bit 168 Advanced Encryption Standard Standard NIST issued call for proposals for an Advanced NIST Encryption Standard (AES) in 1997 Encryption security strength equal to or better than 3DES significantly improved efficiency symmetric block cipher with block length 128 bits block key lengths 128, 192, and 256 bits evaluation include security, computational efficiency, evaluation memory requirements, hardware and software suitability, and flexibility suitability, AES issued as FIPS 197 in 2001 issued AES Description AES assume key length 128 bits input a 128-bit block (square matrix of bytes) copied into state array, modified at each stage after final stage, state copied to output expanded into array of 44 32-bit key schedule words 1st 4 bytes of 128-bit input occupy 1st column 128-bit 1st 4 bytes of expanded key occupy 1st column occupy 128-bit key (square matrix of bytes) byte ordering by column AES Encryption and Decryption Decryption AES Encryption Round AES Location of Encryption Devices Devices Link Encryption Link each communication link equipped at both ends all tr...
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- Spring '09