第5单元 高级加密标准.pptVIP

第5章 高级加密标准 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 June 1998 5 were shortlisted in Aug-99 Rijndael was selected as the AES in Oct-2000 issued as FIPS PUB 197 standard in Nov-2001 AES Requirements private key symmetric block cipher 128-bit data, 128/192/256-bit keys stronger faster than Triple-DES active life of 20-30 years (+ archival use) provide full specification design details both C Java implementations NIST have released all submissions unclassified analyses AES Evaluation Criteria initial criteria security – effort for practical cryptanalysis cost – in terms of computational efficiency algorithm implementation characteristics final criteria general security ease of software hardware implementation implementation attacks flexibility (in en/decrypt, keying, other factors) AES Shortlist after testing and evaluation, shortlist in Aug-99: MARS (IBM) - complex, fast, high security margin RC6 (USA) - v. simple, v. fast, low security margin Rijndael (Belgium) - clean, fast, good security margin Serpent (Euro) - slow, clean, v. high security margin Twofish (USA) - complex, v. fast, high security margin then subject to further analysis comment saw contrast between algorithms with few complex rounds verses many simple rounds refined existing ciphers verses new proposals The AES Cipher - Rijndael designed by Rijmen-Daemen in Belgium, has 128/192/256 bit keys, 128 bit data 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 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