Bài giảng "Mật mã học: Other block ciphers" cung cấp cho người học các kiến thức: What will we learn, electronic codebook mode, CBC critics, cipher feedback mode, counter mode, exhaustive key search revisited, . | Bài giảng Mật mã học Other block ciphers - Huỳnh Trọng Thưa Other Block Ciphers Huỳnh Trọng Thưa htthua@ What will we learn The most important modes of operation for block ciphers in practice Security pitfalls when using modes of operations The principles of key whitening Why double encryption is not a good idea and the meet-in-the-middle attack Triple encryption 2 Encryption with Block Ciphers Modes of Operation Electronic Code Book mode ECB Cipher Block Chaining mode CBC Cipher Feedback mode CFB Output Feedback mode OFB Counter mode CTR . 3 Electronic Codebook Mode ECB 4 ECB critics Advantages Block synchronization is not necessary. Problem identical plaintext blocks result in identical ciphertext blocks as long as the key does not change Replay attack Usage not recommended to encrypt more than one block of data encryption in database 5 Ex of Substitution attack against electronic bank transfer Oscar observes the ciphertexts going through the communication network. After a while he can recognize the five blocks of his own transfer. He now stores blocks 1 3 and 4 of these transfers. The same key is used for several other transfers between bank A and B. By comparing blocks 1 and 3 of all subsequent messages with the ones he has stored Oscar recognizes all transfers that are made from some account at bank A to some account at bank B. He now simply replaces block 4 which contains the receiving account number with the block 4 that he stored before. 6 Encryption of bitmaps in ECB mode 7 Cipher Block Chaining Mode CBC 8 CBC critics Decryption of all subsequent blocks yi i 2 9 CBC critics cont. Good Randomized encryption repeated text gets mapped to different encrypted data. A ciphertext block depends on all preceding plaintext blocks blocks reorder affects decryption Bad Errors in one block propagate to two blocks Sequential encryption cannot use parallel hardware 10 Encryption of bitmaps in CBC mode 11 Output Feedback Mode OFB Key stream is not generated
