tailieunhanh - security fundamentals for e commerce phần 2

Nó mô tả hệ thống thanh toán điện tử cung cấp một cách an toàn để trao đổi giá trị tiền tệ giữa khách hàng và các doanh nghiệp và tập trung vào các nguyên tắc của các kỹ thuật bảo đảm thanh toán chứ không phải là đưa ra một cái nhìn tổng quan hoàn chỉnh các hệ thống thanh toán. | 22 Security Fundamentals for E-Commerce solves this problem by introducing feedback. Each plaintext block Pi is chained to the encryption result Cj-1 of the previous plaintext block Pi- 1. Encryption Ci EK Pi Ci_1 Decryption Pi Ci_1 DK C. The first plaintext block is chained to an initialization vector IV known to both the sender and the receiver . C1 EK P1 IV . Sometimes it is necessary to encrypt data units smaller than the block size for example if there is no time to wait for enough data to fill a block. In such cases CFB is used which also adds feedback and requires an IV. With OFB most of the encryption process can occur off-line before the plaintext message even exists. With both CFB and OFB a block cipher is actually used as a stream cipher. Unlike block ciphers stream ciphers convert plaintext to ciphertext one bit or byte at a time. If it is necessary to encrypt data units smaller than the block size block ciphers can also be applied in counter mode. In counter mode sequence numbers or pseudorandom sequences are used as the input to the encryption algorithm. DES Today The fastest DES chips today achieve an encryption speed of approximately 1 Gbps with a 56-bit key. The fastest software solutions are much slower about 10 Mbps. The latest record in cracking DES as of September 1999 set by the Electronic Frontier Foundation s Deep Crack is 22 hours and 15 minutes 10 . It involved about 100 000 PCs on the Internet. It was performed as a known ciphertext attack based on a challenge from the RSA The task was to find a 56-bit DES key for a given plaintext and a given ciphertext. Other Symmetric Encryption Algorithms IDEA International Data Encryption Algorithm proposed in 1992 was the European answer to DES and to the United States export restrictions on cryptographic algorithms. IDEA is a block cipher that encrypts a 64-bit plaintext block with a 128-bit key. It applies the same basic cryptographic techniques as DES confusion and .