tailieunhanh - Internet Security Cryptographic Principles, Algorithms and Protocols - Chapter 4

Hash Function, Message Digest and Message Authentication Code Khi công nghệ chữ ký kỹ thuật số trở nên rộng rãi hơn hiểu và sử dụng, nhiều quốc gia trên toàn thế giới đang cạnh tranh phát triển các tiêu chuẩn chữ ký riêng của họ để sử dụng và các ứng dụng của họ. Một số ứng dụng điện tử sử dụng chữ ký số trong thương mại điện tử (thương mại điện tử) bao gồm e-mail và các giao dịch tài chính. E-mail có thể cần được ký kỹ thuật số, nơi thông tin nhạy cảm đang được truyền. | 4 Hash Function Message Digest and Message Authentication Code As digital signature technology becomes more widely understood and utilised many countries world-wide are competitively developing their own signature standards for their use and applications. Some electronic applications utilising digital signatures in electronic commerce ecommerce include e-mail and financial transactions. E-mail may need to be digitally signed where sensitive information is being transmitted and security services such as sender authentication message integrity and non-repudiation are desired. Financial transactions in which money is being transferred directly or in exchange for services and goods could also benefit from the use of digital signatures. Signing the message digest rather than the message often improves the efficiency of the process because the message digest is usually much smaller than the message. In e-commerce it is often necessary for communication parties to verify each other s identity. One practical way to do this is with the use of cryptographic authentication protocols employing a one-way hash function. Division into fixed-bit blocks can be accomplished by mapping the variable-length message on to the suitable-bit value by padding with all zeros including one bit flag and the original message length in hex. Appropriate padding is needed to force the message to divide conveniently into certain fixed lengths. Several algorithms are introduced in order to compute message digests by employing several hash functions. The hash functions dealt with in this chapter are DMDC 1994 MD5 1992 and SHA-1 1995 . DMDC Algorithm DES-like Message Digest Computation DMDC uses a DES variant as a one-way hash function. In 1994 this scheme was introduced to compute the 18-bit authentication data Internet Security. Edited by . Rhee 2003 John Wiley Sons Ltd ISBN 0-470-85285-2 124 INTERNET SECURITY with CDMA cellular mobile communications system. DMDC divides messages into blocks