Chức năng Secure Hash Hàm băm an toàn cần thiết cho chữ ký số. Một hàm băm có thể được gọi là an toàn nếu: ML chức năng là một chiều. Nói cách khác, chức năng tạo ra một tổng kiểm tra từ thông tin này nhưng bạn không thể tạo ra các thông tin từ tổng kiểm tra các. Nó là rất khó khăn để xây dựng hai mẩu thông tin cung cấp các kiểm tra tương tự khi chạy qua các hàm. | 226 Network Security A Beginner s Guide Secure Hash Functions Secure hash functions are necessary for digital signatures. A hash function can be called secure if The function is one-way. In other words the function creates a checksum from the information but you cannot create the information from the checksum. It is very difficult to construct two pieces of information that provide the same checksum when run through the function. The second condition is not easy to satisfy. The checksums in question should also be smaller than the information so as to make it easier to sign store and transmit. If this is the case it must also be true that some large number of different pieces of information will map to the same checksum. What makes the functions secure is the way that all the bits in the original information map to all the bits in the checksum. Thus if a single bit in the information is changed a large number of bits in the checksum will also change. Secure hash functions should create a checksum of at least 128 bits. The two most common secure hash functions are MD5 which produces a 128-bit checksum and SHA which produces a 160-bit checksum. There are many other hash functions but most of them have been proven insecure. MD5 has been identified as having weaknesses that may allow a computational attack. This attack may allow a second piece of information to be created that will result in the same checksum. SHA was developed by the United States government and is currently believed to be secure. Most security software offers both MD5 and SHA as available options. KEY MANAGEMENT The management of keys is the bane of all encryption systems. The keys are the most valuable information. If I can get a key I can get decrypt everything that is encrypted by that key. In some cases I may also be able to get succeeding keys. The management of keys is not just about protecting them while in use. It is also about creating strong keys securely distributing keys to remote users .
