chap11-hash函数.ppt

* * 必威体育官网网址：12 认证：3456 数字签名：78 * Stallings Fig 12.1 Stallings Table 11.3 provides a comparison of the various parameters for the SHA hash functions. * * Now examine the structure of SHA-512, noting that the other versions are quite similar. SHA-512 follows the structure depicted in Stallings Figure 11.8. The processing consists of the following steps: ? Step 1: Append padding bits, consists of a single 1-bit followed by the necessary number of 0-bits, so that its length is congruent to 896 modulo 1024 ? Step 2: Append length as an (big-endian) unsigned 128-bit integer ? Step 3: Initialize hash buffer to a set of 64-bit integer constants (see text) ? Step 4: Process the message in 1024-bit (128-word) blocks, which forms the heart of the algorithm. Each round takes as input the 512-bit buffer value Hi, and updates the contents of that buffer. ? Step 5: Output the final state value as the resulting hash See text for more details. * The SHA-512 Compression Function is the heart of the algorithm. In this Step 4, it processes the message in 1024-bit (128-word) blocks, using a module that consists of 80 rounds, labeled F in Stallings Figure 11.8, and is shown in detail in Figure 11.9. Each round takes as input the 512-bit buffer value, and updates the contents of the buffer. At input to the first round, the buffer has the value of the intermediate hash value. Each round t makes use of a 64-bit value Wt derived using a message schedule from the current 1024-bit block being processed. Each round also makes use of an additive constant Kt, based on the fractional parts of the cube roots of the first eighty prime numbers. The constants provide a “randomized” set of 64-bit patterns, which should eliminate any regularities in the input data. The output of the eightieth round is added to the input to the first round to produce the final hash value for this message block, which forms the input to the next iteration of this compression function, as shown on the previous slide. * The s