Question

What is MD5 HASH?
......explain in detail.

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Answer 1

The simplest way to understand MD5 is to implement it from the specification, which is rather simple.

In very crude words:

The data to hash is a sequence of bits. Let's keep things simple, and let's assume that it is a sequence of bytes. A few extra bytes (the "padding") are appended to that sequence, so that the number of extra bytes is between 9 and 72 (inclusive) and the total length after padding is a multiple of 64. The specification explains the contents of the padding; basically a lot of zeros, and an encoding of the input data length.

The padded data is split into blocks of 64 bytes. The blocks will be processed one by one. Processing of each block (64 bytes) takes as input a 128-bit value (16 bytes) which is the output of the processing of the previous block, and outputs a new 128-bit value.

Since the first block has no previous block, a conventional fixed value is used to start the process. The MD5 specification details that value.

.The complete MD5 output is the 128-bit value you get after processing the last block.