MD5 is a cryptographic hash function that works on blocks of data that are 128 bits long. This means that it takes four 32-bit words as input and produces a 128-bit output. To make sure that the input message can be divided into equal blocks of 128 bits, the algorithm adds some padding bits at the end of the message until it reaches a multiple of 512 bits (16 32-bit words). The padding bits are designed to preserve the length and integrity of the original message. After padding, the algorithm processes each block of 128 bits one by one. It applies a series of mathematical operations that involve shifting, rotating, adding, and XORing the bits in different ways. These operations are called transpositions and substitutions, and they are the basic building blocks of encryption techniques. They aim to make the output as unpredictable and irreversible as possible, so that it is hard to find two messages that have the same hash value or to recover the original message from the hash value.
The algorithm uses four auxiliary functions that operate on the 32-bit words in each block. These functions are denoted by F, G, H, and I, and they have different properties. For example, F is a nonlinear function that depends on the bits of all three input words, while G is a linear function that depends on the bits of only one input word. The algorithm also uses four constants that are derived from the fractional parts of the sines of certain angles. These constants are denoted by K[0..63], and they help to introduce more randomness and asymmetry into the output.
For each block of 128 bits, the algorithm performs four rounds of 16 operations each. In each round, it uses one of the auxiliary functions and one of the constants. It also uses a different order of processing the input words in each round. The algorithm maintains four variables that store the intermediate results of the hash computation. These variables are initialized with some fixed values at the beginning of the first block and updated after each round. They are denoted by A, B, C, and D. At the end of the last block, the final hash value is obtained by concatenating these four variables in little-endian order.
The MD5 algorithm is designed to be fast and efficient on 32-bit machines. It can be implemented in software or hardware, and it can be used for various purposes such as digital signatures, message authentication codes, or checksums. However, MD5 is not considered secure anymore, as several attacks have been found that can break its collision resistance or preimage resistance properties. This means that it is possible to find two messages that have the same hash value or to find a message that has a given hash value with less effort than brute force. Therefore, MD5 should not be used for applications that require high security or integrity. 0efd9a6b88