The MD5 function is a cryptographic algorithm that takes an input of arbitrary length and produces a message digest that is 128 bits long. The digest is sometimes also called the "hash" or "fingerprint" of the input.The most common application is the creation and verification of digital signatures.

MD5 Working:

• The MD5 algorithm first divides the input in blocks of 512 bits each. 64 Bits are inserted at the end of the last block. These 64 bits are used to record the length of the original input. If the last block is less than 512 bits, some extra bits are 'padded' to the end.
• Next, each block is divided into 16 words of 32 bits each.These are denoted as M0 ... M15.

• MD5 uses a buffer that is made up of four words that are each 32 bits long. These words are called A, B, C and D.

• MD5 further uses a table K that has 64 elements. Element number i is indicated as K_i. The table is computed beforehand to speed up the computations. The elements are computed using the mathematical sin function:

  K_i = abs(sin(i + 1)) * 2³²

• In addition MD5 uses four auxiliary functions that each take as input three 32-bit words and produce as output one 32-bit word. They apply the logical operators and, or, not and xor to the input bits.

    F(X,Y,Z) = (X and Y) or (not(X) and Z)
    G(X,Y,Z) = (X and Z) or (Y and not(Z))
    H(X,Y,Z) = X xor Y xor Z
    I(X,Y,Z) = Y xor (X or not(Z))
  

• The contents of the four buffers (A, B, C and D) are now mixed with the words of the input, using the four auxiliary functions (F, G, H and I). There are four rounds, each involves 16 basic operations. One operation is illustrated in the figure below.

  

• The figure shows how the auxiliary function F is applied to the four buffers (A, B, C and D), using message word M_i and constant K_i. The item "<<< s" denotes a binary left shift by s bits.

• After all rounds have been performed, the buffers A, B, C and D contain the MD5 digest of the original input.