MD5 explained / How MD5 works?

An easy to understand simple explanation on how MD5 works.

Padding

The data in bit format should be 64 bits short of a 512 bit multiple.

E.g. — Assume that your data consists of 1000 bits.

The nearest multiple of 512 is 1024.

Now let’s check if 1024 satisfies the above mentioned condition.

1024–64 = 960 which is less than 1000 bits but we need at least 1000 bits to store our data.

Moving on to the next multiple which is 1536.

1536–64 = 1472

Adding padding bits to 1000 to make it 1472.

1472 + 64 = 1536 (which is a multiple of 512)

Now the condition has been satisfied.

Appending

Now express the length of the initial input string in terms of 64 bits.

This string is going to be hashed.

Initializing

Divide the above string into 512 bit blocks.

Initialize four different buffers named A, B, C and D.

Each buffer is 32 bits and initialized as follows:

A = 01 23 45 67

B = 89 ab cd ef

C = fe dc ba 98

D = 76 54 32 10

Processing

Take each 512 bit block and further divide into 16-bit blocks (each 16-bit block will contain 32 bits such that 16 x 32 = 512).

Each 16-bit block is assigned its own number ranging from M0 to M15.

An example is given below:

M0–01010100 01101000 01100101 01111001
M1–00100000 01100001 01110010 01100101
M2–00100000 01100100 01100101 01110100
M3–01100101 01110010 01101101 01101001
M4–01101110 01101001 01110011 01110100
M5–01101001 01100011 10000000 00000000
M6–00000000 00000000 00000000 00000000
M7–00000000 00000000 00000000 00000000
M8–00000000 00000000 00000000 00000000
M9–00000000 00000000 00000000 00000000
M10–00000000 00000000 00000000 00000000
M11–00000000 00000000 00000000 00000000
M12–00000000 00000000 00000000 00000000
M13–00000000 00000000 00000000 00000000
M14–00000000 00000000 00000000 00000000
M15–00000000 00000000 00000000 10110000

These numbers are also used in hexadecimal format.

Four rounds of operations are carried out on the 512-bit block.

Each round consists of 16 operations.

4 x 16 = 64 operations per 512-bit block.

An array K on constant values is created after deriving the values from a formula.

Each operation round utilizes all the sub-blocks, buffers and the constant array values.

An abstract representation of the processing

Subsequent adding operations of output to buffer input form new buffer values.

This eventually results in the final buffer value which is the hash.

For more info and to get an in-depth understanding:

The MD5 algorithm (with examples) | Comparitech

MD5 Hash Algorithm in Cryptography: Here's Everything You Should Know