MD5 is a common cryptographically hash function that translates any input into 128 bits. It is important to notice that any arbitrary data (be is a 4 bytes string, an image or a 10GB movie) will be hashed into 128 bits.

MD5 is deterministic, this is, always output same result given same input

MD5 algorithm will hash inputs into 128 bits. Usually the most common way choosen to represent those 128 bits is using hexadecimal notation, that is, grouping the bits in groups of 4 bits, which ends up being a string of 32 characters.

As an example, MD5("world") = 7d793037a0760186574b0282f2f435e7

It has historically been a cryptographic hash function widely used when storing passwords. Instead of saving the password itself in a database,software applications were saving the MD5 hash of the password since it is non reversible. That is, in case of data leaking, the atttacker would not be able to actually know which were the passwords of the users (only the hashes of the passwordss).

Over the last decade, it has been proved to be easily exploited (that is, regular hardware can generate another input with the same hash in less than 1 second and it is relatively easy to find preimages of a hash), that's why its use as cryptographic function is discouraged.

Nevertheless it can still be used as a checker against data tampering and to verify data integrity. As an example, when downloading a file from a website, we are offered the MD5 hash of the file. After downloading the file we can compute the MD5 hash, verify it matches the same MD5 hash and assure the integrity of the data during the download.

History of MD5