MD5 encryption algorithm: What is it and why is it needed?

Every file or line of text can be converted into a unique “fingerprint” that helps protect data and verify its integrity. For a long time, the MD5 algorithm was the standard for these purposes. In this article, we’ll explore what this algorithm is, how it works, and how to decrypt it.

What is MD5?

MD5 (Message Digest 5) is a hashing algorithm developed in the early 1990s. Its primary purpose is to transform an input message into a unique 128-bit string. The hash is this string.

MD5 evolved the ideas of its predecessors: MD2, MD3, and MD4, which addressed security and performance issues at the time. They were used to protect data in digital environments. MD5’s creators addressed MD4’s shortcomings, increasing its resistance to attacks and making data integrity checking more reliable.

It’s important to understand that a hash is not encryption. The resulting data cannot be converted back to the original data. However, the same input will always generate the same hash.

What is MD5 used for?

1. Creating checksums. A checksum is a short code that helps verify that data has not been altered or corrupted. It is created from a file before it is transmitted or written, and then verified by the recipient. If everything is in order, the checksum should match.
2. Password hashing. MD5 was previously used to store passwords as hashes. This helped protect the data, as the database stored not the password itself, but its “fingerprint.” However, due to vulnerabilities, MD5 is now being replaced by more secure algorithms such as SHA-256 or Argon2.
3. Building associative arrays. MD5 is used for quickly searching data by key. This speeds up programs, as the hash serves as a unique identifier for each record.
4. Data comparison and duplicate detection. Instead of analyzing the data itself, its hashes are compared: this helps find matches faster, especially in large data sets.

How MD5 works 

Data Alignment:
To process data using MD5, its length must be a multiple of 512 bits (64 bytes). First, a “1” bit is appended to the end of the message. Then, zeros are added to bring the length within the required processing size.

Specifying the message length:
After alignment, the original length is appended to the data as a 64-bit number. This allows the algorithm to take into account the original amount of information, even if the data has been padded.

The MD5 MD buffer setup
uses four variables—A, B, C, and D—each 32 bits in size. These variables are given fixed initial values ​​and serve as the basis for further calculations.

Cyclic data processing
. Once the data is prepared, processing begins. The algorithm breaks the message into 512-bit (64-byte) blocks, which are processed in a loop. Each block goes through several stages:

• Variables A, B, C, and D are updated using mathematical operations (bitwise calculations, shifts, permutations, and additions);
• Pre-defined constants are used, which were obtained from the fractional parts of the snes of integers (for example, to make the results more random).

Result:
Once everything is calculated, the values ​​of variables A, B, C, and D are combined into a single 128-bit string. This string is the final hash—a unique “fingerprint” of the original data.

Let’s look at an example.

Now let’s see what this all looks like in practice. Let’s choose the simplest possible source string—for example, “1234.” Its length is 32 bits. To make the length a multiple of 512 bits, we’ll append a “1” bit to the original binary message:

00110001 00110010 00110011 00110100 1

Add zeros until the length is 448 bits (you need to subtract 64 bits, as they are reserved to indicate the length of the original message). 

Now we need to set the starting values ​​for the variables. Let’s say they look like this:

A = 0x67452301;

B = 0xEFCDAB89;

C = 0x98BADCFE;

D = 0x10325476.

The message is then divided into blocks (we have only one). 64 steps are performed using bitwise operations, shifts, and constants. The final hash will look like this:

81dc9bdb52d04dc20036dbd8313ed055.

How to decrypt MD5 

Due to its numerous vulnerabilities, scientists consider MD5 an obsolete algorithm that is unsafe for use in cryptography. In fact, it is impossible to “decrypt” MD5: remember, it is not encryption, but a hashing algorithm. However, there are several methods that can be used to determine the original value that corresponds to the MD5 hash.

The easiest way is to use services for working with MD5:

• crackStation is one of the most popular resources for cracking and checking hashes;
• md5.web-max.ca is a service with a simple and user-friendly interface;
• md5decrypter.com is a similar tool that searches for matches in a database;
• 365education.org is an English-language resource that allows you to both decrypt MD5 hashes and generate them.

These services work with databases of known passwords and strings. However, their effectiveness is limited: they only find simple and common values. For more complex cases, more sophisticated methods exist.

For example, the algorithm can be cracked using brute force, since its hash length is only 128 bits. This means the computer can simply try all possible input data until it finds one that produces the desired hash. Another method is to use a pre-prepared list of popular passwords or data, a so-called dictionary. 

The short hash length also makes it possible to decrypt MD5 using RainbowCrack. This tool uses pre-prepared lists of hashes and their source data—so-called rainbow tables. The table contains pre-written hashes for multiple strings—passwords, combinations of letters, numbers, and symbols. Rainbow tables are also used by the crackStation service.

Brute-force attacks and RainbowCrack work well if the hash was not created using a “salt”—an additional random value added to the data before hashing to make the brute-force process more difficult.

For example, if the password “qwerty123” is hashed with the salt “Xy7!”, the resulting hash will be completely different than without the salt. Even if another user also uses the password “qwerty123,” their salt will be different, meaning the hash will be unique. Salt is especially important in environments where many hashes are stored, such as user databases. Without it, data can leak.

Additionally, it’s worth remembering the risk of collisions within MD5 (again, due to the short hash). The problem is that two different sets of data can have the same hash, and the system won’t be able to distinguish the original from the counterfeit—this situation is called a collision. Let’s say the original file contained an important contract. Using a collision, an attacker can generate a contract with completely different text, and the substitution may not be immediately noticeable, since both files have the same hash. In 2004, a group of Chinese cryptographers discovered a way to easily create such coincidences. 

The MD5 algorithm: A quick overview

To summarize: MD5 is a way to transform any data into a unique 128-bit “fingerprint.” For a long time, the algorithm was used to verify file integrity and protect passwords. However, time has shown that MD5 is no longer as reliable: it can be cracked, and its hashes can be forged. This algorithm is unsuitable for tasks that require high security, but in simpler cases where security is not crucial, it is quite adequate.