## About MD2 hash

MD2 (Message Digest Algorithm 2) is a cryptographic hash function designed by Ronald Rivest in 1989. It is part of the MD (Message Digest) family of hash functions, which also includes MD4, MD5, and others. Here are some key characteristics and aspects of MD2:

### Characteristics of MD2

**Fixed-Length Output**:- MD2 produces a fixed-length output of 128 bits (16 bytes), regardless of the size of the input data.

**Input Padding**:- MD2 requires the input message to be padded so that its length is a multiple of 16 bytes. Padding is done by appending bytes to the message, each containing the value of the number of padding bytes added.

**Checksum Calculation**:- MD2 computes a 16-byte checksum of the original message before padding. This checksum is appended to the message and processed along with the original data.

**Block Processing**:- MD2 processes the message in 16-byte blocks. It maintains a 48-byte state array, which is updated for each block processed.

**Substitution and Permutation**:- The algorithm uses a substitution table (S-box) and performs various permutation operations to mix the input data thoroughly.

### Algorithm Steps

**Initialization**:- Initialize a 48-byte state array (X) to zeros.

**Padding**:- Pad the message so that its length is a multiple of 16 bytes.

**Checksum Calculation**:- Calculate a 16-byte checksum of the original message before padding.

**Processing Blocks**:- For each 16-byte block of the padded message:
- Copy the block into the first 16 bytes of X.
- Perform a series of 18 iterations, involving substitutions and permutations, to update the state array.

- For each 16-byte block of the padded message:
**Finalization**:- After processing all blocks, the first 16 bytes of the state array (X) form the MD2 hash output.

### Security and Usage

**Security**:- MD2 was designed to be secure at the time of its creation. However, with advancements in cryptanalysis and computational power, MD2 is no longer considered secure for modern applications. It is vulnerable to collision attacks, where two different inputs produce the same hash output.

**Usage**:- Despite its vulnerabilities, MD2 might still be encountered in legacy systems or for compatibility purposes. However, for new applications, stronger and more secure hash functions like SHA-256 (part of the SHA-2 family) or SHA-3 are recommended.

### Summary

MD2 is a historical cryptographic hash function known for its simplicity and fixed-length output. While it played a significant role in early cryptographic practices, its security weaknesses make it unsuitable for current use in secure applications.