MD4

MD4 (Message Digest Algorithm 4) is a cryptographic hash function designed by Ronald Rivest in 1990. It is part of the MD family of algorithms, which also includes MD2, MD3, MD5, and others. MD4 produces a fixed-length output, or hash value, of 128 bits (16 bytes), which is commonly represented as a 32-character hexadecimal string. Its primary purpose is to generate a unique message digest for data integrity verification, ensuring that data has not been altered.

The algorithm operates on data blocks of 512 bits by processing them through a series of non-linear functions, bitwise operations, and modular additions. It performs three rounds of transformations, each comprising 16 steps, utilizing specific logical functions designed to mix the input data thoroughly. The process begins with an initial internal state and updates this state based on the message block, ultimately producing the hash value.

MD4 was widely used in the 1990s and early 2000s for digital signatures, message integrity verification, and password hashing. However, its cryptographic security was later compromised after discoveries of vulnerabilities that allow hash collisions—two different inputs producing the same hash output. As a result, MD4 is now considered cryptographically insecure and has been largely replaced by more robust algorithms like MD5, SHA-2, and SHA-3.

Despite its deprecation for security purposes, MD4 remains of historical interest and has influenced the design of subsequent hash functions. Its simplicity and speed in software implementations made it notable during its time, though modern security standards recommend avoiding its use in sensitive applications.