What is MD4 and why was it created?

MD4 (Message Digest Algorithm 4) is a cryptographic hash function designed by Ronald Rivest in 1990 as part of the MD (Message Digest) series. It produces a 128-bit (16-byte) hash value, typically displayed as 32 hexadecimal characters. MD4 was designed for speed and efficiency on 32-bit processors, processing data in 512-bit blocks through three rounds of operations. At the time of creation, MD4 represented a significant advancement in hash function design and influenced many later algorithms. However, serious cryptographic weaknesses were discovered within just a few years of its release. In 1996, Hans Dobbertin published the first collision attack against MD4, demonstrating fundamental flaws in its design. Modern computers can generate MD4 collisions in less than a second, making it completely unsuitable for any security application. MD4 was quickly superseded by MD5 (also now broken) and later by the SHA family. Today, MD4 exists only for legacy compatibility with old systems like NTLM authentication in Windows and the eDonkey2000 file sharing protocol. DO NOT use MD4 for passwords, digital signatures, or any security-critical application. Use SHA-256, SHA-3, or BLAKE3 instead.

Why is MD4 considered broken and insecure?

MD4 is cryptographically broken with multiple severe vulnerabilities that make it completely unsuitable for security applications. The most critical flaw is collision weaknessresearchers can generate two different messages with the same MD4 hash in less than one second on modern hardware, compared to the theoretical 2^64 operations that should be required. This allows attackers to create forged documents, fake digital signatures, and bypass integrity checks with trivial effort. MD4 also suffers from preimage vulnerabilities where attackers can find messages matching a target hash faster than brute force. The algorithm uses only three rounds of operations (MD5 uses four, SHA-256 uses 64), providing insufficient diffusion and confusion. The simple structure makes cryptanalysis easier, and many mathematical weaknesses have been discovered in MD4's compression function. First serious attacks were published in 1996, just six years after MD4's creation, and attacks have only gotten faster and easier since. Today, MD4 offers zero security guarantees. Collisions that should take billions of years to find by chance can be generated instantly. Using MD4 for passwords is especially dangerous as rainbow tables and collision attacks make cracking trivial. The only legitimate use for MD4 is legacy system compatibility where security is not required.

Should I use MD4 for password hashing?

Absolutely not! Never use MD4 for password hashing under any circumstances. MD4 is cryptographically broken and provides zero security for passwords. Attackers can generate rainbow tables, use collision attacks, and exploit the algorithm's weaknesses to crack MD4 password hashes in seconds or minutes, even for reasonably complex passwords. MD4 is also extremely fast, which is a critical flaw for password hashingattackers can try billions of password guesses per second on modern hardware. MD4 has no built-in salting mechanism, making rainbow table attacks trivial. Even if you add your own salt, the broken collision resistance means attackers can still forge password hashes. For password hashing, use modern password-specific algorithms like Argon2id (recommended), bcrypt, or scrypt. These algorithms are designed specifically for password security with built-in salting, configurable work factors to slow down attacks, and resistance to GPU/ASIC cracking. If you must use a general hash function, use SHA-256 or SHA-512 with proper salting and key stretching (PBKDF2), though dedicated password algorithms are much better. Some legacy systems like old Windows NTLM authentication used MD4 for passwords, which is why NTLM is considered insecure and has been replaced by Kerberos. If you encounter MD4 password hashes in old systems, migrate to secure password hashing immediately.

Where is MD4 still used today?

MD4 is used only in legacy systems that predate modern cryptographic standards. The most common use is Windows NTLM authentication, which hashes passwords with MD4 as part of the NTLM protocol. Microsoft has deprecated NTLM in favor of Kerberos, but NTLM remains for backward compatibility in some Windows environments. The eDonkey2000 and eMule peer-to-peer file sharing networks use MD4 to generate file hashes for identifying and verifying shared files. Some very old software distribution systems and legacy applications may use MD4 checksums, though these should be migrated to SHA-256. Certain legacy database systems and old backup tools may still generate MD4 hashes for deduplication or integrity checking. Security researchers study MD4 to understand cryptographic attacks and as a teaching tool for hash function vulnerabilities. Some compatibility libraries include MD4 for reading old data formats or communicating with legacy systems. However, all new applications and systems should avoid MD4 entirely. If you encounter MD4 in a modern system, it is a serious security vulnerability that should be fixed by migrating to SHA-256, SHA-3, or BLAKE3. The only legitimate reason to use MD4 today is maintaining compatibility with old systems that cannot be upgraded, and even then, MD4 should never be relied upon for security.

What are MD4 collision attacks and how do they work?

A collision attack finds two different inputs that produce the same MD4 hash output. Cryptographically secure hash functions should make collisions computationally infeasible, requiring approximately 2^64 operations for a 128-bit hash like MD4. However, MD4's weak design allows collisions to be found in less than one second on modern computers using differential cryptanalysis techniques. Attackers exploit mathematical weaknesses in MD4's compression function to construct message pairs that collide. The attack works by carefully choosing message differences that cancel out through MD4's three rounds of operations. Because MD4 has only three rounds (compared to MD5's four or SHA-256's 64), there is insufficient mixing to prevent these differential attacks. Practical collision attacks on MD4 were first demonstrated in 1996 and have been continuously improved. Modern attacks can generate collisions almost instantly with negligible computational cost. This breaks MD4's fundamental security properties. Collision attacks enable serious security threats including forging digital signatures by creating malicious documents with the same hash as legitimate ones, creating fake software updates that pass MD4 checksum verification, bypassing authentication systems that rely on MD4 hash comparison, and generating fake certificates or credentials. This is why MD4 must never be used for security purposes. Even if your specific use case does not seem vulnerable to collisions, the broken cryptography makes MD4 fundamentally untrustworthy.

MD4 vs MD5 - which is more secure?

Neither MD4 nor MD5 should be used for security, but MD5 is significantly more secure than MD4. MD4 is completely broken with collisions findable in under one second on modern hardware. MD5 is also broken, but collision attacks are more expensive, taking minutes to hours depending on the attack. MD5 uses four rounds of operations compared to MD4's three, providing better diffusion and making cryptanalysis harder. MD5 was explicitly designed to fix known weaknesses in MD4 by adding a fourth round and modifying the round functions. However, both algorithms share fundamental structural weaknesses. MD5 collision attacks were first demonstrated in 2004, and by 2008, researchers could generate MD5 collisions in seconds. In 2012, the Flame malware used MD5 collision attacks to forge Microsoft code signing certificates, demonstrating real-world security impact. While MD5 is less broken than MD4, it is still cryptographically insecure and should be avoided. For legacy systems, MD5 is preferable to MD4 if you must use one, but both should be replaced with SHA-256 or stronger algorithms. MD5 is still sometimes used for non-security checksums where collision resistance is not critical, like detecting accidental file corruption. MD4 should not be used even for non-security purposes due to its extreme weakness. For all new applications, use SHA-256, SHA-3, or BLAKE3, which provide strong collision resistance and have no known practical attacks.

Why was MD4 designed and what was its purpose?

MD4 was designed by Ronald Rivest at MIT in 1990 to provide fast cryptographic hashing for digital signatures and message integrity verification. At the time, existing hash functions were either too slow for practical use or not cryptographically secure. Rivest aimed to create a hash function that could run efficiently on the 32-bit processors common in early 1990s computers. MD4 was optimized for speed, using simple bitwise operations (AND, OR, XOR, bit rotation) that execute quickly on CPUs. The three-round structure was chosen to balance security and performance based on the cryptographic knowledge of the time. MD4 influenced many subsequent hash function designs, including MD5 (Rivest's improved version), SHA-0, SHA-1, and RIPEMD. The algorithm introduced several design concepts that became standard in hash functions, such as processing data in blocks, using a compression function with multiple rounds, and the Merkle-Damgård construction. However, cryptographic understanding evolved rapidly in the 1990s, and weaknesses in MD4 were discovered soon after publication. By 1996, collision attacks demonstrated that MD4's three rounds were insufficient. Rivest himself acknowledged these weaknesses and recommended MD5 as a replacement, though MD5 was also later broken. MD4's legacy is as a stepping stone in hash function evolution and a cautionary tale about the difficulty of designing secure cryptographic primitives. It succeeded at its speed goal but failed at security, teaching the cryptographic community important lessons about hash function design.

How fast can MD4 collisions be found?

On modern hardware, MD4 collisions can be generated in under one second, often in milliseconds. The first practical collision attack on MD4 was published by Hans Dobbertin in 1996, requiring significant computational resources at the time. However, cryptanalytic techniques have improved dramatically, and computer hardware has become exponentially more powerful. By the early 2000s, MD4 collisions could be found in minutes on standard PCs. Today, optimized implementations using differential cryptanalysis can generate MD4 collisions almost instantly on consumer hardware. A modern CPU can compute hundreds of millions of MD4 hashes per second, and collision finding algorithms exploit mathematical weaknesses to dramatically reduce the search space. Academic papers have demonstrated collision finding in fractions of a second on laptops. GPU implementations can accelerate attacks even further, generating collisions in milliseconds by parallelizing hash computations across thousands of cores. This makes MD4 collisions essentially free from a computational standpoint. For comparison, a cryptographically secure 128-bit hash should require approximately 2^64 operations to find a collision (the birthday paradox attack), which would take millions of years even on supercomputers. MD4 reduces this to less than one second, representing a security failure of astronomical proportions. The ease of collision finding means attackers can generate forged documents, fake signatures, and bypass integrity checks with negligible effort. This is why MD4 is considered completely broken and must never be used for security purposes.

What should I use instead of MD4?

For modern applications, use SHA-256 (SHA-2 family) as the industry standard for cryptographic hashing. SHA-256 provides 128-bit collision resistance, strong security properties, and wide compatibility across all platforms and programming languages. It is approved by NIST, used in Bitcoin, SSL certificates, and countless security-critical systems. For maximum security, use SHA-512 (256-bit collision resistance) or SHA-3 (Keccak), which provides similar security with a completely different internal design. For highest performance with strong security, use BLAKE3, which is faster than MD4 while providing SHA-256-level security. For password hashing specifically, never use general hash functions like MD4 or even SHA-256. Instead, use Argon2id (recommended), bcrypt, or scrypt, which are designed specifically for password security with configurable work factors, built-in salting, and resistance to GPU/ASIC attacks. For non-cryptographic checksums where collision resistance is not required (detecting accidental file corruption, not malicious tampering), CRC32 or Adler32 are faster than MD4 and explicitly designed for error detection. For legacy compatibility where you must verify MD4 hashes from old systems, calculate MD4 for comparison but migrate to SHA-256 as soon as possible. When replacing MD4 in existing systems, SHA-256 is the safest choice for compatibility and security. Document the migration clearly and use transitional periods where both MD4 and SHA-256 hashes are computed, allowing systems to verify either until the transition is complete.

Is MD4 used in NTLM authentication?

Yes, MD4 is a critical component of Windows NTLM (NT LAN Manager) authentication, which is one reason why NTLM is considered insecure. In NTLM, user passwords are hashed with MD4 to create the NT hash, which is stored on the system and used for authentication. When a user logs in, the password is hashed with MD4 and compared to the stored NT hash. If they match, authentication succeeds. The use of MD4 creates serious security vulnerabilities. Attackers who gain access to NT hash databases can crack passwords much faster than with secure password hashing algorithms, because MD4 is extremely fast and cryptographically broken. Pass-the-hash attacks allow attackers to use captured NT hashes directly for authentication without needing to crack the original password, because the hash itself is the credential. MD4's collision weaknesses theoretically allow attackers to find different passwords that produce the same NT hash, though this is less practical than other attacks. Rainbow tables for NT hashes are widely available, making password cracking even easier for common passwords. Microsoft has acknowledged these security issues and deprecated NTLM in favor of Kerberos authentication, which uses stronger cryptographic primitives. However, NTLM remains enabled in many Windows environments for backward compatibility with legacy systems and older software. Security best practices recommend disabling NTLM when possible, using Kerberos for authentication, implementing multi-factor authentication, and ensuring strong passwords to mitigate NT hash cracking. The use of MD4 in NTLM is a classic example of how legacy cryptography creates long-term security vulnerabilities that persist for decades.

MD4 Hash Generator

Generate MD4 hashes from text. Legacy 128-bit hash function designed by Ronald Rivest in 1990.

⚠️ CRITICAL WARNING: MD4 is cryptographically BROKEN with known collision vulnerabilities. Do NOT use for passwords, signatures, or security. Only for legacy compatibility (NTLM, eDonkey2000). Use SHA-256 or BLAKE3 for new applications.

TextFile

Input Text

Other hashing tools

File to MD4•SHA256 Hash (Secure)•BLAKE3 Hash (Secure)