Hash functions are not inherently "broken," but many commonly used hash functions have vulnerabilities that can be exploited, making them unsuitable for certain applications. Here are the key reasons why some hash functions are considered "broken":
Collision Attacks
A collision attack occurs when two different inputs produce the same hash value. This undermines the integrity of the hash function, as it allows an attacker to substitute one input for another without detection. For example, MD5 and SHA-1 are well-known for their susceptibility to collision attacks. Researchers have demonstrated practical collision attacks against these algorithms, making them unsuitable for security-sensitive applications[2][4][16].
Preimage and Second Preimage Attacks
Preimage attacks involve finding an input that hashes to a specific output, while second preimage attacks involve finding a different input that produces the same hash as a given input. These attacks exploit the one-way nature of hash functions. While these attacks are more computationally intensive than collision attacks, advances in computing power and cryptanalysis have made some hash functions vulnerable. For instance, MD5 and SHA-1 have shown weaknesses in resisting these types of attacks[12][16].
Weaknesses in Older Algorithms
Older hash functions like MD5, SHA-1, and RIPEMD-128 have been found to be insecure due to their vulnerability to various attacks. These algorithms were designed at a time when the computational power available to attackers was much lower. As a result, they are no longer considered secure by modern standards[2][4][12].
Advances in Cryptanalysis
Cryptanalysis techniques have advanced significantly, revealing weaknesses in hash functions that were previously considered secure. For example, the discovery of collision attacks on MD5 and SHA-1 has led to their deprecation in favor of more secure algorithms like SHA-2 and SHA-3[12][16].
Inadequate Hash Length
The length of the hash output is crucial for security. Shorter hash lengths are more susceptible to brute-force attacks and collisions due to the birthday paradox. For instance, a 128-bit hash like MD5 is more vulnerable than a 256-bit hash like SHA-256[12][18].
Practical Exploits
Practical exploits have demonstrated the vulnerabilities of certain hash functions. For example, in 2008, researchers successfully created a rogue Certificate Authority certificate using an MD5 collision, highlighting the real-world risks of using broken hash functions[12][16].
