Cryptographic hash function
Overview
A cryptographic hash function is a special class of hash functions that has certain properties making it suitable for use in cryptography. It is a mathematical algorithm that takes an input (or 'message') and returns a fixed-size string of bytes, typically a digest that is unique to each unique input. It is deterministic, meaning that the same input will always produce the same output.
Properties
A cryptographic hash function must possess the following properties:
1. Determinism: For a given input value, the output value will always be the same. 2. Fixed Output Length: Regardless of the length of the input data, the output length remains constant. 3. Preimage Resistance: It should be computationally infeasible to retrieve the original input value from its output (hash). 4. Small Changes in Input Affect Large Changes in Output: Even a small modification to the input should produce such a drastic change in output that the new hash value appears uncorrelated with the old hash value. 5. Collision Resistance: It should be extremely difficult to find two different inputs that hash to the same output.
Types of Cryptographic Hash Functions
There are several types of cryptographic hash functions, each with varying levels of security and computational efficiency. Some of the most commonly used ones include:
1. MD5 (Message Digest Algorithm 5): This is a widely used cryptographic hash function that produces a 128-bit (16-byte) hash value. It is commonly used to verify data integrity. However, MD5 is not collision-resistant; as such, it is not suitable for functions such as SSL certificates or encryption that require a high level of security.
2. SHA-1 (Secure Hash Algorithm 1): This is a cryptographic hash function which takes an input and produces a 160-bit (20-byte) hash value. This was developed by the National Security Agency (NSA) in the USA. However, SHA-1 is no longer considered secure against well-funded attackers.
3. SHA-2 (Secure Hash Algorithm 2): It is a set of cryptographic hash functions (including SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, SHA-512/256). SHA-2 has a number of security advantages over its predecessors.
4. SHA-3 (Secure Hash Algorithm 3): This is the latest member of the Secure Hash Algorithm family and consists of six hash functions with digests that are 224, 256, 384, 512, and variable lengths.
5. BLAKE: This is a cryptographic hash function that is faster than MD5, SHA-1, SHA-2, and SHA-3, yet is at least as secure as the latest standard, SHA-3.
Applications
Cryptographic hash functions have a wide variety of applications in computer science and cryptography, including:
1. Data Integrity Checks: They can be used to verify the integrity of data during transmission. By comparing the hash value of the received data with the hash value of the original data, one can determine whether the data was altered during transmission.
2. Password Storage: They are used to store user passwords securely. When a user creates an account or changes their password, the password is hashed and the hash value is stored. When the user logs in, the password they enter is hashed again and the hash value is compared to the stored hash value.
3. Digital Signatures: They are used in the creation of digital signatures, which are used to verify the authenticity of a message.
4. Blockchain: They are used in blockchain technology to link blocks together in a chain.
Security
The security of a cryptographic hash function is determined by its ability to resist attacks. The two main types of attacks are preimage attacks and collision attacks.
1. Preimage Attacks: In a preimage attack, an attacker tries to find a message that has a specific hash value. A good cryptographic hash function is resistant to preimage attacks.
2. Collision Attacks: In a collision attack, an attacker tries to find two different messages that hash to the same value. A good cryptographic hash function is resistant to collision attacks.