A Cryptographic Hash Function is a mathematical algorithm that takes an input, or 'message', and returns a fixed-size string of bytes, typically used to index data in hash tables, facilitate data retrieval, and verify the integrity of data. Here are some key points:
Characteristics
- Deterministic: The same input message always produces the same hash output.
- Quick Computation: Hash functions are designed to be computed rapidly on modern hardware.
- Pre-image Resistance: Given a hash value, it should be computationally infeasible to reconstruct the original input.
- Second Pre-image Resistance: Given an input, it should be hard to find another input with the same hash.
- Collision Resistance: It should be hard to find two different inputs that produce the same hash value.
- Fixed Output Size: Regardless of the size of the input, the output hash size remains the same.
History and Development
The concept of Cryptographic Hash Functions can be traced back to:
- 1953 - The invention of hash tables by Hans Peter Luhn, leading to early hash functions.
- 1970s - The introduction of Merkle-Damgård construction, which many modern hash functions utilize.
- 1989 - MD5 was developed by Ronald Rivest, which was widely used until vulnerabilities were discovered.
- 1995 - SHA-1 was published by the National Security Agency (NSA) as part of the Secure Hash Algorithm (SHA) family.
- 2001 - Introduction of SHA-2 to replace SHA-1 after the latter was found to be vulnerable to certain attacks.
- 2012 - NIST selected SHA-3 (Keccak) as the new hash function standard, aiming to provide a more robust alternative to SHA-2.
Applications
- Integrity Verification: Checking whether data has been altered or corrupted.
- Digital Signatures: Creating a hash of a document to be signed, which is then encrypted with a private key.
- Password Hashing: Storing hashed versions of passwords rather than the passwords themselves for security.
- Blockchain and Cryptocurrencies: Used in mining and as part of transaction verification processes.
- Data Structures: In hash tables for efficient data retrieval.
Challenges and Vulnerabilities
- Hash collisions can be exploited if not properly managed.
- Length extension attacks where an attacker can extend a hash without knowing the original input.
- Vulnerabilities in older hash functions like MD5 and SHA-1 have led to their deprecation in security-sensitive applications.
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See Also