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NOTE

1.3 Hash

Hash concepts, properties, use cases, MD5, and SHA-256.

SecurityUpdated 1 min readhistorical

This is a historical learning note and may contain outdated or incomplete understanding.

1. What Is a Hash

A hash function maps input of arbitrary length to a fixed-length digest.

1.1. Hash vs Encryption

  • Encryption is designed to be reversible with a key.
  • A cryptographic hash is designed as a one-way mapping and does not use a decryption key.

2. Hash Properties

For security-sensitive use, a cryptographic hash should provide practical resistance to preimage, second-preimage, and collision attacks.

2.1. Hash Use Cases

  • Integrity checks.
  • Content addressing and fingerprints.
  • As a building block in MACs, signatures, and password-hashing constructions.

3. Types of Hash Functions

3.1. MD5

MD5 maps arbitrary byte strings to a 128-bit digest. It is not encryption. Practical collision attacks exist, so it should not be used for security-sensitive integrity or signature purposes.

3.2. SHA-256

SHA-256 produces a 256-bit digest and remains a common cryptographic hash function.

4. References