Part 3: Hash Functions

Objective: Understand the purpose, functionality, and applications of cryptographic hash functions.

Introduction to Hash Functions

• Definition: A hash function is a cryptographic algorithm that takes an input (or ‘message’) and returns a fixed-size string of bytes. The output is typically a ‘digest’ that represents the input data.
• Key Characteristics:
  • Deterministic: The same input will always produce the same output.
  • Fixed Size: The output (hash) has a fixed size regardless of the input size.
  • Efficiency: Computes the hash value quickly.
  • Pre-image Resistance: Infeasible to reverse the hash value to find the original input.
  • Collision Resistance: Difficult to find two different inputs that produce the same output.

Key Concepts

1. Understanding Hash Values:
   • The output of a hash function, known as the hash value or hash, appears as a seemingly random string of characters.
   • Even a small change in the input will significantly change the hash.
2. Applications of Hash Functions:
   • Data Integrity Checks: Verifying the integrity of data or files.
   • Password Storage: Storing passwords in hashed form for security.
   • Digital Fingerprints: Unique representation of data for identification.

Common Hash Algorithms

1. SHA (Secure Hash Algorithm) Family:
   • Includes SHA-1, SHA-256, and SHA-512 among others.
   • Widely used in various security applications and protocols.
2. MD5 (Message Digest Algorithm 5):
   • Once widely used but now considered vulnerable and not recommended for security-critical applications.

Hands-on Exercise: Generating and Comparing Hash Values

• Goal: Generate hash values for different inputs and understand the unique nature of hash outputs.
• Tools: Online hash calculator, command-line tools like OpenSSL, or programming libraries.

Steps:

1. Choose a piece of text or a file.
2. Generate the hash value using an algorithm like SHA-256.
3. Modify the input slightly (e.g., change a character in the text).
4. Generate the new hash and compare it with the original hash to observe the differences.

Practical Application

• Verifying Downloaded Files: Using hashes to check the integrity of downloaded files.
• Secure Password Storage: Storing user passwords in hashed form in databases.

Further Reading and Resources

• “Cryptography and Network Security” by William Stallings – Coverage on hash functions.
• Experiment with different hash algorithms using online tools or coding libraries.