Hashing Basics

This is a short, clear guide to the essentials of hashing, for anyone learning cybersecurity or cryptography.

What is a Hash?#

A hash is a fixed-length string produced by a hash function from any size input.

• Input → Hash Function → Fixed-size Hash Value
• Same input → always same hash.
• Small input change → completely different hash.

Example Hash Algorithms:

• MD5 (insecure)
• SHA-1 (insecure)
• SHA-256 (recommended)

Why Use Hashing?#

** File Integrity Checking**

• Check if downloaded file matches original.
• Compare hashes:
  • If hashes match → files identical.
  • If hashes differ → something changed (corruption or tampering).

Example:

sha256sum file.iso

Password Storage Never store passwords in plaintext. Store hashes instead. When user logs in: Hash the entered password. Compare to stored hash.

Example:

User password → hash function → stored hash

Data Integrity & Signatures Hash data before signing it. Receiver verifies signature and hash.

Hash Properties#

Deterministic Same input → same output. Quick to Compute Fast even on large files. Pre-Image Resistance Hard to find input from hash. Collision Resistance Hard to find two inputs with same hash. Avalanche Effect Small input change → huge output change.

Collisions#

Collision = two different inputs with same hash. Good hash functions make collisions extremely rare. MD5 and SHA-1 are no longer collision-resistant. Avoid them.

Recognizing Hashes#

Length hints at algorithm: MD5: 32 hex chars. SHA-1: 40 hex chars. SHA-256: 64 hex chars.

Example:

SHA-256 hash
9b74c9897bac770ffc029102a200c5de

Cracking Hashes Hashes are not encryption → they can't "decrypt". To crack a hash → guess input → hash it → compare. Attack Methods Brute Force Dictionary Attacks Rainbow Tables

Popular Tools#

John the Ripper Hashcat

Hashing for Integrity Protection Publish file + its hash. User computes their own hash. If it matches → file is untampered.

Example:

Website offers file.zip and file.zip.sha256 You download both and run:

sha256sum -c file.zip.sha256

Summary#

Hashing ensures data integrity. Used in password storage, signatures, file verification. Secure hash functions resist collisions and pre-image attacks. Always choose modern algorithms like SHA-256 or better.