hashing - dwilson2547/wiki_demo GitHub Wiki

Hashing is a fundamental concept in computer science and cryptography. It transforms input data (of any size) into a fixed-size string of characters, which is typically a hash value or digest. Let’s explore hashing in detail:

1. What is Hashing?

Hashing is a one-way function that:

  • Takes an input (e.g., a password, file, or message).
  • Produces a fixed-length output (the hash).
  • Is deterministic: The same input always produces the same hash.
  • Is irreversible: You cannot reverse-engineer the input from the hash.

2. Key Properties of Hashing

Property Description
Deterministic Same input → same hash.
Fixed-Length Output Regardless of input size, the hash is always the same length (e.g., 256 bits for SHA-256).
Irreversible No practical way to reverse the hash to get the original input.
Collision-Resistant Hard to find two different inputs that produce the same hash.
Avalanche Effect A tiny change in input drastically changes the hash.

3. Common Hashing Algorithms

Algorithm Output Length Use Case
MD5 128 bits Legacy (insecure for cryptography).
SHA-1 160 bits Legacy (vulnerable to collisions).
SHA-256 256 bits Secure (used in Bitcoin, TLS, etc.).
SHA-3 Variable (e.g., 224, 256, 384, 512 bits) Modern alternative to SHA-2.
bcrypt Variable (adaptive) Password hashing (slow by design).
Argon2 Variable Winner of the Password Hashing Competition (PHC).

4. How Hashing Works

Example: SHA-256

  • Input: "hello"
  • Output (SHA-256): 
    2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
  • Even a small change (e.g., "Hello") produces a completely different hash: 
    64ec88ca00b268e5ba1a35678a1b5316d212f4f366b2477232534a8aeca37f3c

5. Use Cases of Hashing

A. Password Storage

  • Never store plaintext passwords!
  • Instead, store the hash of the password.
  • When a user logs in, hash their input and compare it to the stored hash.
  • Example:
    • User’s password: "mypassword"
    • Stored hash: bcrypt("mypassword") → "$2a$10$N9qo8uLOickgx2ZMRZoMy..."

B. Data Integrity

  • Verify that data hasn’t been tampered with.
  • Example: Download a file and check its hash against the published hash.

C. Digital Signatures

  • Combine hashing with encryption to verify authenticity.
  • Example: Sign a message with a private key and verify with a public key.

D. Blockchain

  • Each block contains the hash of the previous block, ensuring immutability.

6. Hashing vs. Encryption

Feature Hashing Encryption
Purpose Integrity, fingerprinting. Confidentiality.
Reversible No. Yes (with a key).
Use Case Passwords, checksums. Secure communication.

7. Security Considerations

  • Rainbow Tables: Precomputed tables for reversing hashes. Mitigate with salting (adding random data to input).
  • Brute Force Attacks: Use slow hashing algorithms (e.g., bcrypt, Argon2) for passwords.
  • Collision Attacks: Use modern algorithms (e.g., SHA-256, SHA-3) to avoid collisions.

8. Example: Password Hashing with Salt

  1. User creates a password: "mypassword".
  2. System generates a random salt: "a1b2c3".
  3. Combine password + salt: "mypassworda1b2c3".
  4. Hash the result with bcrypt: 
    bcrypt("mypassworda1b2c3") → "$2a$10$N9qo8uLOickgx2ZMRZoMy..."
  5. Store both the salt and hash in the database.

9. Common Misconceptions

  • Hashing ≠ Encryption: Hashing is one-way; encryption is two-way.
  • Not All Hashes Are Secure: MD5 and SHA-1 are broken for cryptographic use.
  • Salting Alone Isn’t Enough: Use slow hashing algorithms for passwords.

Would you like to explore a specific hashing algorithm, real-world implementation, or how hashing fits into modern security practices?

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