MD5 and Cryptographic Hash Functions: A Deep Dive

2025-09-25

Introduction

Hash functions are everywhere in computer science: from verifying file downloads, to securing passwords, to powering distributed systems like Git and blockchain. One of the earliest widely-used hash functions is MD5 (Message Digest Algorithm 5). Although MD5 is considered broken today, understanding it provides a foundation for modern hashing.

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What is a Hash Function?

A hash function maps input data of arbitrary size to a fixed-length output (the "digest"). Cryptographic hash functions are designed with extra properties:

  • Determinism: same input → same output.
  • Avalanche effect: small change in input drastically changes output.
  • Preimage resistance: infeasible to reverse.
  • Collision resistance: infeasible to find two inputs with the same output.

MD5: History and Algorithm

  • Designed by Ronald Rivest in 1991.
  • Produces a 128-bit digest (32 hex characters).
  • Processes input in 512-bit blocks with padding.

Example

import hashlib

msg = b"hello world"
print(hashlib.md5(msg).hexdigest())
# Output: 5eb63bbbe01eeed093cb22bb8f5acdc3

Security Weaknesses

  • 2004: Xiaoyun Wang et al. demonstrated practical collisions.
  • 2012: Flame malware used MD5 collision attacks.
  • Today: MD5 is unsuitable for cryptography, but still used for checksums.

Modern Alternatives

  • SHA-2 (SHA-256, SHA-512) – industry standard.
  • SHA-3 – based on Keccak.
  • BLAKE2/BLAKE3 – faster, modern designs.

Applications Beyond Security

  • Hash tables
  • Bloom filters
  • Git version control (SHA-1 → SHA-256 migration)
  • Blockchain (Bitcoin mining)

References