Architecture Diaries

Hash Functions

Hashing

A hash function is a one-way mathematical function that converts a string of bytes into a fixed size representation, known as its hash value.

Modern hash functions optimized for securing passwords are deliberaly designed to be slow and memory-hard (consume a lot of memory) to make them harder to brute-force. A memory-hard function limits the effectiveness of specialized cracking hardware such as ASICs or GPUs.

Properties of a good hash function

  • determinism: the same input always produces the same output
  • avalance effect: minor differences in input produces singificant differences in the hash value
  • pre-image resistance: it’s computationally infeasible for reproduce the original input from the output
  • collision resistance: it’s hard to find two values that produce the same hash value

Main applications

  • check data integrity and checksums
  • store passwords securely
  • data structures like hash tables
  • digital signatures and certificates
  • file deduplication
  • data partitioning

Comparison

Algorithm Size (bits) Features Applications
MD5 128 * fast to compute
* vulnerable to collision attacks and rainbow tables		 non-security critical applications, legacy systems
SHA 1 160 * stronger than MD5 collisions possible, deprecated in favor of SHA-2 and SHA-3
SHA 2 224
256
384
512		 * considered secure
* no practical attack methods yet		 widely used in SSL/TLS certs and digital signatures
SHA 3 224
256
384
512		 * different cryptographic approach from SHA-2
* slower than SHA-2
* quantum resistant, potentially more secure than SHA-2		 emerging applications with specific security requirements
PBKDF2 (Password-Based Key Derivation Function) 1 bits * uses a salt and multiple iterations
* protects against brute force and rainbow tables
* less memory-hard than bcrypt and Argon2		 password protection in older systems with limited memory
bcrypt 1 bits * adaptive hash function with built in salt
* adjusts computational cost as hardware improves to deliberately slow down computation
* less memory-hard than Argon2		 password protection in general web applications
Argon2 1 bits * modern highly secure hash function
* configurable memory, time and parallelism cost		 high security requirements