Cryptographic Hash Functions

Also Known As

TL;DR

Explanation

Cryptographic hash functions must be: deterministic (same input = same output), fast to compute, pre-image resistant (cannot reverse), second pre-image resistant (cannot find collision for a given input), and collision resistant (cannot find any two inputs with the same hash). SHA-256/SHA-512 (SHA-2 family): widely deployed, no known practical attacks. SHA-3 (Keccak): different construction, quantum-resistant design. BLAKE3: fast, secure, modern. MD5 and SHA-1: broken collision resistance — do not use for security. In PHP: hash('sha256', $data), hash('sha3-256', $data).

Common Misconception

Why It Matters

Common Mistakes

• MD5 for file integrity checksums — collisions have been demonstrated; use SHA-256.
• SHA-256 for password hashing — fast algorithm; use password_hash() with Argon2id.
• Not using HMAC for message authentication — a bare hash can be length-extended; use hash_hmac().
• Comparing hashes with == instead of hash_equals() — timing attack vulnerability.

Code Examples

// MD5 for integrity — broken:
$checksum = md5_file($uploadedFile); // Collisions possible

// SHA-256 for password — wrong tool:
$hash = hash('sha256', $password); // Fast = GPU crackable

// Timing-vulnerable comparison:
if ($computed === $provided) { /* vulnerable to timing attack */ }

// SHA-256 for file integrity:
$checksum = hash_file('sha256', $uploadedFile); // Secure for integrity

// Argon2id for passwords:
$hash = password_hash($password, PASSWORD_ARGON2ID);

// HMAC for message authentication:
$mac = hash_hmac('sha256', $message, $secretKey);

// Constant-time comparison:
if (hash_equals($expected, $computed)) { /* safe */ }

References

• ↗ https://www.php.net/manual/en/function.hash.php

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