Hash Algorithms Compared: How to Choose Between MD5, SHA-1, SHA-256, and bcrypt

"Use MD5 to store passwords" still gets written into code by beginners in 2026 — and then waits for the breach that puts them on the front page.

Hash algorithms look like a simple "input to fixed-length string" mapping, but the gap between the right algorithm and the wrong one is huge: the former withstands billions of brute-force attempts, the latter has your users' passwords cracked in 5 minutes.

This article runs through all the mainstream hash algorithms — what can be used where, what can't, and why — plus the most common misuse patterns.

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First Principles: Hash Is Not Encryption

Many people conflate "hash" and "encryption". Lock the concepts down:

Core properties of hashing:

1. Deterministic — same input always produces the same output
2. Irreversible — recovering the original from the hash is mathematically infeasible
3. Avalanche effect — flip one input bit and the output changes completely
4. Collision resistance — finding two different inputs with the same hash should be extremely hard

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General-Purpose Hash Lineup

MD5: The Old Soldier That Should Have Retired

Known problems:

• 2004 — Wang Xiaoyun proved MD5 collision resistance is dead
• 2008 — researchers forged a CA certificate using 200 PS3s in a day
• Today, ordinary PCs produce collisions in seconds

Where it's still OK:

• Non-security integrity checks (a quick MD5 to verify a download isn't corrupted)
• Cache keys, dedup keys (anywhere there's no adversary)

Where it must not be used:

• Password storage
• Digital signatures
• Any security-related purpose

SHA-1: MD5's Wounded Sibling

Known problems:

• 2017 — Google published a real SHA-1 collision (SHAttered)
• Mainstream browsers and CAs stopped accepting SHA-1 certificates years ago
• Git still uses SHA-1 but is migrating to SHA-256

Verdict: same as MD5 — non-security only.

SHA-2 Family: The Current Workhorse

Contains SHA-224 / SHA-256 / SHA-384 / SHA-512:

Uses:

• File integrity verification
• Digital signatures
• Blockchain (Bitcoin uses SHA-256)
• HMAC building block

Note: SHA-256 is not appropriate for password storage (see the password section).

SHA-3: A Modern Backup

Standardized in 2015 (based on Keccak), structurally very different from SHA-2 — if SHA-2 ever breaks, SHA-3 is a seamless replacement.

Uses:

• When you want defense-in-depth against future SHA-2 breaks
• Ecosystem requirements (Ethereum uses Keccak-256)

BLAKE2 / BLAKE3: Fast and Safe

Uses:

• Performance-sensitive verification
• Content-addressable storage (IPFS uses BLAKE2/3)
• Default in modern crypto libraries (e.g., libsodium)

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File Integrity Verification in Practice

After downloading a Linux ISO, verify the official SHA-256:

Compare with the value published on the official site — matching means the file is unaltered.

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Password Hashing: General-Purpose Hashes Don't Belong Here

Why SHA-256 Cannot Store Passwords

Counter-intuitive reason: SHA-256 is too fast.

Modern GPUs compute 10 billion SHA-256 hashes per second (this is what Bitcoin miners do). For password hashing, "fast" is fatal — it means an attacker who steals your hash table can brute-force at insane speed:

Add rainbow tables (precomputed hash databases) and common passwords become near-instant lookups in SHA-256.

Hard Lessons from History

One lesson: passwords must use a purpose-built password hash algorithm, not a general-purpose hash.

Purpose-Built Password Hash Algorithms

Design philosophy: be intentionally slow — make a single hash take tens to hundreds of milliseconds, so attackers' brute-force speed drops by millions of times relative to a fast hash.

Recommendation order:

• New projects: prefer Argon2id
• Existing bcrypt systems: keep using bcrypt, it remains safe
• PBKDF2: avoid unless required by FIPS compliance

bcrypt Usage

Key parameter: rounds (work factor) defaults to 10-12, each +1 doubles the compute time. Recommended:

• General apps: 12
• High-sensitivity: 14+
• Aim for ~250 ms per hash on your current server

Argon2id Usage

OWASP 2026 baseline: memory_cost=19 MiB, time_cost=2, parallelism=1 (minimum), tune upward based on server capacity.

About Salt

bcrypt and Argon2 automatically salt — the salt is baked into the output. You don't store it separately.

But if you see this kind of code, refactor immediately:

Just switch to bcrypt or Argon2.

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HMAC: Hash with a Key

Anyone can compute a regular hash, but HMAC adds a key — only the key holder can compute the correct hash.

Uses:

• API request signing (anti-tampering + identity)
• HS256 algorithm in JWT
• Webhook verification

Security must-knows:

• Compare signatures with hmac.compare_digest(), never with == — avoids timing attacks
• Keys must be at least 256 bits random

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Algorithm Selection Decision Tree

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5 Classic Mistakes

Mistake 1: Storing Passwords with MD5/SHA-1

Replace with bcrypt or Argon2 immediately. For existing data: verify the old MD5 on next login, then immediately rehash with the new algorithm.

Mistake 2: Roll Your Own Salting on a General Hash

General hashes are too fast — no amount of salt defends against GPU brute force. Use a purpose-built password hash.

Mistake 3: HMAC Comparison with Equals

String == exits on the first mismatching character — attackers can infer signatures byte by byte from response time.

Mistake 4: Using Hash When You Need Encryption

Need reversibility? That's not a hash, that's encryption. Use AES-GCM or ChaCha20-Poly1305.

Mistake 5: Truncated Hash as Unique ID

8 hex chars = 32 bits of space = 50% collision probability under birthday attack at ~65k entries. Either use the full hash or use a real UUID.

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Performance Reference

Single-threaded throughput on modern x86_64 CPUs (rough numbers — SHA-NI hardware acceleration and SIMD implementations push these significantly higher):

Takeaway: for general-purpose use BLAKE3 is the speed king; for passwords slowness is the feature, not a bug.

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Summary

Pin this cheat sheet to your brain:

The single most important rule: general hashes (MD5/SHA-2/SHA-3/BLAKE) must never be used directly for password storage. Forget this, and the next breach headline could be your company's.

Generate hashes with the Hash Generator, generate strong passwords and keys with the Password Generator — together they cover roughly 80% of day-to-day crypto needs.