Bcrypt Salt Rounds (Cost Factor) Explained

Bcrypt Salt Rounds (Cost Factor) Explained

The cost factor (often called "salt rounds") is the single most important parameter when using bcrypt. It controls how many iterations the key derivation runs: the hash function executes 2^cost iterations of the Eksblowfish key schedule. Higher values produce slower hashes, making brute-force attacks exponentially harder.

How Cost Affects Speed

Each increment of the cost factor doubles the computation time:

Cost	Iterations	Approx. Time (modern CPU)
8	256	~1 ms
10	1,024	~4 ms
12	4,096	~15 ms
13	8,192	~30 ms
14	16,384	~60 ms
16	65,536	~250 ms

Times vary by CPU. These are approximate values for a modern x86-64 processor.

Choosing the Right Cost

The goal is to make hashing slow enough to deter brute-force attacks but fast enough to avoid degrading user experience:

1. Minimum recommended: 10 — the default in most libraries, but increasingly considered the floor rather than the target
2. Current best practice: 12–13 — provides a good balance between security and responsiveness for most web applications
3. High-security applications: 14+ — suitable when login latency of 100ms+ is acceptable

The Decision Rule

Aim for a hash time of 250 ms or less per password on your production hardware. Measure with a benchmark on your actual server:

const bcrypt = require('bcrypt');
const start = Date.now();
bcrypt.hashSync('test-password', 12);
console.log(`Cost 12: ${Date.now() - start}ms`);

If the result is under 250 ms, you can consider increasing the cost. If it exceeds 1 second, reduce it.

Upgrading Cost Over Time

Hardware improves roughly following Moore’s Law. A cost factor that was adequate in 2015 may be insufficient today. Best practice is to rehash passwords on login when the stored cost is below your current target:

1. User logs in with correct password
2. Check the cost factor in the stored hash
3. If below current target, rehash with the higher cost
4. Store the new hash

This transparent upgrade ensures all active accounts migrate to stronger hashing without requiring password resets.

Common Mistakes

• Cost too low (< 10) — fast enough for attackers to brute-force at scale
• Cost too high (> 16) — causes login timeouts and denial-of-service vulnerabilities
• Hardcoding cost — store it as a configurable value so it can be updated without code changes