bcrypt

Niels Provos's 1999 algorithm. Based on the Blowfish cipher's expensive key schedule.

Format: $2b$<cost>$<22-char-salt><31-char-hash>

$2b$12$N9qo8uLOickgx2ZMRZoMyeIjZAgcfl7p92ldGxad68LJZdL17lhWy
 |   |  |--------salt--------||----------hash----------|
 ver cost

• 2b is the version. Older $2a$ and $2$ exist but $2b$ is current.
• cost is log2(iterations). Cost 12 = 4096 iterations.
• 22 chars salt + 31 chars hash, both base64-ish (modified alphabet).

Cost 10 takes ~10ms on modern hardware. Cost 12 takes ~250ms (industry default). Each cost increment doubles the work.

bcrypt has been BATTLE-TESTED for 25 years. Used by countless web apps. Built into PHP, Ruby (bcrypt-ruby), Node (bcrypt), Python (bcrypt).

Limitations:

• Truncates passwords at 72 BYTES (limitation of Blowfish key schedule)
• Not memory-hard (vulnerable to GPU attacks at high parallelism)

For new code, prefer argon2id. For legacy systems with bcrypt: keep using it, just bump the cost over time.