Bcrypt Hash Not Matching — Password Check Fix

💡If bcrypt.compare() returns false for what should be the correct password, the most likely causes are double-hashing at registration, the hash being stored incorrectly in the database, or comparing the wrong value. Always use bcrypt.compare(plaintext, storedHash) — never compare hash strings directly, because bcrypt uses a unique salt every time.

Quick Diagnosis

If you see “bcrypt.compare returns false” → it means the plaintext password does not match the stored hash → do this: confirm you are comparing the raw password string, not a re-hashed version

If you see “hash stored in DB looks different each time” → it means bcrypt generates a unique salt per hash — this is expected and correct → do this: always use bcrypt.compare() — never compare hash strings directly

If you see “compare always returns false even with correct password” → it means the hash was stored incorrectly (double-hashed, truncated, or encoded) → do this: check if the hash in the DB starts with $2b$ or $2a$ and is 60 characters long

If you see “bcrypt.compare throws an error” → it means the hash argument is not a valid bcrypt hash string → do this: validate that the stored value is a proper bcrypt hash before calling compare

Common Causes and Fixes

Double-hashing on registration

❌ Wrong

// Hash the password twice by mistake
const hash1 = await bcrypt.hash(password, 10);
const hash2 = await bcrypt.hash(hash1, 10);
db.save(hash2);
// Later: bcrypt.compare(password, hash2) → false

✅ Fixed

// Hash exactly once
const hash = await bcrypt.hash(password, 10);
db.save(hash);
// Later: bcrypt.compare(password, hash) → true

Hashing twice means the stored hash no longer corresponds to the original password.

Comparing hash strings directly

❌ Wrong

// Wrong: comparing two different hashes of the same password
const stored = "$2b$10$abc...";
const recomputed = await bcrypt.hash(password, 10);
stored === recomputed  // always false — salts differ

✅ Fixed

// Correct: use bcrypt.compare
const match = await bcrypt.compare(password, stored);
// match → true if password is correct

Bcrypt uses a random salt, so two hashes of the same password always look different. Use bcrypt.compare.

Verify Your Hash

Before debugging code, confirm the stored hash is a valid bcrypt hash. It should start with $2b$ and be 60 characters long. If it does not match this format, the hash was stored incorrectly. Use the Bcrypt Generator to hash a test password and compare the format to what is in your database.

Generate and Verify Bcrypt Hashes

Hash a password or verify that a plaintext matches a stored bcrypt hash — directly in the browser, no code required.

Open Bcrypt Generator →

Related Guides

Frequently Asked Questions

Why does bcrypt.compare return false for the correct password?

The most common causes are double-hashing on registration, storing the hash incorrectly (truncated or re-encoded), or comparing the wrong value. Always pass the original plaintext password to bcrypt.compare.

Can I compare two bcrypt hashes directly?

No. Bcrypt generates a unique salt each time, so two hashes of the same password are always different strings. Use bcrypt.compare(plaintext, storedHash) instead.

What does a valid bcrypt hash look like?

A valid bcrypt hash starts with $2b$ or $2a$, followed by the cost factor, and is exactly 60 characters long. If the stored value differs, the hash was stored incorrectly.

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