Password hashing with bcrypt in Node.js - Step-by-Step Execution

Concept Flow - Password hashing with bcrypt

User inputs password

↓

bcrypt.hash(password, saltRounds)

↓

Generate salt + hash password

↓

Store hashed password

↓

User login attempt

↓

bcrypt.compare(inputPassword, storedHash)

↓

Returns true if match, false if not

↓

Allow or deny access

This flow shows how bcrypt hashes a password, stores it, and later compares input to verify login.

Execution Sample

Node.js

import bcrypt from 'bcrypt';

const password = 'mySecret123';
const saltRounds = 10;

async function run() {
  const hash = await bcrypt.hash(password, saltRounds);
  const match = await bcrypt.compare('mySecret123', hash);
  console.log(match);
}

run();

This code hashes a password and then checks if a given input matches the hashed password.

Execution Table

Step	Action	Input	Output	Notes
1	Receive password input	'mySecret123'	Password string ready	User enters password
2	Call bcrypt.hash	'mySecret123', 10	Generated hash string	Salt generated internally, hash created
3	Store hash	Hash string	Stored securely	Hash saved in database
4	User login input	'mySecret123'	Input password ready	User tries to login
5	Call bcrypt.compare	'mySecret123', stored hash	true	Password matches hash
6	Allow access	true	User logged in	Access granted
7	Call bcrypt.compare	'wrongPass', stored hash	false	Password does not match
8	Deny access	false	Login failed	Access denied

💡 Process ends after allowing or denying access based on password match.

Variable Tracker

Variable	Start	After hash	After compare (correct)	After compare (wrong)
password	'mySecret123'	'mySecret123'	'mySecret123'	'mySecret123'
saltRounds	10	10	10	10
hash	undefined	hashed string	hashed string	hashed string
match	undefined	undefined	true	false

Key Moments - 3 Insights

Why can't we just store the password directly instead of hashing?

Storing passwords directly is unsafe because if the database leaks, attackers get all passwords. Hashing with bcrypt makes stored data safe by hiding the original password (see execution_table step 3).

Why do we need saltRounds in bcrypt.hash?

SaltRounds controls how much work bcrypt does to hash the password, making it slower and harder to crack. This is shown in step 2 where salt is generated internally.

What happens if the input password doesn't match the stored hash?

bcrypt.compare returns false (step 7), so access is denied (step 8). This prevents unauthorized login.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'match' after comparing the correct password?

A false

B true

C undefined

D hashed string

💡 Hint

Check execution_table row 5 where bcrypt.compare returns true for correct password.

At which step does bcrypt generate the salt and hash the password?

A Step 2

B Step 4

C Step 1

D Step 5

💡 Hint

Look at execution_table step 2 where bcrypt.hash is called.

If we increase saltRounds, what changes in the execution process?

A Comparison always returns false

B Password input changes

C Hashing takes more time and is more secure

D Stored hash becomes shorter

💡 Hint

Refer to key_moments about saltRounds controlling hashing work.

Concept Snapshot

Password hashing with bcrypt:
- Use bcrypt.hash(password, saltRounds) to create a secure hash
- saltRounds controls hashing difficulty
- Store only the hash, never the plain password
- Use bcrypt.compare(input, storedHash) to verify passwords
- Returns true if input matches stored hash, else false

Full Transcript

This lesson shows how bcrypt hashes passwords securely in Node.js. First, the user inputs a password. bcrypt.hash creates a salted hash using saltRounds to control difficulty. The hash is stored safely. Later, when the user logs in, bcrypt.compare checks if the input matches the stored hash. If it matches, access is granted; if not, access is denied. This protects passwords from being stolen in plain text and makes cracking very hard.

Practice

1. What is the main purpose of using bcrypt in a Node.js application?

easy

A. To securely hash user passwords before storing them

B. To encrypt user data for faster retrieval

C. To compress files on the server

D. To generate random user IDs

Solution

1. Step 1: Understand bcrypt's role

   Bcrypt is designed to hash passwords securely, making them hard to reverse.

2. Step 2: Identify the correct purpose

   Encrypting data or compressing files are not bcrypt's functions; it focuses on password hashing.

3. Final Answer:

   To securely hash user passwords before storing them -> Option A

4. Quick Check:

   Password hashing = Secure storage [OK]

Hint: Bcrypt is for password security, not encryption or compression [OK]

Common Mistakes:

• Confusing hashing with encryption
• Thinking bcrypt compresses data
• Assuming bcrypt generates IDs

2. Which of the following is the correct way to hash a password asynchronously using bcrypt in Node.js?

easy

A. const hashed = await bcrypt.hash(password, 10);

B. const hashed = bcrypt.hashSync(password, 10);

C. const hashed = bcrypt.compare(password, 10);

D. const hashed = bcrypt.hash(password);

Solution

1. Step 1: Identify asynchronous hashing syntax

   Bcrypt's async hash function requires await and two arguments: password and salt rounds.

2. Step 2: Check each option

   hashSync uses the synchronous method, compare is for verification not hashing, and the hash call misses salt rounds.

3. Final Answer:

   const hashed = await bcrypt.hash(password, 10); -> Option A

4. Quick Check:

   Async hash needs await and salt rounds [OK]

Hint: Async bcrypt hash needs await and salt rounds [OK]

Common Mistakes:

• Using hashSync instead of async hash
• Calling compare instead of hash
• Omitting salt rounds argument

3. What will the following code output if the password matches the hash?

const bcrypt = require('bcrypt');

async function check() {
  const password = 'secret123';
  const hash = await bcrypt.hash(password, 10);
  const result = await bcrypt.compare('secret123', hash);
  console.log(result);
}
check();

medium

A. false

B. true

C. undefined

D. Error

Solution

1. Step 1: Understand bcrypt.compare behavior

   It returns true if the plain password matches the hashed password.

2. Step 2: Analyze the code flow

   The password 'secret123' is hashed and then compared with the same password, so result is true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   Matching password returns true [OK]

Hint: Compare returns true if passwords match [OK]

Common Mistakes:

• Expecting false for matching passwords
• Confusing compare with hash
• Ignoring async/await in code

4. Identify the error in this bcrypt usage:

const bcrypt = require('bcrypt');

async function verify() {
  const password = 'mypassword';
  const hash = bcrypt.hash(password, 10);
  const match = await bcrypt.compare(password, hash);
  console.log(match);
}
verify();

medium

A. Incorrect salt rounds value

B. Using compare instead of hash to create hash

C. Password variable is not defined

D. Missing await before bcrypt.hash causing hash to be a Promise

Solution

1. Step 1: Check bcrypt.hash usage

   Bcrypt.hash is async and returns a Promise, so it needs await to get the hashed string.

2. Step 2: Identify the error impact

   Without await, hash is a Promise, causing compare to fail or behave unexpectedly.

3. Final Answer:

   Missing await before bcrypt.hash causing hash to be a Promise -> Option D

4. Quick Check:

   Async hash needs await [OK]

Hint: Always await async bcrypt.hash calls [OK]

Common Mistakes:

• Forgetting await on async functions
• Confusing compare with hash
• Using invalid salt rounds

5. You want to securely store user passwords and verify them later. Which approach correctly combines hashing and verification using bcrypt in an async function?

hard

A. Hash password synchronously with bcrypt.hashSync(password, 12) and verify asynchronously with await bcrypt.compare(input, storedHash)

B. Store the plain password and verify by hashing input with bcrypt.hashSync(input, 12) and comparing strings

C. Hash the password with await bcrypt.hash(password, 12) and verify with await bcrypt.compare(input, storedHash)

D. Encrypt password with bcrypt.encrypt(password) and decrypt for verification

Solution

1. Step 1: Choose secure async hashing and verification

   Using async bcrypt.hash with salt rounds (12) and async bcrypt.compare is best practice.

2. Step 2: Evaluate other options

   Storing plain passwords and hashing input for string comparison is insecure, mixing sync hashing with async verification is inconsistent, and bcrypt has no encrypt/decrypt methods.

3. Final Answer:

   Hash the password with await bcrypt.hash(password, 12) and verify with await bcrypt.compare(input, storedHash) -> Option C

4. Quick Check:

   Async hash and compare with salt rounds = secure [OK]

Hint: Use async hash and compare with salt rounds for security [OK]

Common Mistakes:

• Storing plain passwords
• Mixing sync and async bcrypt calls
• Assuming bcrypt can encrypt/decrypt