> For the complete documentation index, see [llms.txt](https://alham-rizvi.gitbook.io/alhamrizvi/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alham-rizvi.gitbook.io/alhamrizvi/cryptography/hashing/20.-bcrypt.md).

# 20. bcrypt

### 🔐 bcrypt — Full Understanding

#### What bcrypt actually is

bcrypt is a **one-way hash function** built specifically for passwords.

* Input → password
* Output → fixed-length hash
* You **cannot reverse it back** to the original password

So:

* ❌ No decryption
* ❌ No “decode”
* ✅ Only **verify**

***

### ⚙️ How bcrypt works (internals simplified)

bcrypt is based on:

* Blowfish (modified version)

Process:

1. Takes password + salt
2. Runs **expensive key setup** (slow on purpose)
3. Applies multiple rounds (cost factor)
4. Produces hash

***

### 🔑 Structure of a bcrypt hash

Example:

```
$2b$10$EIXhF0lV8Jt7z1y9lYz1eO8J1nZc7fQ6z5bR6F9sYzTjK8w1Q2e3G
```

Breakdown:

* `$2b$` → version
* `10` → cost factor (2^10 = 1024 rounds)
* next part → salt
* last part → hash

***

### 🔁 “Encryption vs Decryption” in bcrypt terms

bcrypt replaces this concept:

| Concept | In Encryption     | In bcrypt              |
| ------- | ----------------- | ---------------------- |
| Encode  | Encrypt           | Hash                   |
| Decode  | Decrypt           | ❌ Impossible           |
| Check   | Decrypt & compare | ✅ Hash input & compare |

***

### ✅ How verification works (IMPORTANT)

When a user logs in:

1. Stored hash:

```
$2b$10$abc...
```

2. User enters password → `"hello123"`
3. System:

* Extracts salt + cost from stored hash
* Re-hashes `"hello123"` with same settings

4. Compare:

* If same → password correct
* If not → wrong

👉 This is the **only way bcrypt “checks” passwords**

***

### 🧠 Why bcrypt is secure

bcrypt is designed to resist attacks:

#### 1. Slow by design

* Adjustable cost factor (e.g., 10, 12, 14)
* Slows brute force massively

#### 2. Built-in salt

* Every hash is unique
* Prevents rainbow tables

#### 3. Adaptive

* You can increase cost over time

***

### ⚠️ Attacks against bcrypt (not decryption)

Since decryption is impossible, attackers try:

#### 🔹 Brute force

Try passwords one by one

#### 🔹 Dictionary attack

Use common password lists

#### 🔹 GPU cracking (limited)

bcrypt is intentionally slow → GPUs less effective

Tools:

* Hashcat
* John the Ripper

***

### 🔥 Important mindset (CTF / real-world)

If you see bcrypt:

* Don’t think: “how to decrypt?”
* Think:
  * weak password?
  * can I brute-force?
  * can I bypass auth instead?

***

### 🧪 Example (code flow)

```python
import bcrypt

password = b"hello123"

# hash
hashed = bcrypt.hashpw(password, bcrypt.gensalt())

# verify
bcrypt.checkpw(password, hashed)  # True
```


---

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