> 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/18_bcrypt.md).

# bcrypt

## What is bcrypt?

bcrypt is a **password hashing function** designed by Niels Provos and David Mazières in 1999, based on the Blowfish cipher. It is specifically designed to be **slow and computationally expensive**, making brute-force attacks difficult.

***

## Why bcrypt?

* Built-in **salt** generation (prevents rainbow table attacks)
* **Adaptive cost factor** (work factor can be increased as hardware improves)
* Widely supported across languages and frameworks
* Time-tested and battle-hardened

***

## How bcrypt Works

```
password + salt → bcrypt(cost factor) → 60-char hash string
```

### Hash Format

```
$2b$12$SSSSSSSSSSSSSSSSSSSSSSHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH
│   │  │                    │
│   │  └── Salt (22 chars)  └── Hash (31 chars)
│   └── Cost factor (work factor)
└── Version ($2a$, $2b$, $2y$)
```

### Example Output

```
$2b$12$LQv3c1yqBWVHxkd0LHAkCOYz6TtxMQJqhN8/LewdBPj6y8T9G6hm6
```

***

## Cost Factor (Work Factor)

| Cost | Approx. Time (modern CPU) |
| ---- | ------------------------- |
| 10   | \~100ms                   |
| 12   | \~400ms                   |
| 14   | \~1.5s                    |
| 16   | \~6s                      |

Higher cost = slower hashing = harder to crack.

***

## Implementation Examples

### Python

```python
import bcrypt

# Hashing
password = b"mysecretpassword"
salt = bcrypt.gensalt(rounds=12)
hashed = bcrypt.hashpw(password, salt)
print(hashed)

# Verifying
if bcrypt.checkpw(password, hashed):
    print("Password matches!")
```

### Node.js

```javascript
const bcrypt = require('bcrypt');

// Hashing
const saltRounds = 12;
const hash = await bcrypt.hash('mysecretpassword', saltRounds);

// Verifying
const match = await bcrypt.compare('mysecretpassword', hash);
console.log(match); // true
```

### Go

```go
import "golang.org/x/crypto/bcrypt"

// Hashing
hash, err := bcrypt.GenerateFromPassword([]byte("password"), 12)

// Verifying
err = bcrypt.CompareHashAndPassword(hash, []byte("password"))
```

***

## Internals: EksBlowfish Algorithm

1. Takes password + salt + cost
2. Runs `2^cost` rounds of key setup
3. Encrypts the string `"OrpheanBeholderScryDoubt"` 64 times
4. Returns the resulting ciphertext as the hash

***

## Limitations

* Max password length: **72 bytes** (longer passwords are silently truncated)
* Not ideal for bulk data encryption (designed only for passwords)
* Slower than Argon2 in memory-hardness

***

## bcrypt vs Other Password Hashing Functions

| Feature        | bcrypt | scrypt | Argon2  |
| -------------- | ------ | ------ | ------- |
| Salt built-in  | ✅      | ✅      | ✅       |
| Memory-hard    | ❌      | ✅      | ✅       |
| GPU resistance | Medium | High   | Highest |
| Year           | 1999   | 2009   | 2015    |
| PHC Winner     | ❌      | ❌      | ✅       |

***

## Attacks Against bcrypt

| Attack                      | Effectiveness                      |
| --------------------------- | ---------------------------------- |
| Brute Force                 | Very slow due to cost factor       |
| Rainbow Tables              | Prevented by built-in salt         |
| GPU Acceleration            | Limited (bcrypt is GPU-unfriendly) |
| Password truncation exploit | Possible if passwords > 72 bytes   |

***

## Best Practices

* Use cost factor **12+** for production
* Always use the **verify function** (never re-hash to compare)
* Update cost factor periodically as hardware improves
* Handle the **72-byte limit** by pre-hashing with SHA-256 if needed


---

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