Is there any reason not to salt and hash before using bcrypt

Question

I’ve seen some recommendations suggesting that you shouldn’t manually salt and hash a password before passing it into bcrypt, but I’m not sure why.

• Would manually salting interfere with bcrypt’s built-in salting mechanism?
• Could double hashing weaken security instead of strengthening it?
• Is there any scenario where adding an extra layer of hashing before bcrypt is beneficial?

I want to ensure I’m implementing bcrypt correctly without introducing unnecessary complexity or vulnerabilities.

Answer 1

When implementing password hashing with bcrypt, it's important to understand its built-in mechanisms to ensure both security and efficiency.

Bcrypt's Built-in Salting Mechanism

Bcrypt automatically generates a unique salt for each password and incorporates it into the hashing process. This means that even if two users have the same password, their hashed outputs will differ due to the unique salts. The generated salt is embedded within the resulting hash, allowing bcrypt to retrieve it during password verification. Therefore, manually adding a salt before applying bcrypt is unnecessary and can lead to potential issues.

Potential Issues with Manual Pre-Hashing

1. Interference with Bcrypt's Salting: Manually salting and hashing a password before passing it to bcrypt can interfere with bcrypt's internal salting mechanism. This redundancy doesn't enhance security and may complicate the hashing process.

2. Introduction of Null Bytes: Pre-hashing a password using certain hash functions can introduce null bytes into the output. When this pre-hashed output is fed into bcrypt, these null bytes can cause unexpected behavior or vulnerabilities. For instance, if the pre-hashed output contains a leading null byte, it can lead to hash collisions, where different inputs produce the same bcrypt hash.

3. Unintended Reduction in Security: Double hashing or combining multiple hash functions without a thorough understanding can inadvertently weaken security. It's essential to use cryptographic functions as intended rather than creating custom combinations that might introduce vulnerabilities.

Best Practices

• Use Bcrypt as Designed: Rely on bcrypt's built-in salting and hashing mechanisms without adding extra layers of hashing or salting beforehand.

• Avoid Pre-Hashing: Refrain from hashing the password with another algorithm before passing it to bcrypt, as this can introduce vulnerabilities and doesn't provide additional security benefits.

• Stay Updated: Regularly review and adhere to current cryptographic best practices to ensure the security of your password storage mechanisms.

By following these guidelines, you can effectively utilize bcrypt's capabilities to securely hash and store passwords without introducing unnecessary complexity or vulnerabilities.