How Does Bcrypt Verify Hashed Passwords with Random Salts?

Understanding Bcrypt and Randomly Generated Salts

Bcrypt, an industry-standard algorithm for securely hashing passwords, employs a random salt to enhance password protection. However, users may wonder how the verification process works while accounting for this seemingly random element.

Salt's Role

The salt serves as a unique prefix added to the password before hashing. This randomized value ensures that each password produces a distinct hash even if identical. By using an unpredictable salt, attackers cannot precompute password hashes, making it significantly harder to compromise user accounts.

Structure of the Hashed Password

While the salt is randomly generated, it is included in the resulting hashed password. The hashed password comprises several parts, including:

• Algorithm type (in this case, bcrypt)
• Cost parameter (determining the computational intensity of hashing)
• Random salt
• Hashed password

Verification Process

When verifying a password, the hashed password is provided to the bcrypt algorithm. The algorithm extracts the salt portion and uses it to hash the provided password.

This process essentially mirrors the initial hashing operation that generated the hashed password. If the newly generated hash matches the stored hash, the provided password is verified as correct.

Example

Consider a hashed password generated for the password "password":

y$abcdefg...123456789...

• $2y indicates bcrypt algorithm
• 10 is the cost parameter
• abcdefg... is the salt
• 123456789... is the hashed password

To verify if "password" is correct, the following is used:

crypt("password", "y$abcdefg...123456789...")

The result of this operation will be identical to the originally generated hash if "password" is correct. This is because the salt is contained within the hashed password, enabling the recreation of the original hashing operation.

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