Jun 30, 2012

Bcrypt: Choosing a Work Factor

Bcrypt is a Blowfish-based hashing algorithm which is commonly used for password hashing because of its potentially expensive key setup phase. A Bcrypt hash has the following structure:

$2a$(2 chars work)$(22 chars salt)(31 chars hash)

The reason that the key setup phase can be potentially expensive is because it is run 2 times. As password hashing is usually associated with common tasks like logging a user into a system, it’s important to find the right balance between security and performance. Using a high work factor makes it incredibly difficult to execute a brute-force attack, but can put unnecessary load on the system.

Using Marco Arment’s PHP Bcrypt class, I performed some benchmarks to determine how long it takes to hash a string with various work factors:

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