4.2. Iteration Count

4.2. Iteration Count

An iteration count has traditionally served the purpose of increasing the cost of producing keys from a password, thereby also increasing the difficulty of attack. Mathematically, an iteration count of c will increase the security strength of a password by log2(c) bits against trial-based attacks like brute force or dictionary attacks.

Choosing a reasonable value for the iteration count depends on environment and circumstances, and varies from application to application. This document follows the recommendations made in FIPS Special Publication 800-132 [NISTSP132], which says

The iteration count shall be selected as large as possible, as long as the time required to generate the key using the entered password is acceptable for the users. [...] A minimum iteration count of 1,000 is recommended. For especially critical keys, or for very powerful systems or systems where user-perceived performance is not critical, an iteration count of 10,000,000 may be appropriate.