Secure Programming Cookbook for C and C++

8.5. Generating Random Passwords and Passphrases

Problem

You would like to avoid problems with easy-to-guess passwords by randomly generating passwords that are difficult to guess.

Solution

For passwords, choose random characters from an acceptable set of characters using spc_rand_range( ) (see Recipe 11.11). For passphrases, choose random words from a predefined list of acceptable words.

Discussion

In many situations, it may be desirable to present a user with a pregenerated password. For example, if the user is not present at the time of account creation, you will want to generate a reasonably secure password for the account and deliver the password to the user via some secure mechanism such as in person or over the phone.

Randomly generated passwords are also useful when you want to enforce safe password requirements. If the user cannot supply an adequately secure password after a certain number of attempts, it may be best to present her with a randomly generated password to use, which will most likely pass all of the requirements tests.

The primary disadvantage of randomly generated passwords is that they are usually difficult to memorize (and type), which often results in users writing them down. In many cases, however, this is a reasonable trade-off.

The basic strategy for generating a random password is to define a character set that contains all of the characters that are valid for the type of password you are generating, then choose random members of that set until enough characters ...