Secure Programming Cookbook for C and C++

8.9. Performing Password-Based Authentication with crypt( )

Problem

You need to use the standard Unix crypt( ) function for password-based authentication.

Solution

The standard Unix crypt( ) function typically uses a weak one-way algorithm to perform its encryption, which is usually also slow and insecure. You should, therefore, use crypt( ) only for compatibility reasons.

Despite this limitation, you might want to use crypt( ) for compatibility purposes. If so, to encrypt a password, choose a random salt and call crypt( ) with the plaintext password and the chosen salt. To verify a password encrypted with crypt( ), encrypt the plaintext password using the already encrypted password as the salt, then compare the result with the already encrypted password. If they match, the password is correct.

Discussion

Tip

What we are doing here isn’t really encrypting a password. Actually, we are creating a password validator. We use the term encryption because it is in common use and is a more concise way to explain the process.

The crypt( ) function is normally found in use only on older Unix systems that still exclusively use the /etc/passwd file for storing user information. Modern Unix systems typically use stronger algorithms and alternate storage methods for user information, such as the Lightweight Directory Access Protocol (LDAP), Kerberos (see Recipe 8.13), NIS, or some other type of directory service.

The traditional implementation of crypt( ) uses DES (see Recipe 5.2 for a discussion of ...