Secure Programming Cookbook for C and C++

8.7. Prompting for a Password

Problem

You need to prompt an interactive user for a password.

Solution

On Unix systems, you can use the standard C runtime function getpass( ) if you can accept limiting passwords to _PASSWORD_LEN, which is typically defined to be 128 characters. If you want to read longer passwords, you can use the function described in the following Section 8.7.3.

On Windows, you can use the standard EDIT control with ES_PASSWORD specified as a style flag to mask the characters typed by a user.

Discussion

In the following subsections we’ll look at several different approaches to prompting for passwords.

Prompting for a password on Unix using getpass( ) or readpassphrase( )

The standard C runtime function getpass( ) is the most portable way to obtain a password from a user interactively. Unfortunately, it does have several limitations that you may find unacceptable. The first is that only up to _PASSWORD_LEN (typically 128) characters may be entered; any characters after that are simply discarded. The second is that the password is stored in a statically defined buffer, so it is not thread-safe, but ordinarily this is not much of a problem because there is fundamentally no way to read from the terminal in a thread-safe manner anyway.

The getpass( ) function has the following signature:

#include <sys/types.h>
#include <unistd.h>
   
char *getpass(const char *prompt);

The text passed as the function’s only argument is displayed on the terminal, terminal echo is disabled, and input ...