Secure Programming Cookbook for C and C++

4.10. Deriving Symmetric Keys from a Password

Problem

You do not want passwords to be stored on disk. Instead, you would like to convert a password into a cryptographic key.

Solution

Use PBKDF2, the password-based key derivation function 2, specified in PKCS #5.^[3]

Tip

You can also use this recipe to derive keys from other keys. See Recipe 4.1 for considerations; that recipe also discusses considerations for choosing good salt values.

Discussion

Passwords can generally vary in length, whereas symmetric keys are almost always a fixed size. Passwords may be vulnerable to guessing attacks, but ultimately we’d prefer symmetric keys not to be as easily guessable.

The function spc_pbkdf2( ) in the following code is an implementation of PKCS #5, Version 2.0. PKCS #5 stands for "Public Key Cryptography Standard #5,” although there is nothing public-key-specific about this standard. The standard defines a way to turn a password into a symmetric key. The name of the function stands for “password-based key derivation function 2,” where the 2 indicates that the function implements Version 2.0 of PKCS #5.

#include <stdio.h> #include <string.h> #include <openssl/evp.h> #include <openssl/hmac.h> #include <sys/types.h> #include <netinet/in.h> #include <arpa/inet.h> /* for htonl */ #ifdef WIN32 typedef unsigned _ _int64 spc_uint64_t; #else typedef unsigned long long spc_uint64_t; #endif /* This value needs to be the output size of your pseudo-random function (PRF)! */ #define PRF_OUT_LEN 20 /* This ...