Problem - Kevin Tafuro

Another downside is that there is generally more data to send. Systems that use
sequential nonces can often avoid sending the nonce, as it can be calculated from the
sequence number already sent with the message.
Initialization vectors can be generated using the techniques discussed in Chapter 11.
See Also
• Chapter 11
• Recipes 5.6, 6.21, 8.1
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. *
Discussion
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.
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
#include
#include
* This standard is available from RSA Security at http://www.rsasecurity.com/rsalabs/pkcs/pkcs-5/.
136 | Chapter 4: Symmetric Cryptography Fundamentals
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