Problem - Kevin Tafuro

Initialization requires a key to use for encrypting and MAC’ing the data in cookies.
The implementation of CWC described in Recipe 5.10 can use keys that are 128,
192, or 256 bits in size. Before calling spc_cookie_init( ), you should create a key
using spc_rand( ), as defined in Recipe 11.2. If the cookies you are sending to the client
are persistent, you should store the key on the server so that the same key is
always used, rather than generating a new one every time the server starts up. You
can either hardcode the key into your program or store it in a file somewhere that is
inaccessible through the web server so that you are sure it cannot be compromised.
#include
#include
#include
static cwc_t spc_cookie_cwc;
static unsigned char spc_cookie_nonce[11];
int spc_cookie_init(unsigned char *key, size_t keylen) {
memset(spc_cookie_nonce, 0, sizeof(spc_cookie_nonce));
return cwc_init(&spc_cookie_cwc, key, keylen * 8);
}
To encrypt and MAC the data to send in a cookie, use the following spc_cookie_
encode( ) function, which requires two arguments:
cookie
Data to be encrypted and MAC’d. spc_cookie_encode( ) expects the data to be a
C-style string, which means that it should not contain binary data and should be
NULL terminated.
nonce
11-byte buffer that contains the nonce to use (see Recipe 4.9 for a discussion of
nonces). If you specify this argument as NULL, a default buffer that contains all
NULL bytes will be used for the nonce.
The problem with using a nonce with cookies is that the same nonce must be used
for decrypting and verifying the integrity of the data received from the client. To be
able to do this, you need a second plaintext cookie that allows you to recover the
nonce before decrypting and verifying the encrypted cookie data. Typically, this
would be the user’s name, and the server would maintain a list of nonces that it has
encoded for each logged-in user.
If you do not use a nonce, your system will be susceptible to capture
replay attacks. It is worth expending the effort to use a nonce.
The return from spc_cookie_encode( ) will be a dynamically allocated buffer that
contains the base64-encoded ciphertext and MAC of the data passed into it. You are
responsible for freeing the memory by calling free( ).
420 | Chapter 8: Authentication and Key Exchange
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