Problem - Kevin Tafuro

Distinguisher Selection
The basic idea behind a distinguisher is that it must be unique.
If you want to create a particular derived key, we recommend that you string together
in a predetermined order any interesting information about that key, separating data
items with a unique separation character (i.e., not a character that would be valid in
one of the data items). You can use alternate formats, as long as your data representation
is unambiguous, in that each possible distinguisher is generated by a single,
unique set of information.
As an example, let’s say you want to have a different session key that you change once
a day. You could then use the date as a unique distinguisher. If you want to change keys
every time there’s a connection, the date is no longer unique. However, you could use
the date concatenated with the number of times a connection has been established on
that date. The two together constitute a unique value.
There are many potential data items you might want to include in a distinguisher, and
they do not have to be unique to be useful, as long as there is a guarantee that the distinguisher
itself is unique. Here is a list of some common data items you could use:
• The encryption algorithm and any parameters for which the derived key will be
used
• The number of times the base key has been used, either overall or in the context
of other interesting data items
• A unique identifier corresponding to an entity in the system, such as a username
or email address
• The IP addresses of communicating parties
• A timestamp, or at least the current date
• The MAC address associated with the network interface being used
• Any other session-specific information
In addition, to prevent against any possible offline precomputation attacks, we recommend
you add to your differentiator a random salt of at least 64 bits, which you then
communicate to any other party that needs to derive the same key.
Here is an example implementation of a PRF based on HMAC-SHA1, using the
OpenSSL API for HMAC (discussed in Recipe 6.10):
#include
#include
#include
#include
#include
#define HMAC_OUT_LEN 20 /* SHA1 specific */
void spc_make_derived_key(unsigned char *base, size_t bl, unsigned char *dist,
size_t dl, unsigned char *out, size_t ol) {
144 | Chapter 4: Symmetric Cryptography Fundamentals
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