Libcrypt examples?

[Robert J. Hansen]

> I have a program that will frequently need to store modest-size
> chunks of of data on disk, perhaps 1-4kB per chunk.  The data is
> sensitive, but not ultra top secret.  I would like to make a
> reasonable effort to keep it from prying eyes.

Please don't take this the wrong way, but -- please don't.  Libgcrypt is
not particularly friendly to novices.  It exposes a *lot* of dials and
switches in the interests of letting experts do weird and useful things.
 Novices will be better-suited with something like Peter Gutmann's
cryptlib, which is high-quality and well-regarded and is probably more
newbie-friendly.

> P.S.  On my FreeBSD system, un-updated as it may be, there seems
> to be a crypt(3) in the standard C library.

I believe on FreeBSD this is just bog-standard DES, but I could be
mistaken.  DES is not a strong cipher.

> Fortunately, I *do* have something that's a bit faster than A Pentium 166
> :-) but I'm still rather baffled by the meaning of the phrase "designed
> to be time-consuming" in this context.

To help foil brute-force attacks.  crypt(3) is normally used with really
short pieces of text -- passwords.  As such, one way to attack passwords
is to get a large dictionary of words and run each word through crypt(3)
and store the result.  If you want to break a password, look at its
crypt(3)ed value and compare it to your database of computed values.  If
you get a hit, then look back at what the original word was.

To foil these sorts of attacks ("dictionary attacks"), crypt(3) has been
built to be very, very slow.