Re: AES Questions From Another Dummy.

ECB doesn't seem to
be favored by those who post in this group.
A very nice example of limits of ECB mode and why it should not be
considered a suitable mode of operation (for a message longer than
blocksize) can be found here:
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation
see the pictures of Tux!
I join the other posters here and I'll suggest you to use a ready to
use, well known and well mantained crypto library; CBC or CTR modes
will be good and quite simple to use, however don't miss to see some
examples abut them, to understand them and avoid bad trivial errors
(i.e. reuse a nonce...).
If you can, you should prefer to use modes of operations that give
authentication along with privacy, such as EAX (better than older
classic composition method of a cypher and an hash).
However, you will not only need to focus on the encryption stage, you
should select a crypto library that can give you strong ways to derive
the key (to protect you from bruteforce attacks, dictionary attacks and
precomputed dictionary attacks) like PBKDF2 and a good pseudo random
number generator, to be intended as a strong way to derive high quality
pseudo random sequence and as an interface to collect enthropy from the
system (system and hardware events, user actions, system-specific
enthropy collectors etc).
With a system that collect a reasonable amount of enthropy AND uses it
in an efficient way for PRNG AND with a robust key derivation AND with
a robust cypher used in a smart mode of operation, the implementation
should be ok as things can be ok on a general purpouse computer (aka, a
system not meant to resist to hardware tampering, prone to remote EM
interception, running very complex operating systems with a lot of
complex software...).

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