[cryptography] "Combined" cipher modes

Kevin W. Wall kevin.w.wall at gmail.com
Mon Feb 20 02:11:52 EST 2012


Hi list,

This should be a pretty simple question for this list, so please pardon
my ignorance. But better to ask than to continue in ignorance. :-)

NIST refers to "combined" cipher modes as those supporting *both*
authenticity and confidentiality, such as GCM and CCM.

So my first question: Are there ANY "combined" cipher modes
for block ciphers that do not cause the ciphers to act as a key
stream? (That seems to be cause most of the ones I found build
the confidentiality piece around CTR mode.) If "yes", please name
a few (especially those with no patent restrictions).

I know when you have a cipher that acts in a streaming mode,
that you need to be careful to use a unique IV for every encryption
performed with the same key.

So my second question is, if all the "combined" cipher modes all
cause a cipher to act as if it is in a streaming mode, is it okay
to just choose a completely RANDOM IV for each encryption?
Because it sure doesn't seem to be feasible to record all the IVs
for a given key to make sure that an IV isn't reused. If that is not
acceptable, then how does one ever address this?

Thanks,
-kevin
-- 
Blog: http://off-the-wall-security.blogspot.com/
"The most likely way for the world to be destroyed, most experts agree,
is by accident. That's where we come in; we're computer professionals.
We *cause* accidents."        -- Nathaniel Borenstein



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