Re: (semi-OT): Correct definition of the DES OFB?

[Saqib Ali <docbook.xml () gmail com>]

Hello Clement,

> The best reference for any of the DES Modes of operation is FIPS81 at:
> http://www.columbia.edu/~ariel/ssleay/fip81/fip81.html

Thanks for the website. I went through it. After reading the
differences between CFB  and OFB modes as described on the URL above,
I think Shon Harris' book has the the wrong description of the OFB
mode. However the description for the CFB mode is correct. Hopefully
this will be fixed in the 3rd edition of the book. :)


> > I was reading the CISSP Exam Guide by Shon Harris  (2nd Edition). She
> > gives the following definition for DES in the OFB (Output Feedback)
> > Mode.
> >
> > "...if DES is working in Output Feedback (OFB) Mode, it is functioning
> > like a stream cipher by generating a stream of random binary bits to
> > be combined with the plaintext  to create ciphertext. The ciphertext
> > is fed back to the algorithm to form a portion of the next input to
> > encrypt the next stream of bits." (page 486)
> >
> > I thin, the last sentence "The ciphertext is fed back to the algorithm
> > to form a portion of the next input to encrypt the next stream of
> > bits" is incorrect. That is what happen in the Cipher Feeback (CFB)
> > Mode, and NOT theOFB mode.
> >
> > In OFB mode, the "random value" is fed back into the algorithm. i.e.
> > the "Random Value" from the previous operation becomes the IV for the
> > next operation.
-- 
In Peace,
Saqib Ali
http://www.xml-dev.com/blog/