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

["Clement Dupuis" <cdupuis () cccure org>]

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

You cannot go wrong and it is well explained with illustrations.
 
If you wish to get more details and targeted responses, you could post your
questions to CISSP specific mailing list such as the CISSPStudy mailing list
at www.cccure.org
 
Visit http://cccure.org/mailman/listinfo/cisspstudy_cccure.org to subscribe.
 
Take care

Clement
 
Clément Dupuis, CD
President/Security Evangelist/Chief Learning Officer (CLO)
CCCure Enterprise Security & Training Inc.
CISSP, GCFW, GCIA, Security+, CEH, CCSA, MBNS, MBIS, MBHS, CCSE, ACE

Maintainer of :
 
The CISSP and SSCP Open Study Guides Web Site
http://www.cccure.org 

The Professional Security Testers Warehouse
http://www.professionalsecuritytesters.org 

> -----Original Message-----
> From: Saqib Ali [mailto:docbook.xml () gmail com]
> Sent: Sunday, July 24, 2005 9:37 AM
> To: webappsec () securityfocus com; security-basics () securityfocus com!
> Subject: (semi-OT): Correct definition of the DES OFB?
>
> Hello All,
>
> 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.
>
> Am I correct?  Can anyone please validate this? Thanks.
>
> --
> In Peace,
> Saqib Ali
> http://www.xml-dev.com/blog/