WebApp Sec mailing list archives
Re: (semi-OT): Correct definition of the DES OFB?
From: Saqib Ali <docbook.xml () gmail com>
Date: Sun, 24 Jul 2005 14:29:28 -0700
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/
Current thread:
- (semi-OT): Correct definition of the DES OFB? Saqib Ali (Jul 24)
- RE: (semi-OT): Correct definition of the DES OFB? Clement Dupuis (Jul 24)
- Message not available
- Re: (semi-OT): Correct definition of the DES OFB? Saqib Ali (Jul 24)