Global Sales Contact List

Contact   A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

RSA Laboratories

2.1.4.4 What is Cipher Feedback Mode?

In CFB mode (see Figure 2.4), the previous ciphertext block is encrypted and the output produced is combined with the plaintext block using XOR to produce the current ciphertext block. It is possible to define CFB mode so it uses feedback that is less than one full data block. An initialization vector c0 is used as a "seed" for the process.


ci = Ek(ci-1) Åmi     mi = Ek(ci-1) Åci

Figure 2.4: Cipher Feedback mode (click for a larger image)

CFB mode is as secure as the underlying cipher and plaintext patterns are concealed in the ciphertext by the use of the XOR operation. Plaintext cannot be manipulated directly except by the removal of blocks from the beginning or the end of the ciphertext; see next question for some additional comments. With CFB mode and full feedback, when two ciphertext blocks are identical, the outputs from the block cipher operation at the next step are also identical. This allows information about plaintext blocks to leak. The security considerations for the initialization vector are the same as in CBC mode, except that the attack described in Question 2.1.4.5 is not applicable. Instead, the last ciphertext block can be attacked.

When using full feedback, the speed of encryption is identical to that of the block cipher, but the encryption process cannot be easily parallelized.


Top of the page

Notes:
Connect with EMCConnect with EMC
Need help immediately? EMC Sales Specialists are standing by to answer your questions real time.
Use Live Chat for fast, direct access to EMC Customer Service Professionals to resolve your support questions.
Explore and compare EMC products in the EMC Store, and get a price quote from EMC or an EMC partner.
We're here to help. Send us your sales inquiry and an EMC Sales Specialist will get back to you within one business day.
Want to talk? Call us to speak with an EMC Sales Specialist live.