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which mode of DES, will a block of plaintext and a key …

In which mode of DES, will a block of plaintext and a key always give the same ciphertext?

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A.
Electronic Code Book (ECB)

B.
Output Feedback (OFB)

C.
Counter Mode (CTR)

D.
Cipher Feedback (CFB)

Explanation:
Electronic Code Book (ECB) is the “native” mode of DES and is a block cipher. ECB is best suited for use with
small amounts of data. It is usually applied to encrypt initialization vectors or encrypting keys. ECB is applied to
64-bit blocks of plaintext, and it produces corresponding 64-bit blocks of ciphertext.
Electronic Code Book (ECB) mode operates like a code book. A 64-bit data block is entered into the algorithm
with a key, and a block of ciphertext is produced. For a given block of plaintext and a given key, the same block
of ciphertext is always produced.
Incorrect Answers:
B: The DES Output Feedback Mode (OFB) is also a stream cipher that generates the ciphertext key by XORing
the plaintext with a key stream. OFB mode is not the mode described in the question.
C: Counter Mode (CTR) is very similar to OFB mode, but instead of using a randomly unique IV value to
generate the keystream values, this mode uses an IV counter that increments for each plaintext block that
needs to be encrypted. CTR mode is not the mode described in the question.
D: The Cipher Feedback Mode (CFB) of DES is a stream cipher where the ciphertext is used as feedback intothe key generation source to develop the next key stream. CFB mode is not the mode described in the
question.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, 2013, p. 803
Krutz, Ronald L. and Russel Dean Vines, The CISSP Prep Guide: Mastering the Ten Domains of Computer
Security, John Wiley & Sons, New York, 2001, p. 143


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