Distinguish between a block cipher and a stream cipher. What is the strength of Data Encryption Standard(DES)?
Subject Computer and Network Security
NU Year Set: 2.(d) Marks: 3+2 Year: 2014
  1. Block cipher technique involves encryption of one block of text at a time, i.e. singly. Similarly, decrypt the text by taking one block after another. In contrast, Stream cipher technique involves encryption and decryption of one byte of the text at a time.
  2. Block cipher uses both confusion and diffusion while stream cipher relies only on confusion.
  3. The usual size of the block could be 64 or 128 bits in the Block cipher. As against, 1 byte (8 bits) at a time is converted in the stream cipher.
  4. Block cipher uses ECB (Electronic Code Book) and CBC (Cipher Block Chaining) algorithm modes. On the contrary, Stream cipher uses CFB (Cipher Feedback) and OFB (Output Feedback)algorithm modes.
  5. Stream cipher uses XOR function for converting the plain text into cipher text, that is the reason why it is easy to reverse the XORed bits. Whereas Block cipher does not use XOR for doing so.

Strength- The strength of DES lies on two facts:
a. The use of 56-bit keys: a 56-bit key is used in encryption, there are 256 possible keys. A brute force attack on such number of keys is impractical.
b. The nature of the algorithm: Cryptanalyst can perform cryptanalysis by exploiting the characteristic of DES algorithm but no one has succeeded in finding out the weakness.

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