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Table Of Contents  CertiGuide to Security+
 9  Chapter 4:  Basics of Cryptography (Domain 4.0; 15%)
      9  4.1  Algorithms
           9  4.1.2  Symmetric

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4.1.2  Symmetric
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Security Issues with Symmetric Cryptography
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Types of Symmetric Algorithms

There are two primary types of symmetric algorithms: block and stream ciphers (coding methods). In a block cipher, data is broken into blocks of a certain size and encrypted (and later decrypted) block-by-block, with the same key. Stream ciphers work the same way but work on a bit-by-bit basis. A combination of the two, cipher-block-chaining, encrypts block by block, but the encryption of any block is dependent on the results from encrypting the prior block.

DES, Triple DES, IDEA, RC2 and AES are examples of symmetric cryptography. While DES is the most well known, it is also an older algorithm and not as strong as the others. For example, the Electronic Frontier Foundation has broken DES encryption with a brute force attack in less than a day (to read about it, see Cracking DES: Secrets of Encryption Research, Wiretap Politics and Chip Design391 by the Electronic Frontier Foundation), so this is clearly not an encryption technology we’d want our most sensitive government or commercial traffic to use.

Figure 39: The basis of symmetric encryption is the same key is used to cipher and decode the data.

 


Block & Stream Cipher

Block cipher and stream cipher are the two types of symmetric algorithms. Block ciphers encrypt an entire block of data at a time, independent from other message data; stream ciphers encrypt bit-by-bit. A combination of the two, cipher-block-chaining, encrypts block by block, but the encryption of any block is dependent on the results from encrypting the prior block. Block ciphers are best suited for large sets of data, stream ciphers for bit-oriented traffic like streaming audio or video.

Some well-known symmetric algorithms include: DES, Triple DES, RC2, IDEA and AES. DES, which uses a 56-bit key, is considered breakable, and weaker than many other symmetric algorithms.



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391. Electronic Frontier Foundation, Cracking DES: Secrets of Encryption Research, Wiretap Politics and Chip Design, O’Reilly and Associates, August, 1998, http://www.nerdbooks.com/item.html?id=1565925203

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