Confidentiality Using Symmetric Encryption

Confidentiality Using Symmetric Encryption

7.1 Placement of Encryption Function
Potential Locations for Confidentiality Attacks Link versus End-to-End Encryption
7.2 Traffic Confidentiality
Link Encryption Approach End-to-End Encryption Approach
7.3 Key Distribution
A Key Distribution Scenario Hierarchical Key Control Session Key Lifetime A Transparent Key Control Scheme Decentralized Key Control Controlling Key Usage
7.4 Random Number Generation
The Use of Random Numbers Pseudorandom Number Generators (PRNGs) Linear Congruential Generators Cryptographically Generated Random Numbers Blum Blum Shub Generator True Random Number Generators Skew
7.5 Recommended Reading and Web Sites
7.6 Key Terms, Review Questions, and Problems
Key Terms Review Questions Problems

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Amongst the tribes of Central Australia every man, woman, and child has a secret or sacred name which is bestowed by the older men upon him or her soon after birth, and which is known to none but the fully initiated members of the group. This secret name is never mentioned except upon the most solemn occasions; to utter it in the hearing of men of another group would be a most serious breach of tribal custom. When mentioned at all, the name is spoken only in a whisper, and not until the most elaborate precautions have been taken that it shall be heard by no one but members of the group. The native thinks that a stranger knowing his secret name would have special power to work him ill by means of magic.

The Golden Bough, Sir James George Frazer

John wrote the letters of the alphabet under the letters in its first lines and tried it against the message. Immediately he knew that once more he had broken the code. It was extraordinary the feeling of triumph he had. He felt on top of the world. For not only had he done it, had he broken the July code, but he now had the key to every future coded message, since instructions as to the source of the next one must of necessity appear in the current one at the end of each month.

Talking to Strange Men, Ruth Rendell

Key Points In a distributed environment, encryption devices can be placed to support either link encryption or end-to-end encryption. With link encryption, each vulnerable communications link is equipped on both ends with an encryption device. With end-to-end encryption, the encryption process is carried out at the two end systems. Even if all traffic between users is encrypted, a traffic analysis may yield information of value to an opponent. An effective countermeasure is traffic padding, which involves sending random bits during periods when no encrypted data are available for transmission. Key distribution is the function that delivers a key to two parties who wish to exchange secure encrypted data. Some sort of mechanism or protocol is needed to provide for the secure distribution of keys. Key distribution often involves the use of master keys, which are infrequently used and are long lasting, and session keys, which are generated and distributed for temporary use between two parties. A capability with application to a number of cryptographic functions is random or pseudorandom number generation. The principle requirement for this capability is that the generated number stream be unpredictable.

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Historically, the focus of cryptology has been on the use of symmetric encryption to provide confidentiality. It is only in the last several decades that other considerations, such as authentication, integrity, digital signatures, and the use of public-key encryption, have been included in the theory and practice of cryptology.

Before examining some of these more recent topics, we concentrate in this chapter on the use of symmetric encryption to provide confidentiality. This topic remains important in itself. In addition, an understanding of the issues involved here helps to motivate the development of public-key encryption and clarifies the issues involved in other applications of encryption, such as authentication.

We begin with a discussion of the location of encryption logic; the main choice here is between what are known as link encryption and end-to-end encryption. Next, we look at the use of encryption to counter traffic analysis attacks. Then we discuss the difficult problem of key distribution. Finally, we discuss the principles underlying an important tool in providing a confidentiality facility: random number generation.