Focused Practice

I. Answer the following questions:

1. Why do digitized images demand large amounts of computer memory?

2. How many megabytes of memory do computers have nowadays?

3. What ratios can current compression techniques achieve?

4. What terms does traditional computer graphics encode images in?

5. Does a standard graphics system work well when the problem is to encode a sunset or a cloud?

6. What do we need to escape this difficulty?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: Image compression.

Unit 66 Grammar: The Past Simple Tense Word List:

1. cryptography	криптография, шифрование
2. information age	век информации
3. cryptographic algorithm	алгоритм шифрования
4. cryptosystem	система шифрования
5. ad hoc	временный
6. Data Encryption Standard (DES)	стандарт шифрования данных, стандарт DES
7. national	гражданин
8. non-combatant	нестроевой, гражданский
9. work factor	показатель (фактор) трудозатрат, времени на расшифровку

The Advanced Encryption Standard

Cryptography was once the domain of generals and small children, but the advent of the Information Age changed that. In the early 1970s the National Security Agency (NSA) and the National Bureau of Standards (NBS) realized that noncombatant adults needed to protect their sensitive, but unclassified, information.

NBS issued a public solicitation for a cryptographic algorithm. IBM responded. The company submitted a cryptosystem with a 56-bit key. A conventional cryptosystem is considered secure when its work factor – the amount of time needed to decrypt – is about 2^{key length}. The new algorithm became the data Encryption Standard (DES). With narrow exceptions, products incorporating DES could not be exported.

A 1996 National Research Council report on cryptography policy recommended an immediate loosening of export controls. No changes occurred until 1998, when a $250,000 special-purpose machine built by the Electronic Frontier Foundation cracked a DES-encrypted message in 56 hours. At that point U.S. export controls were relaxed to permit DES in exported products. In recent months export controls have been lifted even further, with no limit on number of bits.

A DES replacement was overdue. In 1997 the National Institute of Standards and Technology (NIST, formerly the National Bureau of Standards) announced a competition for the algorithm’s replacement and held public meetings to discuss the criteria for a proposed Advanced Encryption Standard (AES). Key length was most important. A 1996 ad hoc committee argued 90 bits was currently the minimum key length needed to provide data security for twenty years. NIST sought that much security and more – encrypted files should remain confidential well after AES was retired. NIST settled on a minimum key length of 128 bits.

NIST allowed foreign submissions and foreign viewing of the candidates. A foreign national who wanted software implementations of the candidates could have them The person had to register with NIST and promise not to pass on the algorithms (even if obtained from another source). While within the U.S. export-control laws, in spirit, this system formed a contrast to the export rules so recently enforced regarding DES.