II. Basic Cryptographic Algorithms

A method of encryption and decryption is called a cipher. Some cryptographic methods rely on the secrecy of the algorithms; such algorithms are only of historical interest and are... (не соответствуют) for real-world needs. All modern algorithms use a key to control encryption and decryption; a message can be decrypted only if the key matches the encryption key. The key used for decryption... (может отличаться) from the encryption key, but for most algorithms they are the same. There are two classes of key-based algorithms, symmetric (or secret-key) and asymmetric (or public-key) algorithms. The difference is that symmetric algorithms use the same key for encryption and decryption (or the decryption key is easily derived from the encryption key), whereas asymmetric algorithms use a different key for encryption and decryption, and the decryption key cannot be derived from the encryption key.

Symmetric algorithms... (можно разделить) into stream ciphers and block ciphers. Stream ciphers can encrypt a single bit of plaintext at a time, whereas block ciphers take a number of bits (typically 64 bits in modern ciphers), and encrypt them as a single unit.

Asymmetric ciphers... (также называемые) public-key algorithms or generally public-key cryptography) permit the encryption key to be public (it can even be published in a newspaper), allowing anyone to encrypt with the key, whereas only the proper recipient (who knows the decryption key) can decrypt the message. The encryption key is also called the public key and the decryption key the private key or secret key.

Modern cryptographic algorithms cannot really be executed by humans. Strong cryptographic algorithms are designed... (чтобы выполняться) by computers or specialized hardware devices. In most applications, cryptography is done in computer software.

Generally, symmetric algorithms are much faster to execute on a computer than asymmetric ones. In practice they are often... (используются вместе), so that a public-key algorithm is used to encrypt a randomly generated encryption key, and the random key is used to encrypt the actual message using a symmetric algorithm.

Task 2. Answer the following questions:

Задание 2. Ответьте на следующие вопросы:

1. What is a cipher?

2. What are two classes of key-based algorithms?

3. What are symmetric algorithms?

4. What are asymmetric algorithms?

5. What are stream ciphers?

6. What are block ciphers?

7. What is the proper recipient?

8. What is the encryption key?

9. What is the decryption key?

Task 3. Find the English equivalents in the text:

Задание 3 Просматривая текст, найдите эквиваленты следующих выражений:

более не является; с чем не следовало бы иметь дела; теперь время (настал момент); извлечь все преимущества; любые мнения и оценки являются теоретическими (умозрительными); автор не может считаться ответственным; сохранение секретным; представляют только историчес­кий интерес; не имеют ничего общего с реальными нуждами; одновременно; целый ряд (битов); не могут выполняться человеком; гораздо быстрее; в больших книжных магазинах, библиотеках, патентных бюро и в Интернете; хорошо известный.

Task 4. Say whether the statements given below are true or false:

Задание 4. Определите истинность или ложность следующих предложений:

1. Cryptography is still a military term.

2. In cryptographic terminology, the message is called plaintext or cleartext.

3. The process of retrieving the plaintext from the ciphertext is called decryption.

4. Encryption and decryption usually use a key, and decryption can be done only by knowing the proper key.

5. Cryptography deals only with secure messaging.

6. A message can be decrypted only if the key matches the encryption key.

7. Modern cryptographic algorithms can really be executed by humans.

Task 5. Complete the sentences:

Задание 5. Закончите предложения:

1. In cryptographic terminology, the message …..

2. Encoding the contents of the message....

3. Cryptography is the art or science of....

4. Cryptography deals with all aspects of....

5. All modern algorithms use a key to control …..

6. There are two classes of key-based algorithms …..

7. The difference is that symmetric whereas asymmetric algorithms use..., whereas asymmetric algorithms use ….

8. Modern cryptographic algorithms cannot....