Put the verbs in brackets into the correct tenses. Conditional sentences: type 2.

1. If I had a typewriter I (type) it myself. 2. If I (know) his address I'd give it to you. 3. He (look) a lot better if he shaved more often. 4. If he worked more slowly he (not make) so many mistakes. 5. More tourists would come to this country if it (have) a better climate. 6. If someone (give) you a helicopter what would you do with it? 7. If he (clean) his windscreen he'd be able to see where he was going. 8. If he knew that it was dangerous he (not come). 9. The whole machine would fall to pieces if you (remove) that screw. 10. I'd go and see him more often if he (live) on a bus route.

 

Put the verbs in brackets into the correct tenses. Conditional sentences: type 3.

1. If I had known that you were in hospital I (visit) you. 2. If you (arrive) ten minutes earlier you would have got a seat. 3. I shouldn't have believed it if I (not see) it with my own eyes.4. If he had asked you, you (accept)? 5. If I (had) a map I would have been all right. 6. I (offer) to help him if I had realized that he was ill. 7. If I had realized that the traffic lights were red I (stop). 8. But for the fog we (reach) our destination ages ago. 9. If he had known that the river was dangerous he (not try) to swim across it. 10. If you (speak) more slowly he might have understood you. 11. If he had known the whole story he (not be) so angry. 12. If you (look) at the engine for a moment you would have seen what was missing.

 

Translate the sentences.

A. 1. Если бы я знал французский, я бы помог вам. 2. Если бы он был более пунктуален, с ним было бы легче иметь дело.3. Если бы он получил ваше письмо, он бы приехал раньше.4. Если бы вы не вмешались, они, возможно, спорили бы долго. 5. Если бы он был более опытен, он не сделал бы такой ошибки. 6. Если бы вы позвонили ему вчера, сейчас бы вы не волновались.

7. Жаль, что Оскара там не было. Он бы рассказал им много инте­ресных историй.

В. 1. Если бы я знал м-ра Робертсона, я бы поговорил с ним. 2. Если бы Ричард был здесь, он дал бы вам другой совет. 3. Если бы язнал, что Смиты были тогда в Париже, я бы зашел к ним. 4. Если бы старик не купил галерею, его картина не получила бы премию. 5. Если бы у него было чувство юмора, он бы не рассердился 6. Если бы вы извинились перед ним тогда, он бы на вас сейчас не сердился. 7. Почему бы вам не принять участие в завтрашнем соревновании? Вы могли бы получить приз.

 

Summarize the text.

Text B

Substitution Сiphers.

A substitution cipher is one in which the units of the plaintext (usually letters or numbers) are replaced with other symbols or groups of symbols. The actual order of the units of the plaintext is not changed. The simplest substitution cipher is one where the alphabet of the cipher is merely a shift of the plaintext alphabet, for example, A might be encrypted as В, С as D and so forth. Of this type of cipher, the most well-known is Caesar cipher, used by Julius Caesar in which A becomes D etc. It is easy to guess that cyclical-shift substitution ciphers of this sort are not at all secure because individual letter frequencies are left completely intact.

Product ciphers. In the days of manual cryptography i. e. without the aid of the computer product ciphers were a useful device for the cryptographer and double transposition ciphers on keyword-based matrices were, in fact, widely used. There was also some use of a particular class of product ciphers called fractionation systems. In a fractionation system a substitution is first made from symbols in the plaintext to multiple symbols (usually pairs, in which case the cipher is called bilateral cipher) in the ciphertext, which is then superencrypted by a transposi­tion.

Block ciphers. Generally, ciphers transform pieces of the plaintext of a fixed size into ciphertext. In older, manual sys­tems, these pieces were usually single letters or characters (or sometimes, as in the Playfair cipher, digraphs), since these were the largest units that could be easily encrypted or decrypted by hand. Although systems which operated on sets of three charac­ters and other, larger groups of numbers were proposed and un­derstood to potentially be more secure they were implemented because of the extra difficulty in the manual encryption or de­cryption process. In modern, single key cryptography however, the units of information can be much larger.

A block cipher is a type of symmetric-key encryption al­gorithm that changes a fixed-length block of the plain text into the same length of cipher text. The encryption works by means of a key. Decryption is a simply the reverse of the encryption process using the same secret key. The fixed length is called the block size and for modern block ciphers is usually 64 bits. As processors become more sophisticated, however, it is likely that this "block” size will increase to 128 bits.

Stream ciphers. A stream cipher also breaks the plaintext into units, this time it is normally a single character. It then en­crypts the n* unit of the plaintext with the в* unit of the key stream. Stream ciphers can be designed to be exceptionally fast, much faster than any block cipher. While the encryption of any particular plaintext with a block cipher will result in the same ciphertext when the same key is used. With a stream cipher, the transformation of the smaller plaintext units will vary, depending on when they are encountered during the encryption process.

A stream cipher generates what is known as a keystream — a sequence of bits, which is used as a key. The encryption process involves combining the keystream with the plaintext. The keystream can be generated in two ways.

1. Independent of the plaintext and ciphertext (this yields what is known as a synchronous stream cipher).

2. Depending on the data and its encryption(in which case the stream cipher is said to be self-synchronising).

In a chosen plaintext attack the cryptanalyst has the capa­bility to find the ciphertext corresponding to an arbitrary plain­text message of his or her own choosing. The likelihood of this type of attack being possible is not much. Codes which can sur­vive this attack are considered to be very secure.

In a chosen ciphertext attack the cryptanalyst can choose an arbitrary cipher-text and find the corresponding decrypted plaintext. This attack can be used in public key systems, where it may reveal the private key.

In an adaptive chosen plaintext attack the cryptoanalyst can determine the ciphertext of chosen plaintexts in an iterative process based on previous results. This is the general name for a method of attacking product ciphers called "differential cryptoanalysis".