Tham khảo tài liệu 'thuật toán algorithms (phần 32)', khoa học tự nhiên, toán học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 303 Exercises 1. Decrypt the following message which was encrypted with a Vigenere cipher using the pattern CAB repeated as necessary for the key on a 27-letter alphabet with blank preceding A DOBHBUAASXFZWJQQ 2. What table should be used to decrypt messages that have been encrypted using the table substitution method 3. Suppose that a Vigenere cipher with a two-character key is used to encrypt a relatively long message. Write a program to infer the key based on the assumption that the frequency of occurrence of each character in odd positions should be roughly equal to the frequency of occurrence of each character in the even positions. 4. Write matching encryption and decryption procedures which use the exclusive or operation between a binary version of the message with a binary stream from one of the linear congruential random number generators of Chapter 3. 5. Write a program to break the method given in the previous exercise assuming that the first 10 characters of the message are known to be blanks. 6. Could one encrypt plaintext by and ing it bit by bit with the key Explain why or why not. 7. True or false Public-key cryptography makes it convenient to send the same message to several different users. Discuss your answer. 8. What is P S M for the RSA method for public-key cryptography 9. RSA encoding might involve computing Mn where M might be a k digit number represented in an array of k integers say. About how many operations would be required for this computation 10. Implement encryption decryption procedures for the RSA method assume that s p and N are all given and represented in arrays of integers of size 25 . 304 SOURCES for String Processing The best references for further information on many of the algorithms in this section are the original sources. Knuth Morris and Pratt s 1977 paper and Boyer and Moore s 1977 paper form the basis for much of the material from Chapter 19. The 1968 paper by Thompson is the basis for the regularexpression pattern .
