In Example 5.16 we declared but did not implement three operators for the Point3 class. Add implementations for these three operators and add tests to the client code.

In this exercise, we will reuse the random() function from the <cstdlib> (Appendix B).

random() generates a pseudo-random integer in the range from 0 to RAND_MAX (commonly set to 2147483647).

Write a function

 int myRand(int min, int max); 

that returns a pseudo-random int in the range from min to max - 1.

Write the function

QVector<int> randomPerm(int n, unsigned key); 

that uses the myRand() function (seeded with key) to produce a permutation of the numbers 0, ... n.

Encryption and privacy are becoming increasingly important. One way to think of encryption is that we start with a string of text that we pass to one or more transforming functions. The result of these transformations is a string of encrypted text that we can then transmit more safely. The recipient of the encrpted string then applies the inverses of the transforming functions to the string of encrypted text (i.e., decrypts it) and obtains a copy of the original string. The sender of the encrypted string must share some information with the recipient that permits the string to be decrypted (i.e., a key). In the following exercises we explore a few simple designs for the transforming functions.

Implement a Crypto class that encapsulates the functions from the preceding exercises. You can use the following UML diagram to get you started.

m_OpSequence is a QString consisting of the characters 'p' and 's' that represent permute() and shift(). The encrypt() function applies those functions to the given string in the order that they appear in the m_OpSequence string.

Example 5.21 contains some code to test your class.

#include "crypto.h"
#include <QTextStream>
#include <argumentlist.h>
#include <qstd.h>

using namespace qstd;

void usage(const QString& appname) {
  cout << " Usage:\n"
		 << appname 
		 << " actionSwitch "
		 << " -k keyValue "
		 << " -s string\n"
		 << "where\nactionswitch is either -e (encrypt) or -d (decrypt)\n"
		 << "(default is encrypt)."
		 << "keyvalue is the encryption key if you don't use default key.\n"	 
		 << "string is the string to be encrypted or decrypted.\n"
		 << "Converted string is sent to stdout."
		 << endl;
}

int main(int argc, char** argv)  {

     QString str1 ("asdfghjkl;QWERTYUIOP{}}|123456&*()_+zxcvnm,,, ./?"), 
             str2;
     cout << "Original string: " << str1 << endl;
     QString seqstr("pspsp");
     ushort key(12579);
     Crypto crypt(key, seqstr);
     str2 = crypt.encrypt(str1);
     cout << "Encrypted string: " << str2 << endl;
     cout << "Recovered string: " << crypt.decrypt(str2) << endl;

	  /*
  ArgumentList al(argc, argv);
  QString appname(al.takeFirst());
  QString str1(al.getSwitchArg("-s"));
  if(str1 == QString()) {
	 usage(appname);
	 exit(1);
  }
  bool decrypt(al.getSwitch("-d")); 
  QString keystr(al.getSwitchArg("-k"));
  unsigned key(214365);   // default
  if(keystr != QString())
	 key = keystr.toUInt();
  QString seqstr("pspsp");
  Crypto crypt(key, seqstr);

  if(decrypt)
	 cout << crypt.decrypt(str1);
  else
	 cout << crypt.encrypt(str1);
	  */
}

Now that you have developed a Crypto class for encrypting and decrypting strings, let's use that class for managing the passwords for users of some system.

In addition to the Crypto member, the UserManager class also needs members to hold, for each user, the userid and encrypted password. ^[20]

All of the member functions are “silent” (i.e., no interaction with the user). User interactions take place only in the client code.

The constructor instantiates the Crypto member.

Each of the three bool functions returns true if the operation is successful.

loadList() and saveList() both access a file named pwfile. They each return the number of users.