md5class.h

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// md5class.h: interface for the CMD5 class.
//
//////////////////////////////////////////////////////////////////////

#if !defined(AFX_MD51_H__2A1EA377_D065_11D4_A8C8_0050DAC6D85C__INCLUDED_)
#define AFX_MD51_H__2A1EA377_D065_11D4_A8C8_0050DAC6D85C__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000


  /***************************************************************************

   This class is a utility wrapper for 'C' code contained in internet RFC 1321, 
   "The MD5 Message-Digest Algorithm".

   It calculates a cryptological hash value, called a "digest" from a character 
   string.  For every unique character string the MD5 hash is guaranteed to be 
   unique.  The MD5 hash has the property that given the digest, it's 
   thought to be impossible to get back to the plain text string with existing 
   technology. In this implementation the digest is always a 32 digit hex number,
   regardless of the length of the input plaintext.

   This class is helpful for programs which store passwords.  Rather than storing 
   the password directly, the programmer should store the MD5 digest of the password. 
   Then when the user enters a password, compute the MD5 digest of the input password.
   If it is identical to the stored digest, then the user
   has entered the correct password.  It doesn't matter if an evil person sees the 
   digest, since he or she can't get from the digest to the password. At least not 
   unless the user enters a word out of the dictionary, since the evil person could 
   hash the whole dictionary.  One way to defeat a dictionary attack is to append 
   a non-text character onto the password, so that even if the user enters a dumb 
   password like "password", you just append some non alpha character to the entered
   password, i.e. password = "password" + "$".  By always appending a nonalpha 
   character, your stored digest isn't in the attacker's dictionary. You can 
   then safely post the digest of the password on a highway billboard.
    
   Example pseudocode:
  {
	std::string storedPasswordDigest = GetPasswordDigestFromStorage();
	std::string passwordEnteredbyUser;
	cout << "Enter password:" ;
	cin >> passwordEnteredbyUser;
	
	CMD5 md5(passwordEnteredbyUser.c_str()); //note c_str() returns a pointer to the std::string's character buffer, just like CString's "GetBuffer" member function.
	
	if(md5.getMD5Digest != storedPasswordDigest)
	{
		//user has entered an invalid password
		cout << "Incorrect password!";
		exit(1);
	}
	
    //if we get here, then the user entered a valid password
  }
  **************************************************************************
  Use this code as you see fit. It is provided "as is"
  without express or implied warranty of any kind.
  
  Jim Howard, [e-mail]
  ***************************************************************************/

class CMD5  
{
public:
	CMD5();	//default ctor
	CMD5(const char* plainText);  //set plaintext in ctor
	void setPlainText(const char* plainText); //set plaintext with a mutator, it's ok to 
				                              //to call this multiple times, the digest is recalculated after each call.
	const char* getMD5Digest();	  //access message digest (aka hash), return 0 if plaintext has not been set

	virtual ~CMD5();
private:
	bool calcDigest(); //this function computes the digest by calling the RFC 1321 'C' code

	bool m_digestValid; //false until the plaintext has been set and digest computed
	unsigned char  m_digest[16]; //the numerical value of the digest
	char  m_digestString[33];  //Null terminated string value of the digest expressed in hex digits
	char* m_plainText; //a pointer to the plain text.  If casting away the const-ness 
	                   //worries you, you could either make a local copy of the plain 
	                   //text string instead of just pointing at the user's string or 
					   //modify the RFC 1321 code to take 'const' plaintext. 

};

#endif // !defined(AFX_MD51_H__2A1EA377_D065_11D4_A8C8_0050DAC6D85C__INCLUDED_)