Validating Win95 Screen Saver Password

Ed Milczarek -- emilczar@Matrox.COM
Thursday, January 09, 1997

Environment : Win95,VC++ 4.2-flat

Validating Win95 Screen Saver Password

In the screen saver example (saver.scr) that comes with MSDEV 
the author decided to write a screen saver without using the 
standard screen saver library (scrnsave.lib).  Instead, he 
replaced most of the DefScreenSaverProc using his own handles.  
Unfortunately he left out the most important one - dealing
with the case of the SCRM_VERIFYPW message.  This message is set
when password protection is enabled and the user is trying to 
close the screen saver.

The header file (scrnsave.h) makes reference to the Master
Password Router but I haven't found any info on that.

To complete this example,  how would one go about validating 
the password that the user enters.  I haven't seen any functions 
that would help in doing this.

Any ideas?

Ed

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P.J. Tezza -- pj@exemplarsoftware.com
Sunday, January 12, 1997

I seem to remember that this is the algorithm used by the Windows 3.x 
screen saver library to encrypt and decrypt passwords. The encrypted 
password was saved in a .ini file. This may have been changed in Windows 
95. I just thought the algorithm was kind of amusing.

BYTE Xor(BYTE x1, BYTE x2)
{
	BYTE xor = (BYTE)(x1 ^ x2);
	
	if(xor <= 0x20 || (xor >= 0x7f && xor <= 0x90) || (xor >=0x93 && xor <= 
0x9f) || xor == '=' || xor == '[' || xor == ']')
		return x2;
		
	return xor;		
}

void EncryptString(char *szString)
{
	ASSERT(strlen(szString) < UCHAR_MAX);

	BYTE nLen = (BYTE)strlen(szString);
	
	if(nLen == 0)
		return;
		
	AnsiUpper(szString);
	
	// Go forward
	
	for(BYTE nPtr = 0; nPtr < nLen; nPtr++)
		{
		BYTE c = (BYTE)szString[nPtr];
		
		c = Xor(nLen, c);													// Xor with string length
		
		if(nPtr == 0)
			c = Xor(0x2a, c);   										// Xor first byte with constant
		else
			{
			c = Xor(nPtr, c);												// Xor with position
			c = Xor((BYTE)szString[nPtr - 1], c);		// Xor with previous BYTE	
			}
			
		szString[nPtr] = (char)c;			
		}
		
	// Go backward
	
	if(nLen == 1)
		return;
		
	for(nPtr = (BYTE)(nLen - 1); nPtr >= 0; nPtr--)
		{
		BYTE c = (BYTE)szString[nPtr];
		
		c = Xor(nLen, c);
		
		if(nPtr == nLen - 1)
			c = Xor(0x2a, c);
		else
			{
			c = Xor(nPtr, c);
			c = Xor((BYTE)szString[nPtr + 1], c);
			}
			
		szString[nPtr] = (char)c;
		
		if(nPtr == 0)
			break;			// watch out - nPtr is unsiged so for loop won't exit
		}	
}

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