If you have to go the private file route, this may help. It creates a file that can only be read by the current userid, and writes the password to it. It then waits for keyboard input and deletes the file.

I've verified that other users, including administrators cannot read or delete the file, nor even inspect its permissions.

Also, in theory, by using FILE_ATTRIBUTE_TEMPORARY, the contents may never actually be written to the disk, and (assuming a small file and a system file cache that is not overrun) only ever exist in cache. (In theory!)

#include <windows.h>
#include <sddl.h>
#include <stdio.h>
#include <conio.h>
#include <Lmcons.h>
#include "debug.h"

#define TESTFILENAME "PermissionsTest.txt"

int main( int argc, char **argv ) {
    char userName[ UNLEN+1 ];
    char sid[ 400 ];
    char *stringSID;
    char domainName[ 256 ];
    SID_NAME_USE sidType;
    char pswd[] = "The quick brown fox";
    char ssdTemplate[] = "O:%sD:P(A;;FA;;;%s)";
    char ssd[1024];
    SECURITY_DESCRIPTOR *psd = NULL;
    SECURITY_ATTRIBUTES sa = { sizeof( SECURITY_ATTRIBUTES ), NULL, 0 
+};
    ULONG sdSize, unSize = sizeof( userName ), sidSize = sizeof( sid )
+, dnSize = sizeof( domainName ), written;
    HANDLE h;


    DIEIF( !GetUserName( userName, &unSize ), NULL );
    DIEIF( !LookupAccountName( NULL, userName, sid, &sidSize, domainNa
+me, &dnSize, &sidType ), NULL );
    printf( "Got sid\n" );
    DIEIF( !ConvertSidToStringSid( sid, &stringSID ), NULL );
    printf( "SID (as string): '%s'\n", stringSID );

    sprintf_s( ssd, sizeof(ssd), ssdTemplate, stringSID, stringSID );
    printf( "SSD: '%s'\n", ssd );

    DIEIF( !ConvertStringSecurityDescriptorToSecurityDescriptor( ssd, 
+SDDL_REVISION_1, &psd, &sdSize ), NULL );
    printf( "psd:%x sdSize: %d\n", psd, sdSize );
    sa.lpSecurityDescriptor =  psd;
    DIEIF( ( h = CreateFile(
            TESTFILENAME, GENERIC_WRITE, FILE_SHARE_READ, &sa, CREATE_
+ALWAYS,
            FILE_ATTRIBUTE_TEMPORARY, NULL
        )) == INVALID_HANDLE_VALUE, NULL
    );
    printf("File created\n" );
    DIEIF( !WriteFile( h, pswd, sizeof( pswd ), &written, NULL ), NULL
+ );
    printf( "File written '%s'\n", pswd );

    while( !_kbhit() ) Sleep( 1 );

    printf( "Closing file\n" );
    DIEIF( !CloseHandle( h ), NULL );
    DIEIF( !DeleteFile( TESTFILENAME ), NULL );
    return 0;
}
[download]

I am currently trying to make a module from that: https://github.com/salva/p5-Win32-SecretFile