Hacking Windows 95 Screen Saver Passwords

(and a bit of cryptography)
by Lonely Hawk

Courtesy of Fravia's page of reverse engineering

I have been cracking programs ever since I owned a ZX Spectrum. To be 
honest, the only ones I was really interested in at first, were the
games I played and couldn't win. I remember my first crack was at a
game called JetPac, and I was only 15 years old. 
   But then came the horrible times of studying in the Univ. Non stop.
Completely destroying any creative impulse God gave me, by trying to
'guide' it. Oh well. It was until I saw the great +ORC's tutorials
that I realized my supressed 'hobby' could not die. I immediately
scanned the Web for the necessary tools and started spying Win 95
  I litteraly cracked my way in every direction: I created 4 pages of 
program codes, producing my own serials for every shareware program I
had. Some of the programs I cracked needed a little bit more of 'zen' 
thinking, to put it in +ORC's words. Most of them though, were of the
BOZO-PROGRAMMERS-R-LIVE-HERE kind. So I relaxed, and enjoyed my supreme
power over the Windows 95 architecture.
  Then one day, in the lab I work, I wanted to get something from a
friend's hard disk. I already knew the machine's password (not by hacking,
he's my friend), so I sat down in front of the monitor and moved the
mouse to remove the screen saver. Yikes! A dialog box popped up, asking
for the screen saver password. I tried my friend's 'global' password,
but it didn't work. Hmmm. I returned to my machine and searched the
Web for screen saver hacks. I found one for Windows 3.1 and tested it on 
my NT 4.0. Great! I just run the program and it says the password. Good.
Let's run it on my friend's PC (rebooted and logged in - I had already
lost my interest in what I wanted from his disk and wanted to know his
screen saver password). I run it and it says 'screen saver password is 
'BacB'. I try it - nothing. It's wrong. Hmm, time to do some hacking.
  Allright, Microsoft, where do you store these passwords? In Windows 3.1,
says the proggy I downloaded, it's inside control.ini. The same happens
in Windows NT 4.0, but not in Windows 95! Strange... (obviously some mixup
in the sources used inside these monster OS's). I GREP'ed inside all my
friend's PC *.ini files for 'pass'. Nothing related to my needs. In most
of these situations, one must then try the registry. I fired up regedit,
and looked for "saver". After some searching, I got in the following 
interesting place:

\HKEY_CURRENT_USER\Control Panel\desktop\ScreenSave_Data

This hosted a list of numbers, which in the test I immediately did, was
exactly twice the size of the password I gave. If for example I gave the
password "testing", the list had 14 numbers. This looked suspicious enough,
so I fired SoftIce, and got to the dialog box asking for the new password 
two times. I wrote my favorite one, 'POTATOES', in both edit fields, and 
pressed CTRL-D. I then searched the memory to BPR in, so 

s 30:0 lffffffff "POTAT"

BTW, always use a subset of the password, since this way you avoid mixing
up the real password image with the one in the OS dustbin. The real one
will show up in the data window as POTATOES, the false one as POTAT.
I found two occurences, one immediately after the other (remember, you
input it twice in the dialog box), so I BPR there.


Ctrl-D again, and pressing OK, made Softice pop up again, this time in
the well known KERNEL HMEMCPY part. The two password images were copied in
memory, and by BPR in the new positions, I found out that they were 
compared bytewise to see if they match. A normal operation for passwords,
so I cotinued. The next SoftIce popup was inside a REP MOVSB to a new
location, so I BPR it again. I was beggining to feel frustrated - how
many times does this stupid OS copy the string - I already had 5 copies
of it! (no wonder Win 95 needs 16 MB RAM). Oh well, Ctrl-D again (continue
  Luckily, the next popup was right where I wanted it:

        sub ebp, ebp  ; ebp=0, first character of the password
                      ; will be processed
        ....    ; code that produces a special number into eax
        ....    ; eax is in range of 0-255
        mov cl, [7E125010+eax]   ; read from a table of 256 values
        mov eax, offset password
        xor [eax+ebp], al        ; xor the ebp-letter of the password
        inc ebp
        cmp ebp, [length of password]
        jl looop1

After this ridiculous xoring loop, the transformed password was read,
and typed into a string, using wsprintf. If for example POTATOES was
transformed to 8 numbers like a1, 54, a2, 32, ... then with wsprintf
these numbers will be typed in the registry as 41, 31, (asciiz of a1)
35, 34 (asciiz of 54), etc. What a coincidence, when I discovered that
these numbers were the same with the ones in the registry!
  OK, so how do I crack this? Well, first I examined how the table was
getting filled (the table is supposed to be pseudorandom) and I recreated
the table, using C (i mean the table placed at 7E125010 in the above code
snip). Then I started scratching my head to find a way back to the original
password, when I realized this: each output value, was produced from
exactly one character of the password! there was no interleaving in the
XOR's! Each letter of the original password was XOR'ed with a value, and
that's it! This means that by checking every entry in the table to see
which one produces the value you search after XORING, you could find the 
original password!
  For example: Suppose you have a table of random values a(i),i=0..N-1, 
where each a(i) is different from all others a(j), j<>i. You then choose 
i randomly, and XOR password[0] with a(i). The same is done with all 
letters of the password. To crack this code, all you have to do is:

  for each transformed code of the password, password[k]
     find by testing all the matrix elements, matrix[j]
        which ASCII code XOR'd with matrix[j] gives password[k]

This is not computationally expensive brute force, since you check 256
values for each letter! A ZX Spectrum would compute the password in 
half a second. A PC, say, in 1 sec :)

OK, here's the code:


unsigned char matrix[256+2];
unsigned char matrixok[256+2];
unsigned char mystery[4]={ 0xb2, 0xdc, 0x90, 0x8f };
unsigned char h1;
unsigned char pa[79], passwd[80];
unsigned char tofind[30];
int h2=4;
unsigned int lentofind;
int len;

void fixmatrix()
    unsigned char orig, mys, help1, last;
    int i,j, help2;

    for(i=0; i<256; i++) matrix[i]="i;" matrix[256]="0;" matrix[256+1]="0;" h1="0;" last="0;" for(j="0;j<256;j++)" { orig="matrix[j];" mys="mystery[h1];" help2="(mys+last+matrix[j])" & 0xff; help1="matrix[help2];" matrix[j]="help1;" matrix[help2]="orig;" last="help2;" h1++; h1="h1%4;" } memcpy(matrixok, matrix, sizeof(matrix)); } void check(char *test) { unsigned char help1, oldh2; int i; strcpy(passwd, test); strcpy(pa, passwd); len="strlen(pa);" memcpy(matrix, matrixok, sizeof(matrix)); h1="0;" h2="0;" for(i="0;i<len;i++)" { h1++; h1="h1&0xff;" oldh2="matrix[h1];" h2="(h2+matrix[h1])" & 0xff; help1="matrix[h1];" matrix[h1]="matrix[h2];" matrix[h2]="help1;" help1="(matrix[h1]+oldh2)" & 0xff; help1="matrix[help1];" pa[i]^="help1;" } } int is_ok(char a) { if ((a<="9" ) && (a>='0'))
        return 1;
    else if ((a<='f') && (a>='A'))
        return 1;
        return 0;

int nibble(char c)
    if((c>='A') && (c<='f')) return (10+c-'A'); else if((c>='0') && (c<='9')) return (c-'0'); } void parse(char *inpt) { char *tok; char num[2]; lentofind="0;" tok="strtok(inpt," "\t ,\n"); while(tok!="NULL)" { num[0]="tok[0];" num[1]="tok[1];" if ((!is_ok(num[0])) || (!is_ok(num[1]))) { puts("Please input strings like: a1,b1,05,c3,d2,f3"); exit(0); } tofind[lentofind++]="16*nibble(num[0])+nibble(num[1]);" tok="strtok(NULL," "\t ,\n"); } tofind[lentofind]="0;" } void main() { unsigned int i; int j,found="0;" unsigned char tst[80]; char inpt[120]; fixmatrix(); printf("Windows 95 Screen Saver Cracker.\nMade by Lonely Hawk.\n\n"); printf("Give me the codes, separated by commas (in hex):\n>");
This kind of stupid Microsoft coding makes me think a little. What would be
the ideal way to hide these passwords? The reason I cracked Microsoft's code
is NOT because I have SoftIce. The reason is because the algorithm is 
ridiculous. The way I see it, to do real cryptography, you must use an
algorithm that cannot be reversed, but cannot also be cracked by brute
force. Take the UNIX algorithm for instance: everyone has the code for
it, but no one can reverse it. And to brute force it, you need a 
supercomputer running for ages. If Microsoft used such an algorithm,
I would have to crack the system to get in: alter the code to do a
jnz instead of a jz. And if the code is only deletable from administrative
accounts, I would not be able to do anything. 
  Or at least, that's what I would let them think... :)

Lonely Hawk

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