HOW TO CRACK, by +ORC, A TUTORIAL
Lesson 6.1: Funny tricks (1)
LESSON 6 (1) - Funny tricks. Xoring, Junking, Sliding
EXERCISE 01: [LARRY in search of the King]
Before the next step let's resume what you have learned in
the lessons 3-5, beginning with a very simple crack exercise
(again, we'll use the protection scheme of a game, for the
reasons explained in lesson 1): SEARCH FOR THE KING (Version
1.1.). This old "Larry" protection sequence, is a "paper
protection" primitive. It's a very widespread (and therefore easy
to find) program, and one of the first programs that instead of
asking meaningful passwords (which offer us the possibility to
immediately track them down in memory) asked for a random number
that the good buyer could find on the manual, whereby the bad
cracker could not. (Here you choose -with the mouse- one number
out of 5 possible for a "gadget" choosen at random). I don't need
any more to teach you how to find the relevant section of code
(-> see lesson 3). Once you find the protection, this is what you
:C922 8E0614A3 MOV ES,[A314]
:C952 50 0E PUSH AX & CS
:C954 E81BFF CALL C872 <- call protection scheme
:C957 5B POP BX twice
:C959 8B76FA MOV SI,[BP-06] <- prepare store_room
:C95C D1E6 SHL SI,1 <- final prepare
:C95E 8942FC MOV [BP+SI-04],AX <- store AX
:C961 837EFA00 CMP Word Ptr [BP-06],+00 <- good_guy?
:C965 75BB JNZ C922 <- loop, bad guy
:C967 8E0614A3 MOV ES,[A314]
:C96B 26F606BE3501 TEST Byte Ptr ES:[35BE],01 <- bad_guy?
:C971 74AF JZ C922 <- loop, bad guy
:C973 8B46FC MOV AX,[BP-04]... <- go on good guy
Let's see now the protection scheme called from :C954
:C872 55 PUSH BP
:C8F7 90 NOP
:C8F8 0E PUSH CS
:C8F9 E87234 CALL FD6E <- call user input
:C8FC 5B POP BX
:C8FD 5B POP BX
:C8FE 8B5E06 MOV BX,[BP+06]
:C901 D1E3 SHL BX,1
:C903 39872266 CMP [BX+6622],AX <- right answer?
:C907 7505 JNZ C90E <- no, beggar_off
:C909 B80100 MOV AX,0001 <- yes, AX=1
:C90C EB02 JMP C910
:C90E 2BC0 SUB AX,AX <- beggar_off with AX=0
:C910 8BE5 MOV SP,BP
:C912 5D POP BP
:C913 CB RETF <- back to main
Here follow 5 questions, please answer all of them:
1) Where in memory (in which locations) are stored the "right"
passnumbers? Where in memory is the SEGMENT of this
locations stored? How does the scheme get the OFFSET?
2) Would setting NOPs instructions at :C965 and :C971 crack?
Would it be a good idea?
3) Would changing :C907 to JZ crack? Would it be a good idea?
4) Would changing :C907 to JNZ C909 crack? Would it be a good
5) Write down (and try) at least 7 OTHER different patches to
crack this scheme in spades (without using any NOP!).
Uff! By now you should be able to do the above 5 exercises in
less than 15 minutes WITHOUT USING THE DEBUGGER! Just look at the
data above and find the right answers feeling them... (you 'll
now which one are the right one checking with your debugger...
score as many points as you like for each correct answer and sip
a good Martini-Wodka... do you know that the sequence should
ALWAYS be 1) Ice cubes 2) Martini Dry 3) Wodka Moskovskaja 4)
olive 5) lemon 6) Schweppes Indian tonic?
Let's now come to the subject of this lesson:
-----> [Xoring] (Simple encryption methods)
One easy way to encrypt data is the XOR method. XOR is a bit
manipulation instruction that can be used in order to cipher and
decipher data with the same key:
Byte to encrypt key result
FF XOR A1 5E
5E XOR A1 FF
As you can see XOR offers a very easy way to encrypt or to
decrypt data, for instance using the following routine:
mov bx, offset_where_encryption/decryption_starts
mov ah, [bx] <- get current byte
xor ah, encrypt_value <- engage/disengage xor
mov [bx], ah <- back where you got it
inc bx <- ahead one byte
cmp bx, offset_start_+_size <- are we done?
jle xor_loop <- no, then next cycle
ret <- back where we came from
The encrypt_value can be always the same (fixed) or chosen at
random, for instance using INT_21, service 2Ch (get current time)
and choosing as encrypt_value the value reported in DL (but
remembering to discard the eventual value 0, coz otherwise it
would not xor anything at all!)
The problem with XORing (and with many other encryption
methods), is that the part of the code that calls the encryption
routine cannot be itself encrypted. You'll somewhere have, "in
clear" the encryption key.
The protectionist do at times their best to hide the
decrypting routine, here are some common methods:
-----> JUNK FILLING, SLIDING KEYS AND MUTATING DECRYPTORS
These are the more common protection method for the small
decryption part of the program code. This methods, originally
devised to fool signature virus scanners, have been pinched from
the polymorphic virus engines of our fellows viriwriters, and are
still in use for many simple decryption protection schemes. For
parts of the following many thanks go to the [Black Baron], it's
a real pity that so many potential good crackers dedicate so much
time to useless (and pretty repetitive) virus writing instead of
helping in our work. This said, virus studying is VERY important
for crackers coz the code of the viri is
* TIGHT AND EFFECTIVE
* CLOAKED AND CONCEALED.
Let's show as example of the abovementioned protection tactics
the following ultra-simple decryptor:
MOV SI,jumbled_data ;Point to the jumbled data
MOV CX,10 ;Ten bytes to decrypt
mn_loop: XOR BYTE PTR [SI],44 ;XOR (un_scramble!) a byte
INC SI ;Next byte
LOOP mn_loop ;Loop the 9 other bytes
This small program will XOR the ten bytes at the location pointed
to by SI with the value 44. Providing the ten bytes were XORed
with 44 prior to running this decryptor the ten bytes will be
restored to their original state.
In this very simple case the "key" is the value 44. But there are
several tricks involving keys, the simplest one being the use of
a "sliding" key: a key that will be increased, or decreased, or
multiplied, or bit-shifted, or whatever, at every pass of the
A possible protection can also create a true "Polymorph"
decryptor, a whole decryptor ROUTINE that looks completely
different on each generation. The trick is to pepper totally
random amounts of totally random instructions, including JUMPS
and CALLS, that DO NOT AFFECT the registers that are used for the
decryption. Also this kind of protection oft uses a different
main decryptor (possibly from a selection of pre-coded ones) and
oft alters on each generation also all the registers that the
decryptor uses, invariably making sure that the JUNK code that
it generates doesn't destroy any of the registers used by the
real decryptor! So, with these rules in mind, here is our simple
MOV DX,10 ;Real part of the decryptor!
MOV SI,1234 ;junk
AND AX,[SI+1234] ;junk
MOV DI,jumbled_data ;Real part of the decryptor!
TEST [SI+1234],BL ;junk
OR AL,CL ;junk
mn_loop: ADD SI,SI ;junk instr, but real loop!
XOR AX,1234 ;junk
XOR BYTE PTR [DI],44 ;Real part of the decryptor!
SUB SI,123 ;junk
INC DI ;Real part of the decryptor!
TEST DX,1234 ;junk
AND AL,[BP+1234] ;junk
DEC DX ;Real part of the decryptor!
XOR AX,DX ;junk
SBB AX,[SI+1234] ;junk
AND DX,DX ;Real part of the decryptor!
JNZ mn_loop ;Real part of the decryptor!
As you should be able to see, quite a mess! But still executable
code. It is essential that any junk code generated by the
Polymorph protection is executable, as it is going to be peppered
throughout the decryptor. Note, in this example, that some of the
junk instructions use registers that are actually used in the
decryptor! This is fine, providing the values in these
registers aren't destroyed. Also note, that now we have random
registers and random instructions on each generation. So, a
Polymorph protection Engine can be summed up into three major
1 .. The random number generator.
2 .. The junk code generator.
3 .. The decryptor generator.
There are other discrete parts but these three are the ones where
most of the work goes on!
How does it all work? Well a good protection would
* choose a random selection of registers to use for the
decryptor and leave the remaining registers as "junk" registers
for the junk code generator.
* choose one of the compressed pre-coded decryptors.
* go into a loop generating the real decryptor, peppered with
From the protectionist's point of view, the advantages of this
kind of method are mainly:
* the casual cracker will have to sweat to find the decryptor.
* the casual cracker will not be able to prepare a "patch" for
the lamers, unless he locates and patches the generators, (that
may be compressed) coz otherwise the decryptor will vary every
To defeat this kind of protection you need a little "zen" feeling
and a moderate knowledge of assembler language... some of the
junk instructions "feel" quite singular when you look at them
(->see lesson B). Besides, you (now) know what may be going on
and memory breakpoints will immediately trigger on decryption...
the road is open and the rest is easy (->see lessons 3-5).
-----> Starting point number magic
For example, say the encrypted code started at address 10h, the
following could be used to index this address:
MOV SI,10h ;Start address
MOV AL,[SI] ;Index from initial address
But sometimes you'll instead find something like the following,
again based on the encrypted code starting at address 10h:
MOV DI,0BFAAh ;Indirect start address
MOV AL,[DI+4066h) ;4066h + 0BFAAh = 10010h (and FFFF = 10h)!!
The possible combinations are obviously infinite.
[BIG KEYS] (Complicated encryption methods)
Prime number factoring is the encryption used to protect
sensible data and very expensive applications. Obviously for few
digit keys the decoding is much easier than for, say, 129 or 250
digit keys. Nevertheless you can crack those huge encryption too,
using distributed processing of quadratic sieve equations (which
is far superior for cracking purpose to the sequential processing
methods) in order to break the key into prime numbers. To teach
you how to do this sort of "high" cracking is a little outside
the scope of my tutorial: you'll have to write a specific short
dedicated program, linking together more or less half a thousand
PC for a couple of hours, for a 250 bit key, this kind of things
have been done quite often on Internet, were you can also find
many sites that do untangle the mysteries (and vagaries) of such
As References I would advocate the works of Lai Xueejia, those
swiss guys can crack *everything*. Begin with the following:
Xuejia Lai, James Massey, Sean Murphy, "Markov Ciphers and
Differential Cryptanalysis", Advances in Cryptology,
Xuejia Lai, "On the Design and Security of Block Ciphers",
Institute for Signal and Information Processing,
ETH-Zentrum, Zurich, Switzerland, 1992
Factoring and primality testing is obviously very important for
this kind of crack. The most comprehensive work I know of is:
(300 pages with lengthy bibliography!)
W. Bosma & M. van der Hulst
Primality Testing with Cyclotomy
Thesis, University of Amsterdam Press.
A very good old book you can incorporate in your probes to build
very effective crack programs (not only for BBS accesses :=) is
*the* "pomerance" catalog:
Pomerance, Selfridge, & Wagstaff Jr.
The pseudoprimes to 25*10^9
Math. Comp. Vol 35 1980 pp. 1003-1026
Anyway... make a good search with Lykos, and visit the relevant
sites... if encryption really interests you, you'll be back in
two or three (or thirty) years and you'll resume cracking with
deeper erudite knowledge.
[PATENTED PROTECTION SYSTEMS]
The study of the patented enciphering methods is also *quite*
interesting for our aims :=) Here are some interesting patents,
if you want to walk these paths get the complete texts:
[BEST] USPat 4168396 to Best discloses a microprocessor
for executing enciphered programs. Computer programs which have
been enciphered during manufacture to deter the execution of the
programs in unauthorized computers, must be decrypted before
execution. The disclosed microprocessor deciphers and executes
an enciphered program one instruction at a time, instead of on
a continuous basis, through a combination of substitutions,
transpositions, and exclusive OR additions, in which the address
of each instruction is combined with the instruction. Each unit
may use a unique set of substitutions so that a program which can
be executed on one microprocessor cannot be run on any other
microprocessor. Further, Best cannot accommodate a mixture of
encrypted and plain text programs.
[JOHNSTONE] USPat 4120030 to Johnstone describes a
computer in which the data portion of instructions are scrambled
and in which the data is of necessity stored in a separate
memory. There is no disclosure of operating with instructions
which are completely encrypted with both the operation code and
the data address portion being unreadable without a corresponding
[TWINPROGS] USPat 4183085 describes a technique for
protecting software by providing two separate program storages.
The first program storage is a secure storage and the second
program storage is a free storage. Security logic is provided to
check whether an output instruction has originated in the secure
store and to prevent operation of an output unit which receives
output instructions from the free storage. This makes it
difficult to produce information by loading a program into free
[AUTHENTICATOR] USPat 3996449 entitled "Operating System
Authenticator," discloses a technique for authenticating the
validity of a plain text program read into a computer, by
exclusive OR'ing the plain text of the program with a key to
generate a code word which must be a standard recognizable code
word which is successfully compared with a standard corresponding
code word stored in the computer. If there is a successful
compare, then the plain text program is considered to be
authenticated and is allowed to run, otherwise the program
is not allowed to run.
ELEMENTS OF [PGP] CRACKING
In order to try to crack PGP, you need to understand how these
public/private keys systems work. Cracking PGP seems extremely
difficult, though... I have a special dedicated "attack" computer
that runs 24 hours on 24 only to this aim and yet have only begun
to see the light at the famous other end of the tunnel. It's hard, but
good crackers never resign!
We'll see... I publish here the following only in the hope that
somebody else will one day be able to help...
In the public key cryptosystems, like PGP, each user has an
encryption key E=(e,n) and decryption key D=(d,n), wherein the
keys for all users are available in a public file, while the
keys for the users are only known to the respective users. In order
maintain a high level of security a user's decoding key is not
in a practical manner from that user's encoding (public) key.
such systems, since e.multidot.d.ident.1 (mod(1 cm((p-1),(q-1)))),
"1 cm((p-1),(q-1))" is the least common multiple of the numbers p-1
q-1) d can be determined from e provided p and q are also known.
Accordingly, the security of the system is dependent upon the
determine p and q which are the prime factors of n. By selecting p
and q to
be large primes, the resultant composite number n is also large, and
correspondingly difficult to factor.
For example, using known computer-implemented factorization methods,
order of 10.sup.9 years is required to factor a 200 digit long
Thus, as a practical matter, although a user's encryption key
public, the prime factors p and q of n are effectively hidden from
due to the enormous difficulty in factoring n. These aspects are
more fully in the abundant publications on digital signatures and
Public-Key Cryptosystems. Most public/private systems relies on a
A message-digest algorithm maps a message of arbitrary length to a
of fixed length, and has three properties:
Computing the digest is easy, finding a message with a given digest
"inversion" is hard, and finding two messages with the same digest
"collision" is also hard. Message-digest algorithms have many
not only digital signatures and message authentication. RSA Data
MD5 message-digest algorithm, developed by Ron Rivest, maps a
message to a
128-bit message digest. Computing the digest of a one-megabyte
takes as little as a second. While no message-digest algorithm can
secure, MD5 is believed to be at least as good as any other that
maps to a
128-bit digest. As a final gift, I'll tell you that PGP relies on
MD5 for a
secure one-way hash function. For PGP this is troublesome, to say
least, coz an approximate relation exists between any four
additive constants. This means that one of the design principles
(and MD5), namely to design a collision resistant function, is not
satisfied. You can construct two chaining variables (that only
the most significant bit of every word) and a single message block
yield the same hashcode. The attack takes a few minutes on a PC.
you should start, as I did.
[DOS 4GW] cracking - This is only a very provisory part of this
DOS 4GW cracking will be much better described as soon as [Lost
his stuff, if he ever does. For (parts of) the following I thank
Most applications of every OS, and also of DOS 4GW, are written in C
language, coz as you'll have already learned or, either, you'll
C allows you to get the "guts" of a program, almost approaching the
effectiveness of assembler language. C is therefore the LANGUAGE OF
for crackers, when you prepare your tools and do not directly use
Besides... you'll be able to find VERY GOOD books about C for next
nothing in the second hand bookshops. All the lusers are throwing
away in spades buying huge, coloured and absolutely useless books on
unproductive "bloated" languages like Visual basic, C++ and Delphy.
new books are now rare (books on assembler language have always
can be found almost exclusively on the second hand market. Find
them, read them, use them for your/our aims. You can find a lot of C
tutorials and of C material on the Web, by all means DO IT!
Be a conscientious cracker... learn C! It's cheap, lean, mean and
productive (and creative) :=)
Back to the point: most stuff is written in C and therefore you need
find the "main" sub-routine inside the asm. With DOS/4GW programs,
the exe file for "90 90 90 90", almost always it'll be at the start
compiled code. Now search for an INT_21 executed with 4C in AH, the
dos code (if you cannot "BPINT 21 AH=4C" with your tool, then search
the sequence "b4 4c cd 21". This is the equivalent to [mov AH,4C &
it's the most direct call, but as you'll have already learned, there
half a dozen ways to put 4C in AX, try them all in the order of
A few bytes above the INT_21 service 4C, you'll find the call to the
subroutine: "E8 xx xx". Now place a "CC" byte a few bytes above the
the exe and run the exe under a debugger. When the computer tries to
execute the instruction you'll be throw back in the debugger coz the
byte acts as INT_01 instruction. Then proceed as usual.
[THE "STEGONATED" PASSWORD HIDEOUT]
A last, very nice trick should be explained to every wannabe
it would be embarrassing to search for passwords or protection
that (apparently) are not there. They may be hidden INSIDE a picture
*.waw file for that matter). This is steganography, a method of
messages within other media.
Depending on how many shades of grey or hues of colour you want to
pixel can be expressed using 8. 16, 32 or even more bits. If the
significant bit is changed. the shade of the pixel is altered only
one-256th, one-65,OOOth or even less. No human eye could tell the
What the protectionist does, is hijack the least significant bit in
pixel of a picture. It uses that bit to store one bit of a
of a password (or of a file, or of a secret message). Because
pictures have lots of pixels, it's possible to store lots of data in
single picture. A simple algorithm will transfer them to the
of the program when it needs be, and there we'll intercept them.
need to learn very well the zen-cracking techniques to smell this
stuff though (-> see lesson B). Well, that's it for this lesson,