Inside Windows Product Activation - Algorithm Explanation


The current public discussion of Windows Product Activation (WPA) is

characterized by uncertainty and speculation. In this paper we supply

the technical details of WPA - as implemented in Windows XP - that

Microsoft should have published long ago.


While we strongly believe that every software vendor has the right to

enforce the licensing terms governing the use of a piece of licensed

software by technical means, we also do believe that each individual

has the right to detailed knowledge about the full implications of the

employed means and possible limitations imposed by it on software

usage.


In this paper we answer what we think are currently the two most

important open questions related to Windows Product Activation.


* Exactly what information is transmitted during activation?


* How do hardware modifications affect an already activated

installation of Windows XP?


Our answers to these questions are based on Windows XP Release

Candidate 1 (build 2505). Later builds as well as the final version of

Windows XP might differ from build 2505, e.g. in the employed

cryptographic keys or the layout of some of the data

structures.


However, beyond such minor modifications we expect Microsoft to cling

to the general architecture of their activation mechanism. Thus, we

are convinced that the answers provided by this paper will still be

useful when the final version of Windows XP ships.


This paper supplies in-depth technical information about the inner

workings of WPA. Still, the discussion is a little vague at some

points in order not to facilitate the task of an attacker attempting

to circumvent the license enforcement supplied by the activation

mechanism.


XPDec, a command line utility suitable for verifying the presented

information, can be obtained from http://www.licenturion.com/xp/. It

implements the algorithms presented in this paper. Reading its source

code, which is available from the same location, is highly

recommended.


We have removed an important cryptographic key from the XPDec source

code. Recompiling the source code will thus fail to produce a working

executable. The XPDec executable on our website, however, contains

this key and is fully functional.


So, download the source code to learn about the inner workings of WPA,

but obtain the executable to experiment with your installation of

Windows XP.


We expect the reader to be familiar with the general procedure of

Windows Product Activation.


>> INSIDE THE INSTALLATION ID


We focused our research on product activation via telephone. We did

so, because we expected this variant of activation to be the most

straight-forward to analyze.


The first step in activating Windows XP via telephone is supplying the

call-center agent with the Installation ID displayed by msoobe.exe,

the application that guides a user through the activation process. The

Installation ID is a number consisting of 50 decimal digits that are

divided into groups of six digits each, as in


002666-077894-484890-114573-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XX


In this authentic Installation ID we have substituted digits that we

prefer not to disclose by 'X' characters.


If msoobe.exe is invoked more than once, it provides a different

Installation ID each time.


In return, the call-center agent provides a Confirmation ID matching

the given Installation ID. Entering the Confirmation ID completes the

activation process.


Since the Installation ID is the only piece of information revealed

during activation, the above question concerning the information

transmitted during the activation process is equivalent to the

question


'How is the Installation ID generated?'


To find an answer to this question, we trace back each digit of the

Installation ID to its origins.


>>> Check digits


The rightmost digit in each of the groups is a check digit to guard

against simple errors such as the call center agent's mistyping of one

of the digits read to him or her. The value of the check digit is

calculated by adding the other five digits in the group, adding the

digits at even positions a second time, and dividing the sum by

seven. The remainder of the division is the value of the check

digit. In the above example the check digit for the first group (6) is

calculated as follows.


1 | 2 | 3 | 4 | 5 <- position

---+---+---+---+---

0 | 0 | 2 | 6 | 6 <- digits


0 + 0 + 2 + 6 + 6 = 14 (step 1: add all digits)

0 + 6 + 14 = 20 (step 2: add even digits again)


step 3: division

20 / 7 = 2, remainder is 20 - (2 * 7) = 6


=> check digit is 6


Adding the even digits twice is probably intended to guard against the

relatively frequent error of accidentally swapping two digits while

typing, as in 00626 vs. 00266, which yield different check digits.


>>> Decoding


Removing the check digits results in a 41-digit decimal number. A

decimal number of this length roughly corresponds to a 136-bit binary

number. In fact, the 41-digit number is just the decimal encoding of

such a 136-bit multi-precision integer, which is stored in little

endian byte order as a byte array. Hence, the above Installation ID

can also be represented as a sequence of 17 bytes as in


0xXX 0xXX 0xXX 0xXX 0xXX 0xXX 0xXX 0xXX

0x94 0xAA 0x46 0xD6 0x0F 0xBD 0x2C 0xC8

0x00


In this representation of the above Installation ID 'X' characters

again substitute the digits that we prefer not to disclose. The '0x'

prefix denotes hex notation throughout this paper.


>>> Decryption


When decoding arbitrary Installation IDs it can be noticed that the

most significant byte always seems to be 0x00 or 0x01, whereas the

other bytes look random. The reason for this is that the lower 16

bytes of the Installation ID are encrypted, whereas the most

significant byte is kept in plaintext.


The cryptographic algorithm employed to encrypt the Installation ID is

a proprietary four-round Feistel cipher. Since the block of input

bytes passed to a Feistel cipher is divided into two blocks of equal

size, this class of ciphers is typically applied to input blocks

consisting of an even number of bytes - in this case the lower 16 of

the 17 input bytes. The round function of the cipher is the SHA-1

message digest algorithm keyed with a four-byte sequence.


Let + denote the concatenation of two byte sequences, ^ the XOR

operation, L and R the left and right eight-byte input half for one

round, L' and R' the output halves of said round, and First-8() a

function that returns the first eight bytes of an SHA-1 message

digest. Then one round of decryption looks as follows.


L' = R ^ First-8(SHA-1(L + Key))

R' = L


The result of the decryption is 16 bytes of plaintext, which are -

together with the 17th unencrypted byte - from now on interpreted as

four double words in little endian byte order followed by a single

byte as in


name | size | offset

-----+-------------+-------

H1 | double word | 0

H2 | double word | 4

P1 | double word | 8

P2 | double word | 12

P3 | byte | 16


H1 and H2 specify the hardware configuration that the Installation ID

is linked to. P1 and P2 as well as the remaining byte P3 contain the

Product ID associated with the Installation ID.


>>> Product ID


The Product ID consists of five groups of decimal digits, as in


AAAAA-BBB-CCCCCCC-DDEEE


If you search your registry for a value named 'ProductID', you will

discover the ID that applies to your installation. The 'About' window

of Internet Explorer should also yield your Product ID.


>>>> Decoding


The mapping between the Product ID in decimal representation and its

binary encoding in the double words P1 and P2 and the byte P3 is

summarized in the following table.


digits | length | encoding

--------+---------+---------------------------------------

AAAAA | 17 bits | bit 0 to bit 16 of P1

BBB | 10 bits | bit 17 to bit 26 of P1

CCCCCCC | 28 bits | bit 27 to bit 31 of P1 (lower 5 bits)

| | bit 0 to bit 22 of P2 (upper 23 bits)

DDEEE | 17 bits | bit 23 to bit 31 of P2 (lower 9 bits)

| | bit 0 to bit 7 of P3 (upper 8 bits)


The meaning of each of the five groups of digits is documented in the

next table.


digits | meaning

--------+-------------------------------------------------

AAAAA | apparently always 55034 (in Windows XP RC1)

BBB | most significant three digits of Raw Product Key

| (see below)

CCCCCCC | least significant six digits of Raw Product Key

| plus check digit (see below)

DD | index of the public key used to verify the

| Product Key (see below)

EEE | random value


As can be seen, the (Raw) Product Key plays an important role in

generating the Product ID.


>>>> Product Key


The Raw Product Key is buried inside the Product Key that is printed

on the sticker distributed with each Windows XP CD. It consists of

five alphanumeric strings separated by '-' characters, where each

string is composed of five characters, as in


FFFFF-GGGGG-HHHHH-JJJJJ-KKKKK


Each character is one of the following 24 letters and digits:


B C D F G H J K M P Q R T V W X Y 2 3 4 6 7 8 9


Very similar to the decimal encoding of the Installation ID the 25

characters of the Product Key form a base-24 encoding of the binary

representation of the Product Key. Decoding the Product Key yields a

multi-precision integer of roughly 115 bits, which is stored - again

in little endian byte order - in an array of 15 bytes. Decoding the

above Product Key results in the following byte sequence.


0x6F 0xFA 0x95 0x45 0xFC 0x75 0xB5 0x52

0xBB 0xEF 0xB1 0x17 0xDA 0xCD 0x00


Of these 15 bytes the least significant four bytes contain the Raw

Product Key in little endian byte order. The least significant bit is

removed by shifting this 32-bit value (0x4595FA6F - remember the

little endian byte order) to the left by one bit position, resulting

in a Raw Product Key of 0x22CAFD37, or


583728439


in decimal notation.


The eleven remaining bytes form a digital signature, allowing

verification of the authenticity of the Product Key by means of a

hard-coded public key.


>>>> Product Key -> Product ID


The three most significant digits, i.e. 583, of the Raw Product Key's

nine-digit decimal representation directly map to the BBB component of

the Product ID described above.


To obtain the CCCCCCC component, a check digit is appended to the

remaining six digits 728439. The check digit is chosen such that the

sum of all digits - including the check digit - is divisible by

seven. In the given case, the sum of the six digits is


7 + 2 + 8 + 4 + 3 + 9 = 33


which results in a check digit of 2, since


7 + 2 + 8 + 4 + 3 + 9 + 2 = 33 + 2 = 35


which is divisible by seven. The CCCCCCC component of the Product ID

is therefore 7284392.


For verifying a Product Key, more than one public key is available. If

verification with the first public key fails, the second is tried,

etc. The DD component of the Product ID specifies which of the public

keys in this sequence was successfully used to verify the Product Key.


This mechanism might be intended to support several different parties

generating valid Product Keys with different individual private keys.


However, the different private keys might also represent different

versions of a product. A Product Key for the 'professional' release

could then be signed with a different key than a Product Key for the

'server' release. The DD component would then represent the product

version.


Finally, a valid Product ID derived from our example Product Key might

be


55034-583-7284392-00123


which indicates that the first public key (DD = index = 0) matched and

123 was chosen as the random number EEE.


The randomly selected EEE component is the reason for msoobe.exe

presenting a different Installation ID at each invocation. Because of

the applied encryption this small change results in a completely

different Installation ID.


So, the Product ID transmitted during activation will most probably

differ in the last three digits from your Product ID as displayed by

Internet Explorer or as stored in the registry.


>>> Hardware Information


As discussed above, the hardware configuration linked to the

Installation ID is represented by the two double words H1 and H2.


>>>> Bit-fields


For this purpose, the double words are divided into twelve

bit-fields. The relationship between the computer hardware and the

bit-fields is given in the following table.


double word | offset | length | bit-field value based on

------------+--------+--------+----------------------------

H1 | 0 | 10 | volume serial number string

| | | of system volume

H1 | 10 | 10 | network adapter MAC address

| | | string

H1 | 20 | 7 | CD-ROM drive hardware

| | | identification string

H1 | 27 | 5 | graphics adapter hardware

| | | identification string

H2 | 0 | 3 | unused, set to 001

H2 | 3 | 6 | CPU serial number string

H2 | 9 | 7 | harddrive hardware

| | | identification string

H2 | 16 | 5 | SCSI host adapter hardware

| | | identification string

H2 | 21 | 4 | IDE controller hardware

| | | identification string

H2 | 25 | 3 | processor model string

H2 | 28 | 3 | RAM size

H2 | 31 | 1 | 1 = dockable

| | | 0 = not dockable


Bit 31 of H2 specifies, whether the bit-fields represent a notebook

computer that supports a docking station. If docking is possible, the

activation mechanism will be more tolerant with respect to future

hardware modifications. Here, the idea is that plugging a notebook

into its docking station possibly results in changes to its hardware

configuration, e.g. a SCSI host adapter built into the docking station

may become available.


Bits 2 through 0 of H2 are unused and always set to 001.


If the hardware component corresponding to one of the remaining ten

bit-fields is present, the respective bit-field contains a non-zero

value describing the component. A value of zero marks the hardware

component as not present.


All hardware components are identified by a hardware identification

string obtained from the registry. Hashing this string provides the

value for the corresponding bit-field.


>>>> Hashing


The hash result is obtained by feeding the hardware identification

string into the MD5 message digest algorithm and picking the number of

bits required for a bit-field from predetermined locations in the

resulting message digest. Different predetermined locations are used

for different bit-fields. In addition, a hash result of zero is

avoided by calculating


Hash = (Hash % BitFieldMax) + 1


where BitFieldMax is the maximal value that may be stored in the

bit-field in question, e.g. 1023 for a 10-bit bit-field, and 'x % y'

denotes the remainder of the division of x by y. This results in

values between 1 and BitFieldMax. The obtained value is then stored in

the respective bit-field.


>>>> RAM bit-field


The bit-field related to the amount of RAM available to the operating

system is calculated differently. The seven valid values specify the

approximate amount of available RAM as documented in the following

table.


value | amount of RAM available

------+---------------------------

0 | (bit-field unused)

1 | below 32 MB

2 | between 32 MB and 63 MB

3 | between 64 MB and 127 MB

4 | between 128 MB and 255 MB

5 | between 256 MB and 511 MB

6 | between 512 MB and 1023 MB

7 | above 1023 MB


It is important to note that the amount of RAM is retrieved by calling

the GlobalMemoryStatus() function, which reports a few hundred

kilobytes less than the amount of RAM physically installed. So, 128 MB

of RAM would typically be classified as "between 64 MB and 127 MB".


>>>> Real-world example


Let us have a look at a real-world example. On one of our test systems

the hardware information consists of the following eight bytes.


0xC5 0x95 0x12 0xAC 0x01 0x6E 0x2C 0x32


Converting the bytes into H1 and H2, we obtain


H1 = 0xAC1295C5 and H2 = 0x322C6E01


Splitting H1 and H2 yields the next table in which we give the value

of each of the bit-fields and the information from which each value is

derived.


dw & | |

offset | value | derived from

-------+-------+-----------------------------------------------

H1 0 | 0x1C5 | '1234-ABCD'

H1 10 | 0x0A5 | '00C0DF089E44'

H1 20 | 0x37 | 'SCSI\CDROMPLEXTOR_CD-ROM_PX-32TS__1.01'

H1 27 | 0x15 | 'PCI\VEN_102B&DEV_0519&SUBSYS_00000000&REV_01'

H2 0 | 0x1 | (unused, always 0x1)

H2 3 | 0x00 | (CPU serial number not present)

H2 9 | 0x37 | 'SCSI\DISKIBM_____DCAS-34330______S65A'

H2 16 | 0x0C | 'PCI\VEN_9004&DEV_7178&SUBSYS_00000000&REV_03'

H2 21 | 0x1 | 'PCI\VEN_8086&DEV_7111&SUBSYS_00000000&REV_01'

H2 25 | 0x1 | 'GenuineIntel Family 6 Model 3'

H2 28 | 0x3 | (system has 128 MB of RAM)

H2 31 | 0x0 | (system is not dockable)


>>> Using XPDec


XPDec is a utility to be run from the command prompt. It may be

invoked with one of four command line options to carry out one of four

tasks.


>>>> XPDec -i


This option enables you to access the information hidden in an

Installation ID. It decodes the Installation ID, decrypts it, and

displays the values of the hardware bit-fields as well as the Product

ID of your product. Keep in mind that the last three digits of the

Product ID contained in the Installation ID are randomly selected and

differ from the Product ID displayed by Internet Explorer.


The only argument needed for the '-i' option is the Installation ID,

as in


XPDec -i 002666-077894-484890-114573-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XX


>>>> XPDec -p


To help you trace the origin of your Product ID, this option decodes a

Product Key and displays the Raw Product Key as it would be used in a

Product ID.


The only argument needed for the '-p' option is the Product Key, as in


XPDec -p FFFFF-GGGGG-HHHHH-JJJJJ-KKKKK


Note that this option does not verify the digital signature of the

Product Key.


>>>> XPDec -v


This option calculates the hash of a given volume serial number. It

was implemented to illustrate our description of string hashing. First

use '-i' to display the hardware bit-fields. Then use this option to

verify our claims concerning the volume serial number hash.


The only argument needed for the '-v' option is the volume serial

number of your system volume, as in


XPDec -v 1234-ABCD


(The volume serial number is part of the 'dir' command's output.)


>>>> XPDec -m


This option calculates the network adapter bit-field value

corresponding to the given MAC address. Similar to '-v' this option

was implemented as a proof of concept.


The only argument needed for the '-m' option is the MAC address of

your network adapter, as in


XPDec -m 00-C0-DF-08-9E-44


(Use the 'route print' command to obtain the MAC address of your

network adapter.)


>> HARDWARE MODIFICATIONS


When looking at the effects of hardware modifications on an already

activated installation of Windows XP, the file 'wpa.dbl' in the

'system32' directory plays a central role. It is a simple

RC4-encrypted database that stores, among other things like expiration

information and the Confirmation ID of an activated installation,


a) the bit-field values representing the current hardware

configuration,


and

the bit-field values representing the hardware configuration

at the time of product activation.


While a) is automatically updated each time the hardware configuration

is modified in order to reflect the changes, remains fixed. Hence,
can be thought of as a snapshot of the hardware configuration at

the time of product activation.


This snapshot does not exist in the database before product activation

and if we compare the size of 'wpa.dbl' before and after activation,

we will notice an increased file size. This is because the snapshot is

added to the database.


When judging whether re-activation is necessary, the bit-field values

of a) are compared to the bit-field values of , i.e. the current

hardware configuration is compared to the hardware configuration at

the time of activation.


>>> Non-dockable computer


Typically all bit-fields with the exception of the unused field and

the 'dockable' field are compared. If more than three of these ten

bit-fields have changed in a) since product activation, re-activation

is required.


This means, for example, that in our above real-world example, we

could replace the harddrive and the CD-ROM drive and substantially

upgrade our RAM without having to re-activate our Windows XP

installation.


However, if we completely re-installed Windows XP, the information in
would be lost and we would have to re-activate our installation,

even if we had not changed our hardware.


>>> Dockable computer


If bit 31 of H2 indicates that our computer supports a docking

station, however, only seven of the ten bit-fields mentioned above are

compared. The bit-fields corresponding to the SCSI host adapter, the

IDE controller, and the graphics board are omitted. But again, of

these remaining seven bit-fields, only up to three may change without

requiring re-activation.


>> CONCLUSIONS


In this paper we have given a technical overview of Windows Product

Activation as implemented in Windows XP. We have shown what

information the data transmitted during product activation is derived

from and how hardware upgrades affect an already activated

installation.


Looking at the technical details of WPA, we do not think that it is as

problematic as many people have expected. We think so, because WPA is

tolerant with respect to hardware modifications. In addition, it is

likely that more than one hardware component map to a certain value

for a given bit-field. From the above real-world example we know that

the PX-32TS maps to the value 0x37 = 55. But there are probably many

other CD-ROM drives that map to the same value. Hence, it is

impossible to tell from the bit-field value whether it is a PX-32TS

that we are using or one of the other drives that map to the same

value.


In contrast to many critics of Windows Product Activation, we think

that WPA does not prevent typical hardware modifications and,

moreover, respects the user's right to privacy.


>> ABOUT THE AUTHORS


Fully Licensed GmbH is a start-up company focusing on novel approaches

to online software licensing and distribution. Have a look at their

website at

http://www.licenturion.com


for more information.


Their research branch every now and then analyzes licensing solutions

implemented by other companies.


>> COPYRIGHT


Copyright © 2001 Fully Licensed GmbH (www.licenturion.com)

All rights reserved.


You are free to do whatever you want with this paper. However, you

have to supply the URL of its online version

http://www.licenturion.com/xp/


with any work derived from this paper to give credit to its authors.

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