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Microsoft's Digital Rights Management Scheme - Technical Details
By "Beale Screamer"
This document describes version 2 of the Microsoft Digital Rights
Management (MS-DRM), as applied to audio (.wma files). The sources
for this material are varied, and some of the information might be
slightly incomplete; however, the fundamental ideas are solid and
easily verified. There is no attempt at describing the older version
1 of DRM. While version 1 is widely used (probably more widely than
version 2!), and the scheme is somewhat simpler, the purpose of this
is to describe the latest technology and not necessarily allow all
existing systems to be broken. The ideas described here are also
implemented in the software originally distributed with this document
(but as an independent piece, so the software may or may not be
available from where you have obtained this document), so a real
implementation can be examined. Not all of the information here is
needed in order to write the software that removes the encryption, but
some of the more interesting points surrounding the MS-DRM scheme and
software are given even if not necessary. Also note that no code is
included in this document, either real code or pseudo-code. All
that's in this document is a straight mathematical discussion, which
should be fully protected under the 1st Amendment to the
U.S. Constitution. I have no doubt that the corporate entities that
this document offends will attempt to suppress it, but I don't think
any argument they make could hold up to Constitutional scrutiny.
The basic components of MS-DRM involve use of elliptic curve
cryptography (ECC) for public key cryptography, DES for a block
cipher, RC4 for a stream cipher, and SHA-1 for a hash function. There
is also a block cipher which I haven't seen before, used in the MS-DRM
system to build a MAC, or keyed hash function. This cipher will be
explained completely below, and while the remaining algorithms are
well-known, more will be said about Microsoft's use of ECC below.
In the discussion and examples below, all numbers are expressed in
hexadecimal in the standard ordering (most-to-least significant)
unless otherwise stated. The actual bytes comprising large numbers in
any code are stored little endian, so at times it is convenient to
look at data in that ordering, and this will be clearly marked when it
is done.
One confusing item is that binary data sent back and forth is encoded
using Base64, but not using the standard algorithm! For some reason,
Microsoft has decided to use the non-alphanumeric character '*'
instead of '/', and '!' instead of '+' in some places, and in other
places they replace '/' with '@' and '!' with '%'. This means that
any software dealing with these strings cannot use a standard Base64
decoder, but must use a custom-build decoder.
FILES INVOLVED
Several key DLLs are kept in \windows\system that relate to the MS-DRM
scheme.
drmv2clt.dll: Provides basic DRM version 2 functionality
blackbox.dll: Provides basic, machine-specific crypto for MS-DRM.
Functionality replaced by IndivBox.key when the local
system has been "individualized."
The other interesting place for files is in \windows\All Users\DRM
(this location is not necessarily fixed but comes from the registry
entry HKEY_LOCAL_MACHINE\Software\Microsoft\DRM\DataPath). Here's a
sampling of some of the files in this directory (these are hidden
system files, so be sure to turn on "view all files" in order to see
them!):
IndivBox.key: Despite the extension, this is really a DLL
that is an "individualized" version of blackbox.dll
drmv2.lic: The file of licenses (a structured IStorage file)
drmv2.sst: "Secure state" for each of the licenses. Also an IStorage
file, but each stream is RC4 encrypted.
v2ks.bla: The version 2 "key store" - this is where all the public/private
keys are kept (encrypted, of course!).
v2ksndv.bla: The individualized version 2 "key store."
A SIMPLE BLOCK CIPHER (MULTISWAP)
Microsoft is using a very simple block cipher to create a message
authentication code (MAC). As this is not a standard algorithm, I
will describe it fully. The main operations in this cipher are
32-bit multiplications and swaps of the two halves of 32-bit words, so
I have called this cipher the "MultiSwap" cipher.
The MultiSwap cipher works on 64-bit blocks, using a key that consists
of 12 32-bit words, and a current state (or initialization vector)
that is 64-bits long. In the Microsoft implementation, the least
significant bits of all 12 words are set to 1, although once the
cipher is understood it is clear that really only 10 of the words
require this bit to be set. The basic operation of the cipher is a
transformation that is done on the first 32-bit word of the plaintext
block using the first 6 key words, and then repeated for the other
plaintext word and remaining key words.
Let k[0], k[1], k[2], k[3], k[4], and k[5] denote the first 6 key
words, and let s[0] and s[1] denote the two words of the state. To
transform a 32-bit input word x, first define the following function
f(a)=swap(swap(swap(swap(swap(a*k[0])*k[1])*k[2])*k[3])*k[4]) + k[5]
where * is multiplication modulo 2^(32), and "swap" is an operation
which exchanges the two 16-bit halves of a 32-bit word. The complete
transformation of a 32-bit word x then consists of s[1]=s[1]+f(x+s[0])
and s[0]=f(x+s[0]). This is first done with the value x set to the
first 32 bits of the input, and then repeated with x set to the second
32 bits and the using keys k[6] through k[11]. The output of the
block cipher is the new state s[0] and s[1].
The reason this block cipher can be inverted is because all the key
words are odd, which means they have multiplicative inverses modulo
2^(32). To invert f(), just do the operations in the reverse order:
first subtract off k[5], then do the multiply/swap operations with the
inverses of k[4] through k[0]. Notice that only the multiplicative
key words really need to be odd, so there is no reason for the least
significant bit of k[5] or k[11] to be set; however, Microsoft sets
these bits anyway.
This block cipher is never used for encryption, but is used to create
a message authentication code (MAC) in the standard way. Assuming the
length of the message to be hashed is a multiple of 8 bytes (64 bits),
the cipher is initialized with a state of all zeros, and then used to
encrypt the entire data. The output of the last block (the final
state) is the MAC for that message. This is used in computing packet
keys to encrypt protected content by MS-DRM, as will be explained
later.
ELLIPTIC CURVE CRYPTOGRAPHY
For ECC, Microsoft is using an elliptic curve over Zp, where p is a
160 bit prime number (given below). The curve consists of the points
that lie on the curve y^2=x^3+ax+b, where the operations are done over
the field Zp and a and b are coefficients that are given below.
All values are represented as packed binary values: in other words, a
single value over Zp is encoded simply as 20 bytes, stored in little
endian order. A point on the elliptic curve is therefore a 40 byte
block, which consists of two 20 byte little endian values (the x
coordinate followed by the y coordinate). Here are the parameters for
the elliptic curve used in MS-DRM:
p (modulus): 89abcdef012345672718281831415926141424f7
coefficient a: 37a5abccd277bce87632ff3d4780c009ebe41497
coefficient b: 0dd8dabf725e2f3228e85f1ad78fdedf9328239e
generator x: 8723947fd6a3a1e53510c07dba38daf0109fa120
generator y: 445744911075522d8c3c5856d4ed7acda379936f
Order of curve: 89abcdef012345672716b26eec14904428c2a675
These constants are fixed, and used by all parties in the MS-DRM
system. The "nerd appeal" of the modulus is high when you see this
number in hexadecimal: it includes counting in the hexadecimal, as
well as the digits of fundamental constants e, pi, and sqrt(2).
In order to use this public key system, any user must have a
private/public key pair. Since the security of the system relies
pretty heavily on the private keys remaining secret (even from the
user of the system on which they reside), they are carefully hidden.
In fact, there are keys hidden in various files that are used,
including blackbox.dll, v2ks.bla, and IndivBox.key. For example, once
the player has been individualized, IndivBox.key is created, and there
are at least two keys embedded into this file: a 64-bit key used for
RC4, and a 160-bit private key for use in ECC. The ECC private key is
used as the basic client key (the corresponding public key is stored
unencrypted in the key store file, and used as the initial part of the
"client id" sent when requesting a license), and additional key pairs
are stored in part of the keystore file (v2ks.bla or v2ksndv.bla),
encrypted with the RC4 key.
These secret keys are stored in linked lists that contain 32 bits per
node (so the key as a whole is not in contiguous memory), interspersed
with the code in the library (IndivBox.key for example). The idea is
that they can be read by that library, used internally by that
library, and never communicated outside the library. Since the
IndivBox.key file is shuffled in a random way for each client, these
keys would be extremely difficult to extract from the file itself.
Fortunately, we don't have to: these keys are part of the object state
that is maintained by this library, and since the offset within this
object of these secret keys is known, we can let the library itself
extract the secret keys! The code for this simply loads up the "black
box" library, has it initialize an instance of the object, and then
reads the keys right out of that object. This is clearly a weakness
in the code which can be corrected by the DRM software fairly easily,
but for now it is the basis of our exploit.
GETTING A LICENSE
Each protected media file is encrypted with a "content key" that will
unlock the packets of the media stream. We describe briefly how a
license (containing a content key) is obtained for information
purposes, but the license acquisition protocol is not really important
for unlocking that content. Simply use the MS Media player, have it
request and decrypt the licenses, store them in drmv2.lic, and then we
can extract them directly from that file.
A protected media file is apparently recognized by the presence of a
DRMV2 object in the .wma file header. This object has GUID
298ae614-2622-4c17-b935-dae07ee9289c, and contains an XML object 6
bytes into the data part of the object. Among other things, this
header contains a "KID" element identifying the key used to unlock the
content. The drmv2.lic file is then checked to see if a license with
this KID exists locally. If the license doesn't exist, a license
request is formed, which sends an encrypted "client id" to the license
server. This is sent as a "challenge," which consists of 168 bytes in
the MS-Base64 encoding. The first 80 bytes are two ECC points, which
make up an ECC encrypted random session key, and the remaining 88
bytes are the "client id" encrypted using RC4 and the session key.
The ECC encryption is done using a public key that seems to be fixed
for all clients, so it is safe to assume that this corresponds to a
private key that is common to all license servers and built in to that
side of the system (without access to the server side code, it was
impossible to find the corresponding private key).
After some interaction, the license comes back as mime type
application/x-drm-v2, as an escaped XML-encoded license in the
following format
<LICENSERESPONSE><LICENSE version="x.x.x.x">
...base64 encoded license... </LICENSE></LICENSERESPONSE>
where x.x.x.x is most likely "2.0.0.0". To make things tricky for a
sniffer, the license is actually RC4 encrypted using the same session
key that was established by the client when sending the challenge.
The client then decrypts the license and stores it in the drmv2.lic
file.
GETTING THE CONTENT KEY
Getting the content key from a license is pretty easy once the client
knows what its public/private key pairs are, and has a copy of the
license obtained from drmv2.lic. The license entry is an XML object
with an element for "ENABLINGBITS", which has sub-elements ALGORITHM
(which should have type "MSDRM"), PUBKEY, VALUE, and SIGNATURE. The
PUBKEY element should match one of the client's public keys, or else
there a problem! The VALUE element is the ECC-encrypted content key,
which can be decrypted by the private key that corresponds to the
given PUBKEY.
The content key has a specific format: the y coordinate is ignored,
and when the x coordinate is written in storage order (little endian),
the first byte is the length of the content key (which may always be
7), which is followed by that many bytes of the content key. While
the content key is tied to the encoded media file (which may be common
to many users), the enabling bits value will be different for each
user, and tied to that user's public/private keys. Because of this,
licenses are not transferable from one user to another, even though
the media files themselves are (the new user must obtain his own
license from the license server).
We go through an example now of finding a content key. In this
example, we have identified our public and private keys as the
following values:
Public key x: 1957f96f3327a25bba52166ad7fcc74087b9734b
Public key y: 8939e1b1ed988182d34d17ebbcb0e03a82d062e7
Private key: 757ff01b853496452eea0b0646c3a357a6f33509
We're looking at a file RIAALuvsMe.wma, and find in the header the
following bit of XML:
<KID>nA67jM7dNGIUQIkP5v7hSQ==</KID>
The actual KID seems to be a Base64 encoding of a GUID, but it is
treated as a string (uninterpreted) by the software, so the origin
doesn't seem to make much difference.
The license is inside the drmv2.lic file, which is a structured "DOC
file", meaning it can be accessed through the IStorage and IStream
interfaces (and it can be browsed by the Microsoft Visual Studio "DOC
File Viewer" tool if you're curious). The top level drmv2.lic file
has a lower-level IStorage object for each KID, which can contain a
set of licenses for each KID. In order to guarantee valid IStorage
names, the KID is first processed to change all '/' characters to '@',
and all '!' to '%'. The names of the IStream objects containing the
licenses again look like Base64 encoded GUIDs, which turn out to be
the LID (license ID?) element stored in the license. This can be
verified once the license is obtained, but we're not sure how to
generate LID's from the content header information, and so can't
directly open the appropriate LID stream. Instead, we simply
enumerate through all available streams for this KID, testing each one
for a PUBKEY element (see below) that we know. This is taken to be
the license for this content. While this is really just a guess as to
the proper workings, it seems to work fine in all our tests.
Inside the license we find the following XML (this has been formatted
so that it's easier to look at - in the actual file this would all be
on one line).
<ENABLINGBITS>
<ALGORITHM type="MSDRM"></ALGORITHM>
<PUBKEY type="machine">
S3O5h0DH*NdqFlK6W6InM2*5VxnnYtCCOuCwvOsXTdOCgZjtseE5iQ==
</PUBKEY>
<VALUE>
VEsbPedfwrybrpkg0fhoOfe5eB9ef0R7QTxgX7NbtMIFK!h*4Pk7ek
PUqlDIRqYwQkgCGE0r0qtQdCUYszT!b7XedCIpsApQjstaFmafahM=
</VALUE>
<SIGNATURE>
KpxCm6lSXH8dTPI359jToftSEuLiP9v*zpHAy!kDEhlYkw6mkfQzlg==
</SIGNATURE>
</ENABLINGBITS>
The SIGNATURE element above is just random garbage. We didn't make a
real signature for this example (among other things, we don't have a
certified public key, which would have to follow this in a real
license. Requiring such a signature keeps people from creating their
own licenses, since only those that have been issued valid
certificates can do so).
First look at the PUBKEY part. If this is run through a Base64
decoder (modified for the MS character set as described earlier) you
get the following binary values, shown below as a memory dump:
0000: 4B 73 B9 87 40 C7 FC D7 6A 16 52 BA 5B A2 27 33
0010: 6F F9 57 19 E7 62 D0 82 3A E0 B0 BC EB 17 4D D3
0020: 82 81 98 ED B1 E1 39 89
Notice how this is exactly our public key from above, stored in little
endian order! So this license is for our machine.
Next, take the VALUE element above, run it through a Base64 decoder,
and interpret the 80 byte result as 4 20-byte values stored in little
endian order. These four numbers are as follows:
Encrypted u.x: & 1f78b9f73968f8d12099ae9bbcc25fe73d1b4b54
u.y: & 7a3bf9e07fe82b05c2b45bb35f603c417b447f5e
v.x: & 18257450abd22b4d1802484230a646c850aad443
v.y: & 136a9f66165acb8e500ab0292274deb56ffe34b3
To decrypt this value, first we multiply the point u by our private
key, resulting in the point
x: 399c72d525a9b65b7543a3e3adc88ce0f6a38db5
y: 66cfa6bdbfbb93b906b22deb36792363d8e8adc2
and then subtract this point from v to get
x: c91590616b4b3707
y: 753e24e50d437e147b4998376f163dc27b639a7a
Since x is so short, we have almost certainly gotten our content key.
Writing in storage order (little endian), x is
0000: 07 37 4B 6B 61 90 15 C9
which means that the content key has length 7 (from the first byte),
and the actual key is the string of bytes 374B6B619015C9.
DECRYPTING THE CONTENT
The content encryption process is simpler to explain than decryption,
so we start with that. The content key is not used directly, but is
processed for several different uses. First, the content key is
hashed using the SHA-1 hashing algorithm, producing a 20-bit output.
The first 12 bytes of this output are used as an RC4 key, and a block
of 16 words (or 64 bytes) of zeros is encrypted. The least
significant bits of the first 12 words of this output are all set, and
are used as the MultiSwap key. The next 2 words are the encryption
in-whitening mask and the next 2 words are the encryption
out-whitening mask (this will be explained later). The last 8 bytes
of the original SHA-1 hash output are used as a DES key.
To encrypt the content so that packets can be accessed randomly (for
seeks), the content cannot be encrypted as one single stream.
However, to strengthen the cipher we also don't want to re-use the
same key for every packet. To satisfy both of these goals, MS-DRM
uses the following scheme to encrypt a packet: First, the packet (with
size rounded down to a multiple of 8 bytes) is run through
MultiSwapMAC to produce a 64-bit MAC. For some reason, the 32-bit
halves of this MAC are swapped before further processing. Next, the
entire packet is RC4 encrypted using the swapped MAC as an 8-byte RC4
key. The 8-byte MAC (with swapped halves) is then run through a
"whitened DES" by first XORing with the in-whitening mask, then
running through DES (using the DES key from the last paragraph), and
then XORing the result with the out-whitening mask. The resulting 8
bytes are then placed in the final encrypted packet, overwriting the
last full 8-byte block (not the last 8 bytes of the packet, but
blocking the packet from the beginning into 8 byte pieces, and
overwriting the last full piece).
To decrypt such a packet, first locate the last full 8-byte block, run
it through the whitened DES decryption, and the result is used as an
RC4 key to decrypt the packet. This will produce the correct packet
except for 8 bytes: those in the position of the last full 8-byte
block are wrong, since they were overwritten in the last phase of the
encryption. However, by swapping the halves of the RC4 key, we have
the MAC for the original packet up to and including the original bytes
in this position. Since the MAC is actually created out of a block
cipher, we can recover the original 8 bytes as follows: run the entire
packet through MultiSwapMAC up to the block in question, but not
including it. This output is the next-to-last state seen by
MultiSwapMAC in the encryption MAC computation, and we just recovered
the final output of the MAC, so we can put these two values into the
block cipher decryption to obtain the original data of this 8-byte
block. The original 8-bytes are placed back into the packet, and now
the entire original contents are restored!
This is a pretty clever scheme: by using a MAC constructed from a
block cipher, individual packet keys can be computed and encoded into
the encrypted packet with absolutely no increase in space, and since
the size is maintained nothing in the structure of the content file
(describing packet sizes or other parameters) needs to be changed at
all. The encrypted content can be completely transparent to
applications that deal with .wma files in a non-content-sensitive
manner.
We finish this section showing an example of content decryption, using
the content key from the previous paragraph. We first process the
content key by running it through SHA-1 to obtain
15 CB 92 F9 97 2E C8 75 29 4F 12 65 36 B6 C6 DB AC A2 40 35
The first 12 bytes are used as an RC4 key to encrypt a block of all
zeros, giving
0000: 80 0A 2D 48 D1 FD 7E ED 83 69 4A 7D A5 D5 EE C4
0010: 4E E1 64 52 D1 71 98 26 9A F3 14 E3 51 C8 B6 92
0020: D4 93 E4 57 97 6D 63 EF 0E 06 07 54 F7 DD ED 38
0030: E8 CA A0 D0 83 13 F1 DB C1 70 AE 56 61 7D FB 94
The first 48 bytes are interpreted as 12 32-bit words, stored little
endian, and are saved as the MultiSwap key after setting all least
significant bits. So the key values are k[0]=0x482D0A81,
k[1]=0xED7EFDD1, etc. The last 16 bytes of the RC4-encrypted block
are the whitening masks for DES, and the key for DES is given as the
last 8 bytes of the SHA-1 hash value.
Assume we get a packet with size 1450 bytes, which is 181 8-byte
blocks followed by 2 additional bytes. Numbering the byte positions
as 0 through 1449, we look at the bytes in positions 1440 through 1447
(the last full 8-byte block), and find that they are
A8 49 65 36 A2 33 18 09
XORing with the encryption out-whitening mask (the last 8 bytes from
the RC4-encrypted block above) we get
69 39 CB 60 C3 4E E3 9D
and decrypting with DES using key 36 B6 C6 DB AC A2 40 35 gives
FD EF 98 7D 8B 77 72 FD
and finally XORing with the encryption in-whitening mask (the
next-to-last 8-byte piece in the RC4-encrypted block above) gives
15 25 38 AD 08 64 83 26
This is then used as an RC4 key to decrypt the packet. To replace
bytes at positions 1440 through 1447 with the correct values, take
the RC4 value and swap the words around to get
08 64 83 26 15 25 38 AD
This is the MultiSwapMAC of the input packet using bytes 0 through
1447. We run bytes 0 through 1439 through MultiSwapMAC to get
D9 F7 D9 53 A9 6E 14 D9
and then use this as the state input, along with the original MAC
output as the data input to the MultiSwapDecode function to obtain
DA 05 D8 EB 97 FE 1E 7B
These 8 bytes are placed in positions 1440 through 1447, and then the
entire original packet is restored.
OTHER ISSUES
Communication between different DLL modules is encrypted and checked
at multiple points. This works roughly as follows: Objects are
initialized with communication parameters by sending certified public
keys to the object you want to communicate with. The second object
verifies these certificates, generates a random session key (which it
uses to generate a MultiSwap key in addition to the use as a session
key), and sends the encrypted session key back to the calling object.
Future "sensitive communication" is RC4 encrypted with the session key,
and run through MultiSwapMAC to verify integrity (after padding with
zeros to make the data a multiple of 64 bits). This is done for data
sent both to and from the object.
Presumably, this is done so that anyone monitoring parameters passed
between DLL modules wouldn't see any "sensitive data," although its
use for this purpose is pretty limited. However, it does lead to some
interesting and strange situations: when blackbox is sent a packet to
decrypt, it decrypts it, and then immediately *re-encrypts* it using
the session key to send it back to the media player! So in decrypting
a packet, the computer actually goes through a decrypt/encrypt/decrypt
sequence of operations!
One very important effect of this scheme is that Microsoft fully
controls who gets to write modules that interact with the basic
Microsoft media modules. Without a certified public key (and the
corresponding private key) it is impossible to write a compatible DLL
that interfaces with their code. Since Microsoft controls the issuing
of certified public keys, they also have complete control over who is
allowed to make compatible and competing products. Microsoft's
reputation for being generous to competitors is well-known, so this
effectively gives Microsoft a technically guaranteed monopoly power.
Of course, these certificates and private keys must be distributed
with any "Microsoft blessed" software as well, and in fact exist in
the media player and blackbox DLLs. They're not hard to extract, if
you know where to look, but I won't give them here. They would be of
limited use anyway, since Microsoft also has a "revocation list"
mechanism built in to the Media player software, meaning that they can
revoke any of these certificates at their whim, remotely disabling any
software that depends on that certificate for communication.
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