[IWAR] TECH Msoft crypto attack

From: Michael Wilson (MWILSON/0005514706at_private)
Date: Tue Jan 20 1998 - 14:14:56 PST

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    For those listmembers not on the various crypto lists, this came across
    and should be of interest.  --MW
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    Date: Tue, 20 Jan 1998 12:43:26 -0500 (EST)
    From: pgut001 <pgut001at_private>
    Subject: [Long] How to recover private keys for various Microsoft products
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        How to recover private keys for Microsoft Internet Explorer, Internet
                Information Server, Outlook Express, and many others
                                          - or -
                     Where do your encryption keys want to go today?
                        Peter Gutmann, <pgut001at_private>
    Microsoft uses two different file formats to protect users private keys, the
    original (unnamed) format which was used in older versions of MSIE, IIS, and
    other software and which is still supported for backwards-compatibility reasons
    in newer versions, and the newer PFX/PKCS #12 format.  Due to a number of
    design and implementation flaws in Microsofts software, it is possible to break
    the security of both of these formats and recover users private keys, often in
    a matter of seconds.  In addition, a major security hole in Microsofts
    CryptoAPI means that many keys can be recovered without even needing to break
    the encryption.  These attacks do not rely for their success on the presence of
    weak, US-exportable encryption, they also affect US versions.
    As a result of these flaws, no Microsoft internet product is capable of
    protecting a users keys from hostile attack.  By combining the attacks
    described below with widely-publicised bugs in MSIE which allow hostile sites
    to read the contents of users hard drives or with an ActiveX control, a victim
    can have their private key sucked off their machine and the encryption which
    "protects" it broken at a remote site without their knowledge.
    Once an attacker has obtained a users private key in this manner, they have
    effectively stolen their (digitial) identity, and can use it to digitally sign
    contracts and agreements, to recover every encryption session key it's ever
    protected in the past and will ever protect in the future, to access private
    and confidential email, and so on and so on.  The ease with which this attack
    can be carried out represents a critical weakness which compromises all other
    encryption components on web servers and browsers - once the private key is
    compromised, all security services which depend on it are also compromised.
    A really clever attacker might even do the following:
    - Use (say) an MSIE bug to steal someones ActiveX code signing key.
    - Decrypt it using one of the attacks described below.
    - Use it to sign an ActiveX control which steals other peoples keys.
    - Put it on a web page and wait.
    On the remote chance that the ActiveX control is discovered (which is extremely
    unlikely, since it runs and deletes itself almost instantly, and can't be
    stopped even with the highest "security" setting in MSIE), the attack will be
    blamed on the person the key was stolen from rather than the real attacker.
    This demonstrates major problems in both Microsoft's private key security
    (an attacker can decrypt, and therefore misuse, your private key), and ActiveX
    security (an attacker can create an effectively unstoppable malicious ActiveX
    control and, on the remote chance that it's ever discovered, ensure that
    someone else takes the blame).
    About a year ago I posted an article on how to break Netscape's (then) server
    key encryption to the cypherpunks list (Netscape corrected this problem at
    about the same time as I posted the article).  However more than a year after
    the code was published, and 2 1/2 years after a similar problem with Windows
    .PWL file encryption was publicised, Microsoft are still using exactly the
    same weak, easily-broken data format to "protect" users private keys.  To
    break this format I simply dusted off my year-old software, changed the
    "Netscape" strings to "Microsoft", and had an encryption-breaker which would
    recover most private keys "protected" with this format in a matter of seconds.
    In addition to the older format, newer Microsoft products also support the
    PKCS #12 format (which they originally called PFX), which Microsoft render as
    useless as the older format by employing the RC2 cipher with a 40-bit key.  In
    a truly egalitarian manner, this same level of "security" is used worldwide,
    ensuring that even US users get no security whatsoever when storing their
    private keys.  However even RC2/40 can take awhile to break (the exact
    definition of "a while" depends on how much computing power you have available,
    for most non-funded attackers it ranges from a few hours to a few days).
    Fortunately, there are enough design flaws in PKCS #12 and bugs in Microsofts
    implementation to ensure that we can ignore the encryption key size.  This has
    the useful - to an attacker - side-effect that even if Microsoft switch to
    using RC2/128 or triple DES for the encryption, it doesn't make the attackers
    task any more difficult.  By combining the code to break the PKCS #12 format
    with the code mentioned above which breaks the older format, we obtain a single
    program which, when run on either type of key file, should be able to recover
    the users private keys from most files in a matter of seconds.
    A (somewhat limited) example of this type of program is available in source
    code form from http://www.cs.auckland.ac.nz/~pgut001/pubs/breakms.c.  Because
    it's meant as a proof-of-concept program it's somewhat crude, and restricted to
    recovering passwords which are single dictionary words.  Note: This does not
    mean that using (say) two words as a password instead of one will protect your
    private key.  All it means is that I haven't bothered to write anything more
    sophisticated - no doubt anyone who was serious about this could adapt
    something like cracklib's password-generation rules and routines to provide a
    more comprehensive and powerful type of attack.  Similarly, by making trivial
    changes to the key file data format it's possible to fool the program until
    someone makes an equally trivial change to the program to track the format
    change - this is meant as a demonstrator only, not a do-everything encryption
    To use the program, compile and invoke it with:
      breakms <Microsoft key file> <word list file>
    Here's what the output should look like (some of the lines have been trimmed a
      File is a PFX/PKCS #12 key file.
      Encrypted data is 1048 bytes long.
      The password which was used to encrypt this Microsoft PFX/PKCS #12 file is
      Modulus = 00BB6FE79432CC6EA2D8F970675A5A87BFBE1AFF0BE63E879F2AFFB93644D [...]
      Public exponent = 010001
      Private exponent = 6F05EAD2F27FFAEC84BEC360C4B928FD5F3A9865D0FCAAD291E2 [...]
      Prime 1 = 00F3929B9435608F8A22C208D86795271D54EBDFB09DDEF539AB083DA912D [...]
      Prime 2 = 00C50016F89DFF2561347ED1186A46E150E28BF2D0F539A1594BBD7FE4674 [...]
      Exponent 1 = 009E7D4326C924AFC1DEA40B45650134966D6F9DFA3A7F9D698CD4ABEA [...]
      Exponent 2 = 00BA84003BB95355AFB7C50DF140C60513D0BA51D637272E355E397779 [...]
      Coefficient = 30B9E4F2AFA5AC679F920FC83F1F2DF1BAF1779CF989447FABC2F5628 [...]
    Someone sent me a test Microsoft key they had created with MSIE 3.0 and the
    program took just a few seconds to recover the password used to encrypt the
    One excuse offered by Microsoft is that Windows NT has access control lists
    (ACL's) for files which can be used to protect against this attacks and the one
    described below.  However this isn't notably useful: Most users will be running
    Windows '95 which doesn't have ACL's, of the small remainder using NT most
    won't bother setting the ACL's, and in any case since the attack is coming from
    software running as the current user (who has full access to the file), the
    ACL's have no effect.  The ACL issue is merely a red herring, and offers no
    further protection.
    Further Attacks (information provided by Steve Henson <shensonat_private>)
    There is a further attack possible which works because Microsoft's security
    products rely on the presence of the Microsoft CryptoAPI, which has a wonderful
    function called CryptExportKey().  This function hands over a users private key
    to anyone who asks for it.  The key is encrypted under the current user, so any
    other program running under the user can obtain their private key with a single
    function call.  For example an ActiveX control on a web page could ask for the
    current users key, ship it out to a remote site, and then delete itself from
    the system leaving no trace of what happened, a bit like the mail.exe program I
    wrote about 2 years ago which did the same thing for Windows passwords.  If the
    control is signed, there's no way to stop it from running even with the highest
    security level selected in MSIE, and since it immediately erases all traces of
    its existence the code signing is worthless.
    Newer versions of the CryptoAPI which come with MSIE 4 allow the user to set a
    flag (CRYPT_USER_PROTECTED) which specifies that the key export function should
    be protected with no protection (the default), user notification, or password
    protection.  However the way this is implemented makes it pretty much useless.
    Firstly, if the certificate request script used to generate the key doesn't set
    this flag, you end up with the default of "no protection" (and the majority of
    users will just use the default of "no protection" anyway).  Although Microsoft
    claim that "reputable CA's won't forget to set this flag", a number of CA's
    tested (including Verisign) don't bother to set it (does this mean that
    Microsoft regard Verisign as a disreputable CA? :-).  Because of this, they
    don't even provide the user with the option of selecting something other than
    "no security whatsoever".
    In addition at least one version of CryptoAPI would allow the "user
    notification" level of security to be bypassed by deleting the notification
    dialog resource from memory so that the call would quietly fail and the key
    would be exported anyway (this is fairly tricky to do and involves playing with
    the CPU's page protection mechanism, there are easier ways to get the key than
    Finally, the "password protection" level of security asks for the password a
    whopping 16 (yes, *sixteen*) times when exporting the key, even though it only
    needs to do this once.  After about the fifth time the user will probably click
    on the "remember password" box, moving them back to zero security until they
    reboot the machine and clear the setting, since the key will be exported with
    no notification or password check once the box is clicked.
    To check which level of security you have, try exporting your key certificate.
    If there's no warning/password dialog, you have zero security for your key, and
    don't even need to use the encryption-breaking technique I describe elsewhere
    in this article.  Any web page you browse could be stealing your key (through
    an embedded ActiveX control) without you ever being aware of it.
    Details on Breaking the Older Format
    The Microsoft key format is very susceptible to both a dictionary attack and to
    keystream recovery.  It uses the PKCS #8 format for private keys, which
    provides a large amount of known plaintext at the start of the data, in
    combination with RC4 without any form of IV or other preprocessing (even though
    PKCS #8 recommends that PKCS #5 password-based encryption be used), which means
    you can recover the first 100-odd bytes of key stream with a simple XOR (the
    same mistake they made with their .PWL files, which was publicised 2 1/2 years
    earlier).  Although the password is hashed with MD5 (allowing them to claim the
    use of a 128-bit key), the way the key is applied provides almost no security.
    This means two things:
    1. It's very simple to write a program to perform a dictionary attack on the
       server key (it originally took me about half an hour using cryptlib,
       http://www.cs.auckland.ac.nz/~pgut001/cryptlib/, another half hour to rip
       the appropriate code out of cryptlib to create a standalone program, and a
       few minutes to retarget the program from Netscape to Microsoft).
    2. The recovered key stream from the encrypted server key can be used to
       decrypt any other resource encrypted with the server password, *without
       knowing the password*.  This is because there's enough known plaintext
       (ASN.1 objects, object identifiers, and public key components) at the start
       of the encrypted data to recover large quantities of key stream.  This means
       that even if you use a million-bit encryption key, an attacker can still
       recover at least the first 100 bytes of anything you encrypt without needing
       to know your key (Frank Stevenson's glide.exe program uses this to recover
       passwords from Windows .PWL files in a fraction of a second).
    The problem here is caused by a combination of the PKCS #8 format (which is
    rather nonoptimal for protecting private keys) and the use of RC4 to encryt
    fixed, known plaintext.  Since everything is constant, you don't even need to
    run the password-transformation process more than once - just store a
    dictionary of the resulting key stream for each password in a database, and
    you can break the encryption with a single lookup (this would be avoided by
    the use of PKCS #5 password-based encryption, which iterates the key setup and
    uses a salt to make a precomputed dictionary attack impossible.  PKCS #5
    states that its primary intended application is for protecting private keys,
    but Microsoft (and Netscape) chose not to use this and went with straight RC4
    encryption instead).  This is exactly the same problem which came up with
    Microsoft's .PWL file encryption in 1995, and yet in the 2 1/2 years since I
    exposed this problem they still haven't learnt from their previous mistakes.
    For the curious (and ASN.1-aware), here's what the data formats look like.
    First there's the outer encapsulation which Microsoft use to wrap up the
    encrypted key:
      MicrosoftKey ::= SEQUENCE {
        identifier          OCTET STRING ('private-key'),
    Inside this is a PKCS #8 private key:
      EncryptedPrivateKeyInfo ::= SEQUENCE {
        encryptionAlgorithm EncryptionAlgorithmIdentifier,
        encryptedData       EncryptedData
      EncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
      EncryptedData = OCTET STRING
    Now the EncryptionAlgorithmIdentifier is supposed to be something like
    pbeWithMD5AndDES, with an associated 64-bit salt and iteration count, but
    Microsoft (and Netscape) ignored this and used straight rc4 with no salt or
    iteration count.  The EncryptedData decrypts to:
      PrivateKeyInfo ::= SEQUENCE {
        version             Version
        privateKeyAlgorithm PrivateKeyAlgorithmIdentifier
        privateKey          PrivateKey
        attributes    [ 0 ] IMPLICIT Attributes OPTIONAL
      Version ::= INTEGER
      PrivateKeyAlgorithmIdentifier ::= AlgorithmIdentifier
      PrivateKey ::= OCTET STRING
      Attributes ::= SET OF Attribute
    (and so on and so on, I haven't bothered going down any further).  One thing
    worth noting is that Microsoft encode the AlgorithmIdentifier incorrectly by
    omitting the parameters, these should be encoded as a NULL value if there are
    no parameters. In this they differ from Netscape, indicating that both
    companies managed to independently come up with the same broken key storage
    format.  Wow.
    For people picking apart the inner key, Microsoft also encode their ASN.1
    INTEGERs incorrectly, so you need to be aware of this when reading out the
    Details on Breaking the PFX/PKCS #12 Format
    The PFX/PKCS #12 format is vastly more complex (and braindamaged) than the
    older format.  You can find an overview of some of the bletcherousness in this
    format at http://www.cs.auckland.ac.nz/~pgut001/pfx.html.  After Microsoft
    originally designed the format (calling it PFX) and presented it to the world
    as a fait accompli, cleanup crews from other companies rushed in and fixed some
    of the worst problems and security flaws.  However by this time Microsoft had
    already shipped implementations which were based on the earlier version with
    all its flaws and holes, and didn't want to change their code any more.  A
    side-effect of this was that to be compatible, other vendors had to copy
    Microsofts bugs rather than produce an implementation in accordance with the
    standard.  Newer versions of the standard have now been amended to define the
    implementation bugs as a part of the standard.
    Anyway, as a result of this it's possible to mount three independant types of
    attack on Microsoft's PFX/PKCS #12 keys:
    1. Attack the RC2/40 encryption used in all versions, even the US-only one.
    2. Attack the MAC used to protect the entire file.  Since the same password is
       used for the MAC and the encrypted key, recovering the MAC password also
       recovers the password used to encrypt the private key.  The cleanup crews
       added a MAC iteration count to make this attack harder, but Microsoft
       ignored it.
    3. Attack the private key encryption key directly.  Like the MAC's, this also
       has an interation count.  Microsoft don't use it.
    Even if one of these flaws is fixed, an attacker can simply switch over and
    concentrate on a different flaw.
    I decided to see which one could be implemented the most efficiently.
    Obviously (1) was out (you need to perform 2^39 RC2 key schedules on average
    to find the key), which left (2) and (3).  With the refinements I'm about to
    describe, it turns out that an attack on the private key encryption is
    significantly more efficient than an attack on the MAC.
    To understand how the attack works, you need to look at how PKCS #12 does its
    key processing.  The original PFX spec included only some very vague thoughts
    on how to do this.  In later PKCS #12 versions this evolved into a somewhat
    garbled offshoot of the PKCS #5 and TLS key processing methods.  To decrypt
    data which is "protected" using the PKCS #12 key processing, you need to do the
      construct a 64-byte "diversifier" (which differs depending on whether you
            want to set up a key or an IV) and hash it;
      stretch the salt out to 64 bytes and hash it after the diversifier hash;
      stretch the password out to 64 bytes (using incorrect processing of the
            text string, this is one of Microsofts implementation bugs which has
            now become enshrined in the standard) and hash it after the salt hash;
      complete the hash and return the resulting value as either the key or the
            IV, depending on the diversifier setting;
    (it's actually rather more complex than that, this is a stripped-down version
    which is equivalent to what Microsoft use).
    This process is carried out twice, once for the key and once for the IV.  The
    hashing is performed using SHA-1, and each of the two invocations of the
    process require 4 passes through the SHA-1 compression function, for a total
    of 8 passes through the function.  Because the PKCS #12 spec conveniently
    requires that all data be stretched out to 64 bytes, which happens to be the
    data block size for SHA-1, there's no need for the input processing which is
    usually required for SHA-1 so we can strip this code out and feed the data
    directly into the compression function.  Thus the compression function (along
    with the RC2 key setup) is the limiting factor for the speed of an attack.
    Obviously we want to reduce the effort required as much as possible.
    As it turns out, we can eliminate 6 of the 8 passes, cutting our workload by
    75%.  First, we observe that the the diversifier is a constant value, so
    instead of setting it up and hashing it, we precompute the hash and store the
    hash value.  This eliminates the diversifier, and one pass through SHA-1.
    Next, we observe that the salt never changes for the file being attacked, so
    again instead of setting it up and hashing it, we precompute the hash and
    store the hash value.  This eliminates the diversifier, and another pass
    through SHA-1.
    Finally, all that's left is the password.  This requires two passes through
    the compression function, one for the password (again conveniently stretched
    to 64 bytes) and a second one to wrap up the hashing.
    In theory we'd need to repeat this process twice, once to generate the
    decryption key and a second time to generate the decryption IV which is used
    to encrypt the data in CBC mode.  However the start of the decrypted plaintext
      SEQUENCE {
        SEQUENCE {
    and the SEQUENCE is encoded as 30 82 xx xx (where xx xx are the length
    bytes).  This means the first 8 bytes will be 30 82 xx xx 30 82 xx xx, and
    will be followed by the object identifier.  We can therefore skip the first 8
    bytes and, using them as the IV, decrypt the second 8 bytes and check for the
    object identifier.  This eliminates the second PKCS #12 key initialisation
    call which is normally required to generate the IV.
    As this analysis (and the program) shows, Microsoft managed to design a
    "security" format in which you can eliminate 75% of the encryption processing
    work while still allowing an attack on the encrypted data.  To make it even
    easier for an attacker, they then dumbed the key down to only 40 bits, even in
    the US-only version of the software.  In fact this doesn't really have any
    effect on security, even if they used 128-bit RC2 or triple DES or whatever,
    it would provide no extra security thanks to the broken key processing.
    --Boundary (ID i.g+01I?SM6BM1/RQ0B'620I+NDGN3IG)--

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