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, Summary ------- 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). Background ---------- 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 breaker. 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 bit): 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 'orthogonality'. 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 file. 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 ) --------------- 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 this). 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'), encryptedPrivateKeyInfo EncryptedPrivateKeyInfo } 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 data. 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 following: 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 is: SEQUENCE { SEQUENCE { OBJECT IDENTIFIER, ... 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.