Tales from the Crypto – Page 49 – Alun Jones (Security MVP Reconnect) writes about security, cryptography, SSL, PKI, and pretty much anything else that bothers him enough.

NTFS Alternate Data Streams (ADS) & Win2003.

I just noticed a new pair of functions in the Win32 API for Windows Server 2003 – FindFirstStreamW and FindNextStreamW. These are interesting from a security perspective, if for no other reason than that alternate data streams are useful places to hide data.

Before this function, a programmer had to open files with “Backup semantics”, and work his way laboriously through the structure of the streams within the file in order to find out what streams there are. Here’s the sort of code you had to write:

int EnumStreams(const WCHAR *file, StreamEnumProc *func, void *funcarg)
    WCHAR wszStreamName[_MAX_PATH+sizeof(WIN32_STREAM_ID)];
    WIN32_STREAM_ID &wsId=*((WIN32_STREAM_ID *)wszStreamName);
    DWORD dwRead, dwStreamHeaderSize, dw1, dw2;
    LPVOID lpContext=NULL;
    BOOL bResult=TRUE;
    while (bResult)
        bResult=BackupRead(hFile, (LPBYTE)&wsId, dwStreamHeaderSize,
                           &dwRead, FALSE, TRUE, &lpContext);
        if (!bResult || dwRead!=dwStreamHeaderSize)
        if (wsId.dwStreamNameSize+dwStreamHeaderSize>=
            return -1;
        bResult=(!wsId.dwStreamNameSize ||
            BackupRead(hFile, (LPBYTE)&wsId.cStreamName[0],
                wsId.dwStreamNameSize, &dwRead, FALSE, TRUE,
        if (!bResult || dwRead!=wsId.dwStreamNameSize)
        if (bResult)
            int nStrLen=wsId.dwStreamNameSize/sizeof(WCHAR);
            func(file, &wsId, funcarg);
            if (wsId.Size.LowPart || wsId.Size.HighPart)
                BackupSeek(hFile, ULONG_MAX, LONG_MAX, &dw1,
                    &dw2, &lpContext);
    BackupRead(hFile, NULL, 0, &dwRead, TRUE, FALSE, &lpContext);
    if (!bResult)
        return GetLastError();
    return 0;

Since the FindFirst/NextStreamW functions are only in Windows Server 2003, you’ll still have to do something like that mess on a Windows XP or previous system.

There are still no tools, however, in the base operating system that allow an IT Professional to search for alternate data streams that might be attached to files on his workstation. So, a while back, I created “sdir”, a program that allows you to list alternate data streams on a file or a directory, or recursively through a directory tree. You can find it at http://www.wftpd.com/downloads.htm

Several people have suggested that alternate data streams (or ADS, as they are often referred to) are ideal for infecting a system such that the virus scanner will not find the virus. That’d be true, except for a couple of things – first, that the virus would still need to arrive on the system using a method that would allow virus scanners to look for virus signatures. The virus would have to travel through a download – which doesn’t have a hidden stream – or an email – which doesn’t have a hidden stream – etc, etc. Second, the virus scanners have already added ADS scanning to their repertoire.

IIS – Advanced Digest Authentication is MD5-SESS

Reviewing the security for another application today, I find that it relies on Digest Authentication, which is a horrible thing to do to a secure system.

Why is that? Because it requires that you enable the check-box labeled “Store Passwords Using Reversible Encryption” (and once you’ve done so, any users who want to use Digest Authentication have to change their password, so that their password can be stored decryptably).

This is such a horrible thing to do that Microsoft frequently refers to this as storing passwords “in plaintext”. There’s really not much difference – anyone who can get access to the encrypted store will be able to decrypt the passwords.

Fortunately in IIS 6, along comes Advanced Digest Authentication. Now, this is not exactly described very clearly, and in some cases, the description says some really bad things – one description I found implies that this method hashes the user name, domain, and password, and then waits for the browser to send exactly that same hash in order to identify itself.

Fortunately, that’s not the case – the people in IIS are not idiots. What appears to be the case, and it’s almost impossible to find documentation backing this up (probably because of the “Not Invented Here” syndrome), is that what Microsoft terms “Advanced Digest Authentication” is nothing more complicated than the MD5-SESS Digest Authentication described in RFC 2617.

That does hash the username, password and domain name (or realm, if you want the proper term), and stores that at the server. But it’s not what it looks for from the client. The client takes that hash, appends a nonce provided by the server, and one provided by the client, and hashes that string.

This process of “take a hash, add something random to it, and hash it again” is a fairly common procedure in security protocols, and is designed to avoid replay attacks while simultaneously avoiding the use of stored passwords.

This is great… with one caveat. Now, the hash of the username, password and realm are essentially the password to that realm. If you were the sort of nasty person that could get a hold of the reversibly encrypted password, and decrypt it, you could just as easily get a hold of the hash – and that’s all you need to generate the Advanced Digest Authentication message.

All is not lost, though – this only allows an administrator of the realm to get access to his own realm as if he were one of his own users. The “rogue administrator” problem is one that doesn’t have a good solution (except for “trust your administrators not to go rogue”), and is rightly treated as a problem not worth investigating for most systems.

What was allowed under the old Digest Authentication is that the administrator could fetch the clear-text version of the user’s password, which is almost certainly the same as that user’s password on another system. Now this is a problem worth tackling, and the Advanced Digest Authentication method adequately prevents this from occurring. The administrator can only fetch a hash, and that hash is no use outside of his domain.

Oh, and those hashes are still only generated when the password is created, set, or changed, so as a result, if you change the realm, all users have to change their password again. I’m not quite sure if you have to do this when enabling the Advanced Digest Authentication feature.

Programmer Hubris Part 1 – He’s Just Not That Into You

Programmers are, by nature, a very arrogant bunch. We know this – and it comes from the nature of what we do. In our own little world inside the computer, we are a god.

For this reason (and perhaps a few others), it becomes very easy for us to forget to think outside of our little world, and remember that we are also acting as servant to the people that own the box we’re writing our software for.

This is particularly true of developers of off-the-shelf software, who spend next to no time actually dealing with the people that use, or will use, their programs.

So, I’m going to start a topic on the arrogance of developers.

My first example is multiple – Real Networks, Apple Quicktime, and several other programs insist on placing their icons on the system tray – down in the bottom right-hand corner, with the clock.

Now, if these were just icons, that would not be such a bad thing – after all, your system is littered with icons that represent shortcuts, data files, executables, and so on.

Unfortunately, the icons in the system tray are special. Each one represents a running program. Each one is placed there by a programmer who believes that his or her program is so important to all users that it should remain permanently running.

Me, I play a Quicktime Movie, or a Real Audio file, about once every couple of months. It can be a month or more before I notice that the icon is on my system tray, taking up time, communicating who knows what, and exposing goodness knows how many application-related flaws to the Internet.

So, a plea to developers – unless your software positively requires to be run all the time in all its possible installation modes, make it go away when I’m done using it.

No offence, but I’m just not that into your program.

There’s no folk without some ire

[I was going to title this “PATRIOT – Piddling Around The Real Issues Of Terrorism”, but I figured that’d be a little too inflammatory.]

The other day, I was listening to good-old-fashioned talk radio, and something the host said surprised me. He was blathering about how Democrats wanted to make friends with terrorists.

It sounds really stupid when you put it in those terms, but yes – that’s essentially the approach that has to happen. Like a pyramid scheme, the terrorists at the top feed hatred down, and get power back up the chain. While that feed of hatred is accepted by their “down-line”, the feed of power up the line continues. You don’t stop terrorism by making friends with the guys at the top, you stop terrorism by making nice to the guys at the bottom; you remove the power-base by making it difficult for people to hate you.

So, how does that remotely connect to the usual topic of this blog, computer security? Like this:

Vendors [think Microsoft, but it also applies to small vendors like me] face this sort of behaviour, on a smaller level, when it comes to vulnerability reports. Rightly or not, there’s a whole pile of hatred built up among some security researchers against vendors, initially because over the years vendors have ignored and dismissed vulnerability reports on a regular basis. As a result, those researchers believe that the only way they can cause vendors to fix their vulnerabilities is to publicly shame the vendors by posting vulnerability announcements in public without first contacting the vendor.

I’m really not trying to suggest that vulnerability researchers are akin to terrorists. They’re akin to an oppressed and misunderstood minority, some members of which have been known to engage in acts which are inadvertently destructive.

Microsoft and others have been reaching out of late to vulnerability researchers, introducing them to the processes that a vendor must take when receiving a vulnerability report, and before a patch or official bulletin can be released. Some researchers are still adamant that immediate public disclosure is the only acceptable way; others have been brought over to what I think is the correct way of thinking – that it helps protect the users if the first evidence that exists in public is a bulletin and a patch.

The security industry gets regularly excited by the idea of a “zero-day exploit” – a piece of malware that exploits a vulnerability from the moment that the vulnerability is first reported. I think it’s about time we got excited about every release of a “zero-day patch”.

How many kinds of secret are there?

Trick question:
How many different classifications of document should you have?
The answer: two.

Documents should be “public” or “private”.

Public documents need not necessarily be published public documents, but contain information that is not important to keep from the public. By fact, any document that has been published is already public, no matter what you’d like it to be.

Private documents should be attached to an explicit or implicit list of people who are entitled to view them, and there should be policies, procedures, practices and phreakin’ ACLs in place to make sure that their privacy is not broken.

Can you think of a document secrecy category that isn’t covered by this?

Help-desk snafu

So, I started my new job last week.

I spent much of the first week trying to stop the “message waiting” light from flashing. I knew what I had to do – call the voice-mail system, listen to all the old messages and dump them.

So, I press the button for voice-mail and get an alternating tone. What does that mean? Does it mean I’m in the voice mail system? Does it mean “enter your password”? I have no idea, so I enter my password, and it makes a different beep, so maybe that means “no, wrong password”.

I go to the “self-help” page, and the “phone training” pages. They disagree as to which is the default password. Great.

Now I have to do the thing I hate – I have to call the help-desk. So I call, and I let them know what the problem is. I give them my email account and all the other information that they need.

Finally, I come into work after the weekend, and I think I’ve figured it out. I leave the voice-mail button alone, and dial the voice-mail extension by hand. This time, it says something like “welcome to the voice-mail system, please enter your password”.

Seventeen messages later, fifteen of which are from before I started at the company, I reach the cracker. A message from the help-desk, telling me that maybe my voice-mail button isn’t programmed yet, and detailing the default password. They end by telling me “if you are still unable to access your voice-mail, please call the help-desk”.

I call the help-desk in return, and suggest that when people are having trouble with the phone system, that the phone system is not necessarily the best method of contacting them.

What is a fingerprint?

Okay, so we should all be well aware as to what a fingerprint is – it’s the pattern of ridges on most people’s fingers that get left in smudges on glass doors.

What can it be used for?

The question arises as I look at my Microsoft Fingerprint Reader, and try to explain why a fingerprint reader is purposely disabled from authenticating an account to a domain.

Let’s first get into what is needed to log on to a system.  In computer science terms, you need a claim of identity, and you need one or more pieces of evidence, that together will suffice as proof of identity.

Think of the bank ATM as an example – your debit card is the claim of identity (because it contains your account number), and it’s also a piece of evidence (because you cannot use the ATM without the card).  Your PIN is a second form of evidence; with the card and your PIN, you claim and prove your identity for the purposes of the ATM’s operations.

Logging on to a domain is similar – you provide a username, which is a claim of identity, and you provide a password, which is the evidence used as proof of identity.

What differentiates a claim of identity from a proof of identity?  That’s a little subtle.

A claim of identity is any information that uniquely identifies a person, or a role, or an identity, such that it can be used by the computer to look up that identity.  Your ATM card is a claim of identity, because it contains the account number(s) to which you are allowed access, in a form that the ATM can use to supply as your identifier to your bank.

A proof of identity is made up of one or more pieces of evidence that can be relied on to demonstrate that the claimed identity is matched by the person or process presenting themselves for identification.  It’s “something you are, something you have, or something you know.”  The evidence should consist of items which, in conjunction with one another, can only be presented by the authorised user(s) whose identity is being claimed.

So, what is a fingerprint?

Is it a proof of identity?

Not as far as the Microsoft Fingerprint Reader (or any other low-resolution fingerprint reader) is concerned.  Give me a couple of warm gummy bears, a freezer, five minutes, and the use of your finger, and I can produce a replica “finger” that will authenticate to the reader.  What’s more, if someone can give me a glass door you’ve pushed open, or a cup or glass that you’ve held, within a couple of hours I can make as many gummy fingers as I need, that will all authenticate as you on any low-resolution reader.  [I won’t go into the process here].  In more grisly methods, I don’t even have to go to all that effort.

Higher-quality fingerprint readers will look for a finger’s warmth (yeah, a warm gummy bear will beat you there), or pulse, translucency, capillary patterns, or other features that are supposedly only going to be present in a real finger attached to a live human, but those are expensive.

So, because this fingerprint reader is a basic one, to it, a fingerprint alone is not evidence sufficient for a proof of identity – combined with a guard manning the station, trained to check for gummy bears and severed fingers, and who can deny suspicious attempts, it may be enough, but that’s not its designed method of operation.

Is a fingerprint, then, a claim of identity?

Not in general, no.  The fingerprint can be matched against stored fingerprints to see how closely it matches, but the fingerprint alone is not capable of generating the user ID, which is what you’d want.  The fingerprint has to be almost exhaustively matched – this is why cops on TV seem to spend days getting a fingerprint match.  It is very quick to say “here are two fingerprints, do they match” (which would be evidence of identity), but extremely slow to say “here’s a fingerprint, whose is it?”

Then there’s the issue of uniqueness.

I’ve searched and I’ve searched, and I’m surprised to find that there are as many as zero good scientific reviews of large fingerprint databases to check for uniqueness.  So, when a “fingerprint expert” testifies that the fingerprint found at a crime scene matches the defendant, and the defendant only, they’re relying on a guess that hasn’t been reliably tested, and which has been proven false (or at least, badly collected and analysed) on some celebrated occasions:

[Note that these are culled from a very quick search of only one news agency’s recent output.]

Obviously, a fingerprint can be used to refute identity, in much the same way as “the suspect had red hair” will refute the identity of a suspect who does not have red hair, but there’s still significant doubt in my mind as to whether it can be relied upon in any way to prove identity – not without extra layers of evidence to increase the reliability.

Use other, more reliable, measurable, and provable means to protect your networks.  Passwords – strong passwords – will serve you far better than a low-resolution fingerprint reader.

Top ten lists and low-hanging fruit.

I wrote this in response to a question that asked what would be the best firewall to install on a Windows 98 machine.


I like to advise people that they should look at security measures and ask “is this on my top ten list?”, and not do anything that isn’t on the list.  Obviously, as you work through the list and discard items, something that wasn’t on the top ten list before may come back onto the list and deserve to be done.


When you’re on Windows 98, I think that your top ten list starts with:

1. Unplug the network cable.

2. Upgrade to Windows XP.

3. Install Service Pack 2.

4. Convert your hard drive from FAT to NTFS.

5. Upgrade your applications.

6. As much as possible, stop running as an administrator, run as a “restricted user”.

7. Check that the Windows XP Firewall is enabled.

8. Plug the network cable back in.

9. Upgrade (at http://windowsupdate.microsoft.com) from Windows Update to Microsoft Update (look in the bottom right for the link).

10. Download and install patches for everything.


As you can imagine, several of the top-ten list items are “once only”, and others are “every month” or similarly require regular re-visiting.


The key here is to build your list on the basis of what the low-hanging fruit is.


Obviously the original question was posed by someone who was looking for the low-hanging fruit, but was labouring under the misconception that the low-hanging fruit in this case was that part of his system that he could most easily address.  That’s not a good approach, because you end up spending a lot of time making easy fixes, while the attackers are going to come in and get you through the gaping hole that you’ve labeled “difficult to fix”.


You have to address the low-hanging fruit as seen by your attackers.  What’s the easiest way to get into your system?  Address that, no matter how hard it is, because that’s the way that you will be breached.

"New Nigerian law would jail spammers" – MSNBC story.

I don’t know how I missed this story when it first appeared, but apparently the country of Nigeria is so upset with its well-earned reputation as the source of an unfeasibly large number of fraudulent spams, that they are now trying to enact a law that would cause spammers, phishers, fraudsters, child pornographers, and terrorists to spend six months to five years in jail, and pay the equivalent of $77 – $7700 in fines.  Oh, and the government could seize any profits made from the schemes in question.


Having seen how badly our own (USA) attempts to “curtail spam” with laws that do nothing of the sort have gone, I wish the Nigerians the best of luck.

DRM – safe for work, but please not at home.

Here’s a theme you’ll have heard from me a dozen times if you’ve been following my Usenet traffic:

“When I buy software, or music, or videos, I want to buy the content, not just the plastic it comes on.”

What do I mean by this?

Simply that I don’t want to find myself restricted as to what I can do with the software, music, videos, etc.  If I buy a DVD, I want to be able to watch it on my choice of device, in my choice of country, and (if necessary) in my choice of format.

With the recent news of Sony’s unpleasant intrusion into home computers (or this link for an American version), it’s a reminder for me to say this again – my computer is my computer, and I’ll thank you – any of you – to leave me to decide and actively accept what software to install on it.

Yes, Sony may include a licence on their CDs – but who reads them?  Who even expects that an audio CD (not a software title) will install software on their machine?

The key point to my mind is that I, the system administrator on my home computer, cannot hope to maintain the security and reliability of my system if I cannot know when software is installed, and be able to remove what software I choose to no longer be there.  If Mark Russinovich, a hugely capable developer, cannot remove the software from his system without losing access to parts of his system, what hope do the rest of us have?

Digital Rights Management, or DRM, is frequently put forward by music companies as the next best thing since sliced bread.  It’s not, and it’s not even remotely appropriate for home use, or for preventing privacy.

DRM works in exactly one scenario: when the owner of the rights also controls the behaviour of those subject to DRM.  That almost always means “work”, where the rights owner can discipline, and eventually terminate, those that refuse to respect the DRM restrictions on content.  To attempt to apply it to home use, where there is no such control, is to ignore that basic limitation of DRM.

And, quite frankly, it’s insulting.  I don’t feel like pulling out the “innocent until proven guilty” argument in its entirety, but as a legal and honest purchaser of all manner of electronic content, I feel insulted that I am then limited as to my use – not merely limited as to illegal copying and distribution, but limited as to what should be legal – copying for my own use in different devices.

I believe in this so strongly that I have made sure that the software I sell is controlled by those who pay for it.  You can move our software from one machine to another, and we ask only that you use no more copies than you have paid for.  We assume that we can trust our legitimate customers.  We put a few limits into the freely-distributed version, only because if we don’t, nobody buys (trust us, we’ve tried).  Even the honest need a few reminders some times.