February 15, 2006
A free monthly newsletter providing summaries, analyses, insights, and commentaries on security: computer and otherwise.
For back issues, or to subscribe, visit <http://www.schneier.com/crypto-gram.html>.
You can read this issue on the web at <http://www.schneier.com/crypto-gram-0602.html>. These same essays appear in the "Schneier on Security" blog: <http://www.schneier.com/blog>. An RSS feed is available.
In this issue:
- Risks of Losing Portable Devices
- Multi-Use ID Cards
- Ben Franklin on the Feeling of Security
- Valentine's Day Security
- Crypto-Gram Reprints
- U.S. Customs Opening International Mail
- The Failure of US-VISIT
- Identity Theft in the UK
- Passlogix Misquotes Me in Their PR Material
- The Doghouse: Super Cipher P2P Messenger
- Privatizing Registered Traveler
- Counterpane News
- Security Problems with Controlled Access Systems
- Security in Cartoons
- Countering "Trusting Trust"
- Security in the Cloud
- Comments from Readers
Some years ago, I left my laptop computer on a train from Washington to New York. Replacing the computer was expensive, but at the time I was more worried about the data.
Of course I had good backups, but now a copy of all my e-mail, client files, personal writings and book manuscripts were...well, somewhere. Probably the drive would be erased by the computer's new owner, but maybe my personal and professional life would end up in places I didn't want them to be.
If anything, this problem has gotten worse. Our digital devices have all gotten smaller, while at the same time they're carrying more and more sensitive information.
My laptop is my primary computer. It could easily contain every e-mail I've sent and received over the past 12 years, an enormous amount of work-related documents, and my personal everything.
I have several USB thumb drives, including a 2-gig drive that serves as my primary backup. The one I carry with me contains a complete dump of the past 12 months of my life, in a device so easy to lose some people I know buy them in bulk.
My cell phone is a Treo. It holds not only my frequently called phone numbers, but my entire address book -- including any personal notes I've made -- my calendar for the past six years, hundreds of e-mails, all my SMS messages, and a log of every phone call I've made and received. At least, it would if I didn't take specific pains to clean that information out once in a while.
A friend of mine has a habit of leaving his iPod on airplanes; he's been through three so far. The most recent one he lost contained not only his full music library, but his address book and calendar as well. And the press regularly reports stories about lost and stolen laptops with sensitive corporate documents or personal information of hundreds of thousands of individuals.
I could go on forever.
The point is that it's now amazingly easy to lose an enormous amount of information. Twenty years ago, someone could break into my office and copy every customer file, every piece of correspondence, everything about my professional life. Today, all he has to do is steal my computer. Or my portable backup drive. Or my small stack of DVD backups. Furthermore, he could sneak into my office and copy all this data, and I'd never know it.
This problem isn't going away anytime soon.
There are two solutions that make sense. The first is to protect the data. Hard-disk encryption programs like PGP Disk allow you to encrypt individual files, folders or entire disk partitions. Several manufacturers market USB thumb drives with built-in encryption. Some PDA manufacturers are starting to add password protection -- not as good as encryption, but at least it's something -- to their devices, and there are some aftermarket PDA encryption programs.
The second solution is to remotely delete the data if the device is lost. This is still a new idea, but I believe it will gain traction in the corporate market. If you give an employee a BlackBerry for business use, you want to be able to wipe the device's memory if he loses it. And since the device is online all the time, it's a pretty easy feature to add.
But until these two solutions become ubiquitous, the best option is to pay attention and erase data. Delete old e-mails from your BlackBerry, SMSs from your cell phone and old data from your address books -- regularly. Find that call log and purge it once in a while. Don't store everything on your laptop, only the files you might actually need.
I don't think we can make these devices harder to lose; that's a human problem and not a technological one. But we can make the loss just cost money, not privacy.
This essay originally appeared on Wired.com:
A Dutch army officer lost a memory stick with classified details of an Afghan mission.
I don't know about your wallet, but mine contains a driver's license, three credit cards, two bank ATM cards, frequent-flier cards for three airlines and frequent-guest cards for three hotel chains, memberships cards to two airline clubs, a library card, a AAA card, a Costco membership, and a bunch of other ID-type cards.
Any technologist who looks at the pile would reasonably ask: why all those cards? Most of them are not intended to be hard-to-forge identification cards; they're simply ways of carrying around unique numbers that are pointers into a database. Why does Visa bother issuing credit cards in the first place? Clearly you don't need the physical card in order to complete the transaction, as anyone who has bought something over the phone or the internet knows. Your bank could just use your driver's license number as an account number.
The same with those airline, hotel and rental car affinity cards. Or any of the discount cards given out by supermarkets, office supply stores, hardware stores and -- it seems -- everyone else. They could use any of your existing account numbers. Or simply your name and address. In fact, if you forget your card, they'll look up your account number if you give them your phone number. Why go to the trouble and expense of issuing unique cards at all?
A single, centralized authentication system has long been the dream of many technologists. Those involved in computer security will remember the promise of public-key infrastructure, or PKI. Everyone was going to have a single digital "certificate" that would be accepted by all sorts of different applications. It never happened.
And today the most far-reaching proposals for national ID cards -- including a recent South African proposal -- envision a world where a single ID would be used for everything. It won't happen either.
And neither will a world of biometrics. It's the obvious next step: Why carry a driver's license? Use your face or fingerprint.
But the truth is, neither a national ID nor a biometric system will ever replace the decks of plastic and paper that crowd our wallets.
For starters, the uniqueness of the cards provides important security to the issuers. Everyone has different rules for card issuance, expiration and revocation, and everyone wants to be in control of their own cards. If you lose control, you lose security. So airline clubs ask for a photo ID with your membership card, and merchants want to see it when you use your credit card, but neither will replace their cards with that photo ID.
Another reason is reliability. Your credit card company doesn't want your ability to make purchases to disappear if you have your driver's license revoked. Your airline doesn't want your frequent-flier account to depend on a particular credit card. And no one wants the liability of having their application depend on someone else's infrastructure, or having their infrastructure support someone else's application.
But security and reliability are only secondary concerns. If it made smart business sense for companies to piggyback on existing cards, they would find a way around the security concerns. The reason they don't boils down to one word: branding.
My airline wants a card with its logo on it in my wallet. So does my rental car company, my supermarket and everyone else I do business with. My credit card company wants me to open up my wallet and notice its card; I'm far more likely to use a physical card than a virtual one that I have to remember is attached to my driver's license number. And I'm more likely to feel important if I have a card, especially a card that recognizes me as a frequent flier or a preferred customer.
Some years ago, when credit cards with embedded chips were new, the card manufacturers designed a secure, multi-application operating system for these smartcards. The idea was that a single physical card could be used for everything: multiple credit card accounts, airline affinity memberships, public-transportation payment cards, etc. Nobody bought into the system: not because of security concerns, but because of branding concerns. Whose logo would get to be on the card? When the manufacturers envisioned a card with multiple small logos, one for each application, everyone wanted to know: Whose logo would be first? On top? In color?
The companies give you their own card partly because they want complete control of the rules around their own system, but mostly because they want you to carry around a small piece of advertising in your wallet. An American Express Gold Card is supposed to make you feel powerful and everyone else feel green. They want you to wave it around.
That's why you still have a dozen different cards in your wallet. And countries that have national IDs give their citizens yet another card to carry around in their wallets -- and not a replacement for something else.
This essay originally appeared on Wired.com:
January 17 was Ben Franklin's 300th birthday. Among many other discoveries and inventions, Franklin worked out a way of protecting buildings from lightning strikes, by providing a conducting path to ground -- outside a building -- from one or more pointed rods high atop the structure. People tried this, and it worked. Franklin became a celebrity, not just among "electricians," but among the general public.
An article in January's issue of "Physics Today" has a great 1769 quote by Franklin about lightning rods, and the reality vs. the feeling of security:
"Those who calculate chances may perhaps find that not one death (or the destruction of one house) in a hundred thousand happens from that cause, and that therefore it is scarce worth while to be at any expense to guard against it. But in all countries there are particular situations of buildings more exposed than others to such accidents, and there are minds so strongly impressed with the apprehension of them, as to be very unhappy every time a little thunder is within their hearing; it may therefore be well to render this little piece of new knowledge as general and well understood as possible, since to make us safe is not all its advantage, it is some to make us easy. And as the stroke it secures us from might have chanced perhaps but once in our lives, while it may relieve us a hundred times from those painful apprehensions, the latter may possibly on the whole contribute more to the happiness of mankind than the former."
Last Friday, the Wall Street Journal ran an article about how Valentine's Day is the day when cheating spouses are most likely to trip up:
"Valentine's Day is the biggest single 24-hour period for florists, a huge event for greeting-card companies and a boon for candy makers. But it's also a major crisis day for anyone who is having an affair. After all, Valentine's Day is the one holiday when everyone is expected to do something romantic for their spouse or lover -- and if someone has both, it's a serious problem."
So, of course, private detectives work overtime.
"'If anything is going on, it will be happening on that day,' says Irene Smith, who says business at her Discreet Investigations detective agency in Golden, Colo., as much as doubles -- to as many as 12 cases some years -- on Valentine's Day."
Private detectives are expensive -- about $100 per hour, according to the article -- and might not be worth it.
The article suggests some surveillance tools you can buy at home: a real-time GPS tracking system you can hide in your spouse's car, a Home Evidence Collection Kit you can use to analyze stains on "clothing, car seats or elsewhere," Internet spying software, a telephone recorder, and a really cool buttonhole camera.
But even that stuff may be overkill:
"Ruth Houston, author of a book called _Is He Cheating on You? -- 829 Telltale Signs,_ says she generally recommends against spending money on private detectives to catch cheaters because the indications are so easy to read. (Sign No. 3 under "Gifts": He tries to convince you he bought expensive chocolates for himself.)"
I hope I don't need to remind you that cheaters should also be reading that book, familiarizing themselves with the 829 telltale signs they should avoid making.
The article has several interesting personal stories, and warns that "planning a 'business trip' that falls over Valentine's Day is a typical mistake cheaters make."
So now I'm wondering why the RSA Conference is being held over Valentine's Day.
Wall Street Journal article (unfortunately, the link is only for paid subscribers):
Real-time GPS tracking system:
Home Evidence Collection Kit:
Internet spying software:
Crypto-Gram is currently in its ninth year of publication. Back issues cover a variety of security-related topics, and can all be found on <http://www.schneier.com/crypto-gram-back.html>. These are a selection of articles that appeared in this calendar month in other years.
TSA's Secure Flight
The Curse of the Secret Question:
Authentication and Expiration:
Toward Universal Surveillance:
The Politicization of Security:
Identification and Security:
The Economics of Spam:
Militaries and Cyber-War:
The RMAC Authentication Mode:
Microsoft and "Trustworthy Computing":
Hard-drive-embedded copy protection:
A semantic attack on URLs:
E-mail filter idiocy:
Internet voting vs. large-value e-commerce:
Distributed denial-of-service attacks:
Recognizing crypto snake-oil:
The press is reporting that Customs and Border Protection is opening international mail coming into the U.S. without warrant.
Sadly, this is legal.
Congress passed a trade act in 2002, 107 H.R. 3009, that expanded the Custom Service's ability to open international mail. Here's the beginning of Section 344:
"(1) In general.--For purposes of ensuring compliance with the Customs laws of the United States and other laws enforced by the Customs Service, including the provisions of law described in paragraph (2), a Customs officer may, subject to the provisions of this section, stop and search at the border, without a search warrant, mail of domestic origin transmitted for export by the United States Postal Service and foreign mail transiting the United States that is being imported or exported by the United States Postal Service."
If I remember correctly, the ACLU was able to temper the amendment, and this language is better than what the government originally wanted.
Domestic First Class mail is still private; the police need a warrant is to open it. But there is a lower standard for Media Mail and the like, and a lower standard for "mail covers": the practice of collecting address information from the outside of the envelope.
107 H.R. 3009:
US-VISIT is the program to program to fingerprint and otherwise keep tabs on foreign visitors to the U.S. A recent article talks about how the program is being rolled out, but the last paragraph is the most interesting:
"Since January 2004, US-VISIT has processed more than 44 million visitors. It has spotted and apprehended nearly 1,000 people with criminal or immigration violations, according to a DHS press release."
I wrote about US-VISIT in 2004, and back then I said that it was too expensive and a bad trade-off. The price tag for "the next phase" was $15B; I'm sure the total cost is much higher.
But take that $15B number. One thousand bad guys, most of them not very bad, caught through US-VISIT. That's $15M per bad guy caught.
Surely there's a more cost-effective way to catch bad guys?
My previous essay on the topic:
Recently there was some serious tax credit fraud in the UK. Basically, there is a tax-credit system that allows taxpayers to get a refund for some of their taxes if they meet certain criteria. Politically, this was a major objective of the Labour Party. So the Inland Revenue (the UK version of the IRS) made it as easy as possible to apply for this refund. One of the ways taxpayers could apply was via a Web portal.
Unfortunately, the only details necessary when applying were the applicant's National Insurance number (the UK version of the Social Security number) and mother's maiden name. The refund was then paid directly into any bank account specified on the application form. Anyone who knows anything about security can guess what happened. Estimates are that fifteen millions pounds has been stolen by criminal syndicates.
The press has been treating this as an issue of identity theft, talking about how criminals went Dumpster diving to get National Insurance numbers and so forth. I have seen very little about how the authentication scheme failed. The system tried -- using semi-secret information like NI number and mother's maiden name -- to authenticate the person. Instead, the system should have tried to authenticate the transaction. Even a simple verification step -- does the name on the account match the name of the person who should receive the refund -- would have gone a long way to preventing this type of fraud.
One problem with cameras is that you can't trust the watchers not to misuse them. This is a story of two CCTV camera operators who have been jailed for spying on a naked woman in her own home.
The Department of Homeland Security is funding the security of open-source products, including Linux, Apache, MySQL FreeBSD, Mozilla, and Sendmail. I think this is a great use of public funds. One of the limitations of open-source development is that it's hard to fund tools like Coverity. And this kind of thing improves security for a lot of different organizations against a wide variety of threats. And it increases competition with Microsoft, which will force it to improve its OS as well. Everybody wins.
All of that extra-legal NSA eavesdropping resulted in a whole lot of dead ends. This should come as a surprise to nobody. False alarms are what you get when institute a wholesale surveillance program with computers in charge; actual terrorist plots are simply too rare for anything else to happen. Good security has people in charge, and technology as a tool -- not the other way around.
A lot of the above article reads like a turf war between the NSA and the FBI, but the "inside baseball" aspects are interesting.
Interesting essay on suicide bombers. The conclusion: civil liberties increase security.
Great counterfeiting story that illustrates how criminals adapt to security measures. As a security measure, the merchants use a chemical pen that determines if the bills are counterfeit. But that's not exactly what the pen does. The pen only verifies that the paper is legitimate. So criminals take low-value bills, bleach them, and turn them into high-value bills.
Some detail about how bomb-sniffing dogs work:
Anonym.OS is an anonymous operating system. It's CD-based, designed so that you never touch the hard drive. You can walk up to a public computer and be anonymous on the Internet. I think this kind of thing is important, and am pleased to see its development.
The U.S. Defense Department wants to develop a lie detector that can be used surreptitiously. (Sure, who wouldn't want one of those?)
In this article about RFID cards, there's a paragraph that claims the chip can be read "from several yards away at border crossings." I thought the government was still claiming that the chips could only be read from inches away?
The 43rd Mersenne prime was found: 2^30,402,457 - 1, in a massively parallel search. It's 9,152,052 decimal digits long.
Article on how the French utilize domestic spying as a counterterrorism tool. I like reading how a judge is intimately involved in the process.
How to survive a robot uprising:
If you have a moment, take this survey on vulnerability disclosure. The researchers are trying to understand how secrecy and openness can be balanced in the analysis and alerting of security vulnerabilities:
Fascinating information about the information of espionage in New Zealand:
EPIC has documents that show how no-bid contracts for work on voting system standards go to vendors of those machines.
This is a sad story of the U.S. no-fly list. The person in question was flying from Canada to Mexico; his plane didn't land in the U.S., only flew over it. Another case of mistaken identity, of course.
And here's a story of a four-year-old boy on the watch list:
I get e-mail, occasionally weird e-mail. Every once in a while I get an e-mail from someone who needs a handwritten real-world cryptogram solved. This one is from 2004, and involves a multiple murder and suicide. The cryptogram was left by the murderer, and is on my blog. Take a look, at the note and the comments, if you are interested in trying to solve the mystery. But please be respectful of the relatives and friends of the victims; they're also following the progress on the blog.
The 2005 Information Security Salary and Career Advancement Survey is interesting to read.
High-tech wireless dead drop discovered in Russia. Cool stuff.
I am reminded of a dead drop technique used by, I think, the 9/11 terrorists. They used Hotmail (or some other anonymous e-mail service) accounts, but instead of e-mailing messages to each other, one would save a message as "draft" and the recipient would retrieve it from the same account later. I thought that was pretty clever, actually.
A couple of Dutch hacker have cracked their country's biometric passport. Two points stand out. One, the RFID chip in the passport can be read from ten meters. Two, lots of predictability in the encryption key -- sloppy, sloppy -- makes the brute-force attack much easier. But the references are from last summer. Why is this being reported now?
The NSA on how to redact (MS Word and PDF):
Read other NSA Security Configuration Guides here:
Interesting article about someone convicted for running a for-profit botnet:
A Dutch high-tech prison where "inmates wear electronic wristbands that track their every movement and guards monitor cells using emotion-recognition software." Emotion recognition software? Wow. Remember, new surveillance technologies are first used on populations with limited rights: inmates, children, military personnel, and the mentally ill.
Interesting white paper from the ACLU: "Eavesdropping 101: What Can The NSA Do?"
Unknowns tapped the mobile phones of about 100 Greek politicians and offices, including the U.S. embassy in Athens and the Greek prime minister. Details are sketchy, but it seems that a piece of malicious code was discovered by Ericsson technicians in Vodafone's mobile phone software. The code tapped into the conference call system. It "conference called" phone calls to 14 prepaid mobile phones where the calls were recorded.
More information in Greek:
Interesting research paper by Shishir Nagaraja and Ross Anderson, "The Topology of Covert Conflict." Implications for warfare, terrorism, and peer-to-peer file sharing:
Last year I wrote about an article by Daniel J. Solove and Chris Hoofnagle titled "A Model Regime of Privacy Protection." The paper has been revised a few times based on comments -- some of them from readers of my blog and Crypto-Gram -- and published.
The odds of this turning into law are, unfortunately, close to zero.
This Barcelona club requires an embedded RFID chip for VIP status:
And this company requires the same for access to the data center:
This article by Malcolm Gladwell on profiling and generalizations is excellent:
Here's an interesting rebuttal of Laszlo Kish's theoretically secure classical communications scheme.
And a response from Kish:
My original essay on the topic:
Check washing is a form of fraud. The criminal uses various solvents to remove data from a signed check -- the "pay to" name, the amount -- and replace it with data more beneficial to the criminal: his own name, a larger amount. This webpage -- I know nothing about who these people are, but they seem a bit amateurish -- talks about check fraud, and then gives advice on pens and inks to check writers.
Interesting paper on "petnames," which tries to solve the security problems inherent in naming.
The militarization of police work: more and more, police are using military-style weapons and tactics. "Eastern Kentucky University's Peter Kraska -- a widely cited expert on police militarization -- estimates that SWAT teams are called out about 40,000 times a year in the United States; in the 1980s, that figure was 3,000 times a year. Most 'call-outs' were to serve warrants on nonviolent drug offenders."
The TSA has announced that Secure Flight, its comprehensive program to match airline passengers against terrorist watch lists, has been suspended. I have written about this program extensively. It's an absolute mess in every way, and doesn't make us safer. But don't think this is the end. Under Section 4012 of the Intelligence Reform and Terrorism Prevention Act, Congress mandated the TSA put in place a program to screen every domestic passenger against the watch list. Until Congress repeals that mandate, these postponements and suspensions are the best we can hope for. Expect it all to come back under a different name -- and a clean record in the eyes of those not paying close attention -- soon.
Me on Secure Flight:
I just found an interesting paper: "Windows Access Control Demystified," by Sudhakar Govindavajhala and Andrew W. Appel. Basically, they show that companies like Adobe, Macromedia, etc., have mistakes in their Access Control Programming that open security holes in Windows XP.
Ed Felten has some good commentary about the paper on his blog.
Forget RFID; you can be tracked with WiFi from several hundred meters away.
In other news, Apple is adding WiFi to its iPod:
And don't forget, you can be tracked by your cell phone:
Gary T. Marx is a sociology professor at MIT, and a frequent writer on privacy issues. I find him both clear and insightful, as well as interesting and entertaining. This new paper is worth reading: "Soft Surveillance: The Growth of Mandatory Volunteerism in Collecting Personal Information -- 'Hey Buddy Can You Spare a DNA?'"
You can read a whole bunch of his other articles here:
Real fake ID cards. Or maybe they're fake real ID cards. This website sells ID cards. They're not ID cards for anything in particular, but they look official. If you need to fool someone who really doesn't know what an ID card is supposed to look like, these are likely to work.
I recently received a PR e-mail from a company called Passlogix. In part, it said: "Password security is still a very prevalent threat, 2005 had security gurus like Bruce Schneier publicly suggest that you actually write them down on sticky-notes. A recent survey stated 78% of employees use passwords as their primary forms of security, 52% use the same password for their accounts -- yet 77% struggle to remember their passwords."
Actually, I don't. I recommend writing your passwords down and keeping them in your wallet.
I know nothing about this company, but I am unhappy at their misrepresentation of what I said.
Super Cipher P2P Messenger uses "unbreakable Infinity bit Triple Layer Socket Encryption for completely secure communication."
In mid-January, the TSA announced details of its Registered Traveler program (sometimes known as "Trusted Traveler"). Basically, you pay money for a background check and get a biometric ID -- a fingerprint -- that gets you through airline security faster.
I've already written about why this is a bad idea for security:
"What the Trusted Traveler program does is create two different access paths into the airport: high security and low security. The intent is that only good guys will take the low-security path, and the bad guys will be forced to take the high-security path, but it rarely works out that way. You have to assume that the bad guys will find a way to take the low-security path.
"The Trusted Traveler program is based on the dangerous myth that terrorists match a particular profile and that we can somehow pick terrorists out of a crowd if we only can identify everyone. That's simply not true. Most of the 9/11 terrorists were unknown and not on any watch list. Timothy McVeigh was an upstanding US citizen before he blew up the Oklahoma City Federal Building. Palestinian suicide bombers in Israel are normal, nondescript people. Intelligence reports indicate that Al Qaeda is recruiting non-Arab terrorists for US operations."
But what the TSA is actually doing is even more bizarre. The TSA is privatizing this system. They want the companies that *sell* for-profit, Registered Traveler passes to do the background checks. They want the companies to use error-filled commercial databases to do this. What incentive do these companies have to not sell someone a pass? Who is liable for mistakes?
I thought airline security was important.
My previous essay:
This is an excellent discussion of the problems:
"What's worse than having identity thieves impersonate you to Chase Bank? Having terrorists impersonate you to the TSA."
Counterpane monitored someting like 100 billion network events, world-wide, in 2005. These are the attack trends that we're seeing.
For the RSA Conference, my wife and I have written a 110-page restaurant guidebook for the downtown San Jose area. It's a fun read, even if you aren't looking for a San Jose restaurant. (Do people know that I write restaurant reviews for the Minneapolis Star Tribune?) The restaurant guide will be available at the conference -- and of course you can download it -- but I have a few hundred to give away here. I'll send a copy to anyone who wants one, in exchange for postage. (It's not about the money, but I need some sort of gating function so that only those actually interested get a copy.) Cost is $2.50 if you live in the U.S., $3.00 for Canada/Mexico, and $6.00 elsewhere. I'll accept PayPal to my e-mail address -- firstname.lastname@example.org -- or a check to Bruce Schneier, Counterpane Internet Security, Inc., 1090A La Avenida, Mountain View, CA 94043. Sorry, but I can't accept credit cards directly.
Last weekend I spoke at the ACLU Washington Annual Membership Conference. The Seattle Times covered my speech:
There was an interesting security tidbit in an article on the recent post office shooting. "The shooter's pass to access the facility had been expired, officials said, but she apparently used her knowledge of how security at the facility worked to gain entrance, following another vehicle in through the outer gate and getting other employees to open security doors."
This is a failure of both technology and procedure. The gate was configured to allow multiple vehicles to enter on only one person's authorization -- that's a technology failure. And people are programmed to be polite -- to hold the door for others.
Note: There is a common myth that workplace homicides are prevalent in the United States Postal Service. (Note the phrase "going postal.") But not counting this event, there has been less than one shooting fatality per year at Postal Service facilities over the last 20 years. As the USPS has more than 700,000 employees, this is a lower rate than the average workplace.
NOTE: Some news reports say that she got another employee's badge at gunpoint, which is a failure of a completely different kind.
Airline security checkpoints:
Way back in 1974, Paul Karger and Roger Schell discovered a devastating attack against computer systems. Ken Thompson described it in his classic 1984 speech, "Reflections on Trusting Trust" Basically, an attacker changes a compiler binary to produce malicious versions of some programs, INCLUDING ITSELF. Once this is done, the attack perpetuates, essentially undetectably. Thompson demonstrated the attack in a devastating way: he subverted a compiler of an experimental victim, allowing Thompson to log in as root without using a password. The victim never noticed the attack, even when they disassembled the binaries -- the compiler rigged the disassembler, too.
This attack has long been part of the lore of computer security, and everyone knows that there's no defense. And that makes a new paper by David A. Wheeler so interesting. It's called "Countering Trusting Trust through Diverse Double-Compiling," and it describes a technique called diverse double-compiling (DDC) that detects this attack. From the abstract: "Simply recompile the purported source code twice: once with a second (trusted) compiler, and again using the result of the first compilation. If the result is bit-for-bit identical with the untrusted binary, then the source code accurately represents the binary. This technique has been mentioned informally, but its issues and ramifications have not been identified or discussed in a peer-reviewed work, nor has a public demonstration been made. This paper describes the technique, justifies it, describes how to overcome practical challenges, and demonstrates it."
To see how this works, look at the attack. In a simple form, the attacker modifies the compiler binary so that whenever some targeted security code like a password check is compiled, the compiler emits the attacker's backdoor code in the executable.
Now, this would be easy to get around by just recompiling the compiler. Since that will be done from time to time as bugs are fixed or features are added, a more robust form of the attack adds a step: Whenever the compiler is itself compiled, it emits the code to insert malicious code into various programs, including itself.
Assuming broadly that the compiler source is updated, but not completely rewritten, this attack is undetectable.
Wheeler explains how to defeat this more robust attack. Suppose we have two completely independent compilers: A and T. More specifically, we have source code S_A of compiler A, and executable code E_A and E_T. We want to determine if the binary of compiler A -- E_A -- contains this trusting trust attack.
Here's Wheeler's trick:
Step 1: Compile S_A with E_A, yielding new executable X.
Step 2: Compile S_A with E_T, yielding new executable Y.
Since X and Y were generated by two different compilers, they should have different executable code but be functionally equivalent. So far, so good. Now:
Step 3: Compile S_A with X, yielding new executable V.
Step 4: Compile S_A with Y, yielding new executable W.
Since X and Y are functionally equivalent, V and W should be bit-for-bit equivalent.
And that's how to detect the attack. If E_A is infected with the robust form of the attack, then X and Y will be functionally different. And if X and Y are functionally different, then V and W will be bitwise different. So all you have to do is to run a binary compare between V and W; if they're different, then E_A is infected.
Now you might read this and think: "What's the big deal? All I need to test if I have a trusted compiler is...another trusted compiler. Isn't it turtles all the way down?"
Not really. You do have to trust a compiler, but you don't have to know beforehand which one you must trust. If you have the source code for compiler T, you can test it against compiler A. Basically, you still have to have at least one executable compiler you trust. But you don't have to know which one you should start trusting.
And the definition of "trust" is much looser. This countermeasure will only fail if both A and T are infected in exactly the same way. The second compiler can be malicious; it just has to be malicious in some different way: i.e., it can't have the same triggers and payloads of the first. You can greatly increase the odds that the triggers/payloads are not identical by increasing diversity: using a compiler from a different era, on a different platform, without a common heritage, transforming the code, etc.
Also, the *only* thing compiler B has to do is compile the compiler-under-test. It can be hideously slow, produce code that is hideously slow, or only work on a machine that hasn't been produced in a decade. You could create a compiler specifically for this task. And if you're *really* worried about "turtles all the way down," you can write Compiler B yourself for a computer you built yourself from vacuum tubes that you made yourself. Since Compiler B only has to occasionally recompile your "real" compiler, you can impose a lot of restrictions that you would never accept in a typical production-use compiler. And you can periodically check Compiler B's integrity using every other compiler out there.
Now, this technique only detects when the binary doesn't match the source, so someone still needs to examine the compiler source code. But now you only have to examine the source code (a much easier task), not the binary.
It's interesting: the "trusting trust" attack has actually gotten easier over time, because compilers have gotten increasingly complex, giving attackers more places to hide their attacks. Here's how you can use a simpler compiler -- that you can trust more -- to act as a watchdog on the more sophisticated and more complex compiler.
"Reflections on Trusting Trust"
One of the basic philosophies of security is defense in depth: overlapping systems designed to provide security even if one of them fails. An example is a firewall coupled with an intrusion-detection system (IDS). Defense in depth provides security, because there's no single point of failure and no assumed single vector for attacks.
It is for this reason that a choice between implementing network security in the middle of the network -- in the cloud -- or at the endpoints is a false dichotomy. No single security system is a panacea, and it's far better to do both.
This kind of layered security is precisely what we're seeing develop. Traditionally, security was implemented at the endpoints, because that's what the user controlled. An organization had no choice but to put its firewalls, IDSs, and anti-virus software inside its network. Today, with the rise of managed security services and other outsourced network services, additional security can be provided inside the cloud.
I'm all in favor of security in the cloud. If we could build a new Internet today from scratch, we would embed a lot of security functionality in the cloud. But even that wouldn't substitute for security at the endpoints. Defense in depth beats a single point of failure, and security in the cloud is only part of a layered approach.
For example, consider the various network-based e-mail filtering services available. They do a great job of filtering out spam and viruses, but it would be folly to consider them a substitute for anti-virus security on the desktop. Many e-mails are internal only, never entering the cloud at all. Worse, an attacker might open up a message gateway inside the enterprise's infrastructure. Smart organizations build defense in depth: e-mail filtering inside the cloud plus anti-virus on the desktop.
The same reasoning applies to network-based firewalls and intrusion-prevention systems (IPS). Security would be vastly improved if the major carriers implemented cloud-based solutions, but they're no substitute for traditional firewalls, IDSs, and IPSs.
This should not be an either/or decision. At Counterpane, for example, we offer cloud services and more traditional network and desktop services. The real trick is making everything work together.
Security is about technology, people, and processes. Regardless of where your security systems are, they're not going to work unless human experts are paying attention. Real-time monitoring and response is what's most important; where the equipment goes is secondary.
Security is always a trade-off. Budgets are limited and economic considerations regularly trump security concerns. Traditional security products and services are centered on the internal network, because that's the target of attack. Compliance focuses on that for the same reason. Security in the cloud is a good addition, but it's not a replacement for more traditional network and desktop security.
This was published as a "Face-Off" in "Network World":
The opposing view is here:
There are hundreds of comments -- many of them interesting -- on these topics on my blog. Search for the story you want to comment on, and join in.
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CRYPTO-GRAM is written by Bruce Schneier. Schneier is the author of the best sellers "Beyond Fear," "Secrets and Lies," and "Applied Cryptography," and an inventor of the Blowfish and Twofish algorithms. He is founder and CTO of Counterpane Internet Security Inc., and is a member of the Advisory Board of the Electronic Privacy Information Center (EPIC). He is a frequent writer and lecturer on security topics. See <http://www.schneier.com>.
Counterpane is the world's leading protector of networked information - the inventor of outsourced security monitoring and the foremost authority on effective mitigation of emerging IT threats. Counterpane protects networks for Fortune 1000 companies and governments world-wide. See <http://www.counterpane.com>.
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Copyright (c) 2006 by Bruce Schneier.
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