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New ZeuS Botnet No Longer Needs Central Command Servers 137

Posted by timothy
from the andromeda-strain dept.
c0mpliant writes "Researchers at Symantec have identified a new variant of the ZeuS botnet which no longer requires a Command and Control server. The new variant uses a P2P system, which means that each bot acts like a C&C server, but none of them really are. The effect of which is that takedowns of such a network will be extremely difficult because there is no one central source to attack."
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New ZeuS Botnet No Longer Needs Central Command Servers

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  • If you want to actually control the botnet, you do need a C&C. What this setup might achieve is the obfuscation of the command flow so the C&C is much harder to identify.

    • by Anonymous Coward

      Don't confuse logic with sensationalism. "Security journalists" understand computers about as well as the average Facebook user.

      • by errandum (2014454)

        The article is not wrong. There is no need for a C&C server, which doesn't mean there aren't people with computers controlling it...

        • The article is not wrong. There is no need for a C&C server, which doesn't mean there aren't people with computers controlling it...

          Exactly. They can put instructions out on the P2P network and it will just look like another infected machine "sharing" with it's brother bots.

      • by Anonymous Coward

        While we're on the topic, don't confuse malware writers with "security experts". Security experts understand security about as well as Symantec.

    • by neokushan (932374) on Saturday February 25, 2012 @10:44AM (#39158125)

      If my understanding is correct, the entire Zeus network now communicates amongst itself. There's no intermediate sites, IRC channels, twitter accounts, etc.
      This also means that any infected machine can act as the C&C. If that machine gets taken down, all the zeus authors need to do is use another node and keep going. It'll be extremely difficult to trace where the commands are genuinely coming from unless they happen to have access to the C&C server that originally sent the command, then hope that some sort of trail has been left - not an easy task, really

      • by errandum (2014454) on Saturday February 25, 2012 @10:56AM (#39158187)

        I think it's worse than that. If it works with the scheme fasttrack (for example) uses, you'd need to get the people behind the computer to actually kill it. Even if they get the original machine, they can just switch places and keep going (since there is no single point of failure, from what I read).

        • by Ramin_HAL9001 (1677134) on Saturday February 25, 2012 @11:07AM (#39158237)

          But on the other hand, you still need to issue commands to the C&C. If you can figure out the communication protocol used to assign C&C powers to a node, then security researchers can easily toss-out the command to become a C&C to all nodes and then sink-hole it.

          Further, I am not aware of any way to encrypt communications between the botnet's controllers and the botnet's nodes because every node will need to have the private key to decrypt incoming communications. So anyone can analyze a node and just pick out the private key, and then start issuing commands to it as though they were the operators. It just adds bulk to the botnet code, and doesn't prevent anyone from sink-holing it.

          I think the real difficulty is simply containment. If the virus is designed to spread as rapidly as possible, then you need to spend a lot of time finding nodes and taking control of them to shut them down. I think the designers of ZueS are counting on that, and hope sheer numbers will be better than more precise control.

          • by jonamous++ (1687704) on Saturday February 25, 2012 @11:12AM (#39158259)
            What if the commands need to be signed?
            • If the signatures need to be verified by a signature authority controlled by the attackers, it would be much easier to find out who is issuing the commands, just trace all communications back to the signature authority. And a communication to the signature authority would happen every time a command message needs to be verified by one of the nodes.

              Otherwise, the commands must be self-signed, so an ordinary man-in-the-middle attack on any one the nodes could reveal the signature to you. You could do it as so

              • by irtza (893217) on Saturday February 25, 2012 @12:00PM (#39158491) Homepage

                There is no need for a private key for the signature nor the need for a signature authority. If I were to give you a public key and I sent you a signed message, you could verify the message came from me as long as my private key was hidden from a third party.

                This setup still requries C&C software, but as long as the C&C software is not distributed, each node can not initiate a command, but can propogate an already signed one. There would need to be a program that can insert a new signed command, but that need not be on every node. It would be much like gnutella - maintain a list of nodes to connect to and if you get in, you isue your command - disconnect from the network and you can reconnect at will from another IP address.

              • by 1s44c (552956)

                You really are not thinking in peer2peer terms. It's not peer to peer if there is any central authority, and there is no need for one anyway.

              • by Lennie (16154)

                "signature authority" How do you trace that ? It would be similair to a selfsigned certificate used with HTTPS.

                The public key is obviously part of the software, you can't man-in-the-middle that. Why would there be a private key in the bot software ?

                The issuer of the commands just connect to one of the nodes in the P2P-network and creates a command and signs his/her command.

                It is much more likely they made an implementation mistake though, that is usually how these things get cleaned up.

              • by Anonymous Coward

                if you don't know what you're talking about, why talk?

              • Re: (Score:2, Insightful)

                by Anonymous Coward

                If the signatures need to be verified by a signature authority controlled by the attackers

                Red Flag 1: You don't know how Public Key Cryptography works.

                There is nothing magical about Certificate Authorities like Verisign. All they do is generate a random N-bit public/private key pair that meets certain mathematical rules (must be prime) then stick it inside a certificate (X.509 standard) then sign that certificate using the Verisign private key. How do people know what the Verisign public key is? The key is built-in to Windows, Firefox, Chrome, etc. All these programs have nothing more than a

          • by Kjella (173770) on Saturday February 25, 2012 @11:29AM (#39158355) Homepage

            You're still thinking in terms of a C&C, when it doesn't apply anymore. Think of it more like a contagion, there's no "C&C" humans only people in contact with other people in contact with yet more people. There is no command to become a C&C. Commands are encrypted but also signed by the operators and nodes only have the public key to that so you can't fake one. They can just introduce a command anywhere, to any node and it'll relay it to its peers, that'll relay it to it's peers again amd so on until everyone got the command. You probably use a unique ID to avoid loops, like command 0xfe36735b I've already relayed, no need to relay it again.

            • I don't know about this.

              It seems like "Campaign Contribution" is the command to become a Human C&C.

            • by mycroft16 (848585)
              So, would they be able to monitor the traffic and watch for new commands spreading and track the route those are coming from to find the computer that the new C&C stuff is flowing from? Seems like it would be hella difficult and time consuming, but possible.
            • by CxDoo (918501)

              I get the p2p part, encryption & so on, but how does one peer find out where are the others?

          • by errandum (2014454)

            You can simply send the public key in order to verify a signature by a private key. And there are ways to negotiate a secure exchange using only that. If they did things right, this thing is unstoppable unless ISP's get in on the action by disconnecting the infected nodes.

          • by Wierdy1024 (902573) on Saturday February 25, 2012 @11:53AM (#39158459)

            I'm not sure about your comments re: keys.

            It seems relatively easy to design a botnet to be peer to peer and yet not able to be taken over by a rogue node. Consider a P2P overlay network where each node plays "chineese whispers" and forwards any packet to all neighbours (with some TTL limit).

            The botnet owner creates a public private keypair, and uses his private key to sign control messages. Each host takes each incoming packet and checks if it is signed by the botnet-owner, which requires the public key of the botnet owner, and is built into the code. If someone reverse engineers a node, all they have is the public key, so can't sign messages (since signing requires a private key).

            An attacker could still DoS this network with unsigned Control messages, but that can easily be thwarted by:
            a) never forward any unsigned message
            b) forward signed messages only if it's version number is higher than the last forwarded message.

            To hide himself and operate the network, the botnet owner can use TOR or some other anonymising service to connect randomly to any node in the network (rather like utorrent DHT does), and send a signed control message with a version number higher than any seen before by the network.

          • By the way, I think you were mixing up encryption with authentication. You are right that the control messages can't be encrypted, since they must be able to be decrypted by any node in the network, and hence security researchers have access to whatever key they are encrypted with, and can also decrypt them.

            They can however be signed (authenticated) to prevent anyone but the real botnet owner from sending them.

            (note, all of this is assuming assymetric (eg. RSA) cryptography - where one key is used for encr

            • by Lennie (16154)

              They could still use Diffie Hellman key exchange to bootstrap the encryption. You might be able to decrypt the traffic to/from a node that you have control over, but you won't be able to see the traffic between other nodes.

          • by jonwil (467024)

            If they were smart, they would have used public key cryptography to ensure that only commands signed by the bot-net author will be accepted. Assuming the RSA key is strong enough, it would be impossible for anyone else to send commands short of an as-yet-unknown weakness in RSA or a bug in the bot-net code.

            • "If they were smart"
              I think the designers who created the bot have already shown how smart they are. These guys are also continually trying to improve their bot designs based on the number of computer systems they have successfully penetrated and the feedback they get back from those who detected it.
              The real question is can someone design a bot that nobody can detect until after it has already infected millions of machines? There is a already a window of time between detection and creating counter measure

            • by symbolset (646467) *
              They've been at this for quite a few years and have gotten very good at it.
          • by wer32r (2556798)

            Further, I am not aware of any way to encrypt communications between the botnet's controllers and the botnet's nodes because every node will need to have the private key to decrypt incoming communications. So anyone can analyze a node and just pick out the private key, and then start issuing commands to it as though they were the operators.

            The botnet nodes/controllers would theoretically only need the public key of the person supplying the commands. If the commands are issued with a unique, verifiable sequence number (i.e. concatenated with the node's id), it would be practically impossible to issue false commands.

          • by 1s44c (552956)

            You seem to be missing the fact that the nodes don't all need the same keypair, each node can generate it's own keypair like every other bit of software which uses private key encryption.

          • by DarkOx (621550)

            Wrong -- the way to do is every node has the public key. The owners keep the private key. The nodes pass instructions around the network. Any node can submit an instruction to the network so the owners can use any node. Each node verifies the instruction is legit by the fact that it decrypts with the public key. Ideally the owners would just many different nodes to place new instructions on the network; that would help prevent security people from finding the source so easily. Another feature would b

      • by Skapare (16644)

        Probably ANY machine that sends a C&C message ... which is properly signed ... can control it. Instead of the botnet "phoning home", the C&C has to find them. They could probably be spraying their scent around at random, and C&C messages sprayed at random are likely to find an eventual target.

      • by Tom (822)

        I'm still studying the details, but either by now or by the next iteration, you can strike the term "C&C" from your vocabulary.

        Basically, if you used signed commands, the humans controlling the network can inject their commands anywhere and it'll simply spread through the network.

      • by pmontra (738736)
        ZeuS became self aware on February 25, 2012. That's what they'll teach. Are they sending terminators back in time to try winning the war?
      • If ANY node can take control then we can shut down the whole network from a single node. Everything you need to know is available on any single node.
        • by neokushan (932374)

          Nope. Well, not necessarily - I can't say I know the ins and outs of the ZeuS network but for a system like this, they'll likely sign commands so that only the author can submit them. Sure, you know how to send commands to all the nodes, but without the private signature key, there's no way to send commands they'll accept. And the private key will never be stored on the C&C node that sent the command.

    • by errandum (2014454)

      Fasttrack (Kazaa) or skype are not tied down to one server. The only centralized source in those, I believe, it's the log in... But I don't think these require that.

      On the other hand, spreading information through the network could be slow, making it less efficient.

      The only way the RIAA stopped kazaa was by exploiting their checksum algorithm to difuse bogus info. But I don't think that is an option. On the other hand, it should be possible for anyone to give orders to this botnet if they know the "key" to

      • by Hentes (2461350)

        True, but unlike a p2p network a botnet has to be directed. If the network were truly decentralized, any machine could command it, thus anyone could steal it. My guess is that in order to prevent hijacking the commands are signed by the C&C, and then distributed through the network. This way, the origin of the commands becomes very hard to trace, but this method also introduces some vulnerabilities. For example, as each infected machine connects to a number of other ones, getting a list of infected mach

        • by errandum (2014454)

          But knowing the infected machines was never the problem. Spamm e-mail brings with it the address... But you can't really go knocking down doors forcing people to scan their computers. That's why the black holes were designed, you wouldn't invade anyone's privacy but at the same time take them down.

          Now, either the ISP's start disconnecting people that are infected, or you seem to stand no chance against it. But I believe new legislation will have to be drafted if we are to start disconnecting people off the

          • by DarkOx (621550)

            The answer is people need to be held accountable for their machines. The Internet is a public good just like roads. We don't let you operate an unsafe vehicle on our public roads, you can't operate an unsafe computer on our Internet.

            If you machine is spamming or propagating malware yes your access should be terminated until you fix it. Just because someone else may have done the damage by infecting your box does not mean you are not still obligated to fix the problem, just like if someone smashes your he

            • by neonKow (1239288)

              This would be a good system if we could GET to that point where we're not taking down 30% of machines because they're infected by malware, but given our current situation, I think the biggest problem is not how we maintain a safer internet, but how we can clean up all those dirty machines in the first place.

              You need a decent amount of technical expertise to secure machines and keep them secure. Unlike cars, the default configuration of computers is bad, there is no cheap way to maintain their security excep

    • by errandum (2014454)

      Oh, and before you answer, not that it mentions a C&C server, there is no need for a server in this - just someone with a computer and the right access.

    • The question is how easily will the ZeuS botnet be hijacked by someone else?
      • by errandum (2014454)

        Most likely depends on the key scheme they used and how they "protect" it. But standard public key encryption schemes should give anyone a run for their money for a few years...

        • Indeed. Furthermore, I look forward to the version that is able to make use of trusted computing architecture to get remote attestation that what it is talking to is an untampered-with bot. That should be an interesting day.

          • by neonKow (1239288)

            That seems completely unlikely. That would require their software to be signed. The whole point of trusted computing (well, at least one of the points) is that random rogue players can't release "trusted" software.

    • by pinkeen (1804300)
      Actually, I don't think so, because the "Master C&C" can change constantly and still maintain the connectivity.
  • The scary thing is that they are about a decade behind with this step. This is just a logical evolution they likely found in the literature and implemented because the conventional way did not work very well. Of course this just means the the C&C control flow is obscured with techniques from anonymity technology.

      It is time for some more drastic legal measures, like punishing operators and makers of insecure software and systems.

    • by segin (883667)

      That would punish me for running Mac OS X 10.3.9 on my iMac G3. Why should the law require me to use Linux? Hell, with some of the legal suggestions floating around on here, it would be illegal for me to use this machine for anything at all, due to an low watt-per-FLOP ratio.

      Slashdotters are killing my childhood, not 4chan.

      • by Skapare (16644)

        I was not aware that Mac OS X 10.3.9 running on iMac G3 was vulnerable to botnets. But if it is, then you need to take that up with the maker who left you vulnerable to these legal liabilities. If you buy a car with no brakes and drive it out on the highway and crash into someone else, YOU are at least equally liable. Drivers on the road have the responsibility assigned by law to be sure they are operating a safe vehicle. Operating an unsafe computer on the internet should be just as much a responsibili

        • by segin (883667)

          If I interpret your remarks correctly, you're suggesting I should unplug (or heavily firewall, even more so than the NAT I use today) my iMac because Apple no longer pushes security updates for it, or be or else be criminally liable.

          Those laws are ideal, but would never be enforced anyways. What police officer wants to spend hours at a time checking the versions of each and every installed software application to verify that a machine is "secure"? And how many of the 245 million Internet users in the United

          • by DarkOx (621550)

            If I interpret your remarks correctly, you're suggesting I should unplug (or heavily firewall, even more so than the NAT I use today) my iMac because Apple no longer pushes security updates for it, or be or else be criminally liable.

            Criminally liable no but it should be against the civil code just equipment violations on a motor vehicle are. Firewall, patch, replace the equipment, disable vulnerable services, fix the problem however you like; but you are not entitled to degrade the public network. If you are found to be than you should be made to do something about it or stop using it.

            • by errandum (2014454)

              So, you'll punish everyone in the US, but then you discover the world is round and actually quite big. And that it doesn't really matter if you control the US computers when everywhere else no one passed legislation on that and the unsecure versions would keep existing.

              The only way to stop something like this is to heavily educate people. But who's going to pay for that? Who's going to profit? No one

            • by neonKow (1239288)

              So is Microsoft still responsible for existing Win 95 machines that can't be patched?

              Computer security isn't that mature yet. When cars started out, they really WEREN'T that safe, and it would've been unenforceable to force car makers to make cars all that safe.

              I'll agree that unsafe computers these days are a much more widespread and possibly more harmful issue now thanks to the internet, but you shouldn't assume you can fix it just by choosing someone to take responsibility and punishing them if they don'

    • Since no 'user controlled hardware' is 100% safe you propose TPM *everywhere* so users can no longer control anything. ( or have no computers at all )

      No thanks.

      • by gweihir (88907)

        Nobody competent talks about 100% security (whatever that means), but systems that are very easy to hack are just not acceptable and those making ans operating them should be liable for any and all damage caused.

        • by nurb432 (527695)

          "easy" is an abstract concept that can only lead to complete TPM.

          • by gweihir (88907)

            Nonsense. TPM does not make software more secure by the way. Maybe read up on the concepts you are throwing around here?

    • Re: (Score:2, Insightful)

      by Anonymous Coward

      Insecure is relative. Computers and systems have the same problems as the security of a country. You are calling for a TSA like approach for software and systems. The only 100% secure device is one with no human interface device, no ports to allow new data (no net, USB, CD-rom, etc.) and maybe not even a power cord.

      The most popular systems will have the most viruses written for them. Look at Windows. Now look at the reports of Apple OS viruses popping up as that system was becoming more popular. If ev

      • by gweihir (88907)

        Nobody requires 100% security. That is just stupid. But systems that can be automatically hacked and easily enough to make large bot-nets a reality are just a disgrace. These systems are so easy to hack, it does not even require intelligence and that is what needs to change. Without laws to enforce that change, vendors like Microsoft will always only deliver the worst quality they can still get away with, and that is pretty bad. Software with reasonable security level would require attackers to invest years

        • by Rockoon (1252108)
          What people like you do not seem to remember (or maybe you are too young?) is that before Windows had a TCP/IP stack, even before Trumpet Winsock, that Unix and VMS systems were notoriously exploited. Check the history of CERT advisory listings and its nothing but Unix and VMS systems being exploited until a phase change occurred when Windows PC's began to so overwhelmingly dominate the internet.

          History proves it. [] Some of the folks here born before 1975 know this to be true, because we were the ones break
          • by gweihir (88907)

            What are you blabbering about? Because VMS and traditional Unix could be broken into, we do not need better security today? What kind of broken reasoning is that? And what about all the advances in software engineering and also secure software engineering (mostly ignored in practice and academic curricula)? People will not start to do better until there is significant incentive to do so. Software can be written so that it is really hard to break into. It is just more expensive.

            So, while you may consider you

            • by Rockoon (1252108)

              What are you blabbering about? Because VMS and traditional Unix could be broken into, we do not need better security today?

              Care to actually counter the evidence that refutes your world view about "vendors like Microsoft", or do you just want to change your tune at the drop of a hat like a fucking fundamentalist christian?

      • Re: (Score:3, Insightful)

        by Anonymous Coward

        Popularity only means attractive target. Vulnerable is not related to popular except that it also makes the target more attractive.

        The gold in Fort Knox is attractive. However, the security of Fort Knox is so unattractive that it offsets the attraction the gold has to would be thieves. The result? Crooks knock off small banks instead. The money is only attractive if it's reasonably easy enough to get.

        Microsoft's market share on the desktop has not changed in a significant way. Yet, most agree that

        • It's also not reasonable to expect every bank in the US to have the security of fort knox. It would both be impossible for the bank to do business with customers if it was a three hour ordeal anytime someone wanted to make a deposit in the bank. As well as impossible to fund the amount of time and resources to secure them all.

          In the same way, it's completely unreasonable to expect every windows machine in every home in the world to have the kind of security say... the personal desktop of a security res
        • by neonKow (1239288)

          Nothing you say is exclusive of anything he/she's saying.

          He's comparing Windows to other OSes. You're comparing Windows to programs that run on Windows. He is saying people write exploits that run on Windows (which is true). You are saying people write exploits that target flaws in Flash. . . that run on Windows (which is also true).

          Chrome is not the next big attack vector because it still has a tiny tiny part of the market compared to IE, and it's definitely a smaller surface area to attack than Flash, so

  • by nurb432 (527695) on Saturday February 25, 2012 @11:07AM (#39158235) Homepage Journal

    The more you press on shady people the more they will work around the restrictions.

    Its an endless cat-mouse game. And we are the losers.

  • It's the daemon! []

    Excellent book by the way. :)

  • *yawn* (Score:5, Insightful)

    by Tom (822) on Saturday February 25, 2012 @12:24PM (#39158589) Homepage Journal

    This comes as a surprise to anyone? Really? I attended conferences almost 10 years ago listening to and giving speeches about stuff like this. The technology is trivial, the only reason the bad guys haven't moved to the hardened networks stuff yet is because there simply was no need.

    If you want to know what's next, I can dig out my old slides. A guy from Britain and I came up with several highly resistant network designs. I think our final one would remain largely intact if you took out 90% of its nodes.

    Like all things in fighting spam and large-scale scams, eliminating the C&C servers was one step that was useful for a short span in time. There are still old botnets out there that you can take out with this approach, but the more advanced ones have left that window of opportunity now.

    As long as our politicians refuse to tackle the fundamental problem - that of tiny crimes in massive quantities - we're stuck. Our legal system still works by "cases", adapted to a physical world where the crime has an easily enumerated set of victims, each of which having suffered considerable damage. The legal and political systems still don't understand both the tiny and massive scales they need to deal with in a virtual world. Scam 10 people out of $1000 each and you'll get a court case and jail time. Scam 1,000,000 people out of a cent each and nobody in law enforcement will care, even though the damage to society is the same.

    • by Solandri (704621)
      It's essentially the same thing as centralized peer-to-peer file sharing services like Napster being shut down, and decentralized ones like Grokster popping up to replace them. That happened ~10 years ago.
    • "As long as our politicians refuse to tackle the fundamental problem - that of tiny crimes in massive quantities - we're stuck"

      I don't agree, just build `computers' that can't be compromised by clicking on an URL or opening an email attachment.
      • by Lennie (16154)

        It is very likely that the user is a large part of the problem, who do you intent to solve that ?

        There are still people who download a piece of software just based on an ad on a website (free anti virus or whatever) and install that on their machine.

        • by Tom (822)

          It is very likely that the user is a large part of the problem

          Yes, but in an entirely different way than you mean it. The user isn't the dumb fool who is responsible for the whole mess - he is the weak link being exploited, and we blast him with "dumb user" ridicule instead of helping him out. Any surprise that users don't trust the geeks who should know better anymore? The IT department is not exactly admired in most companies. How it treats the users is one big reason why.

          There are still people who download a piece of software just based on an ad on a website (free anti virus or whatever) and install that on their machine.

          Yes, and
          a) aside from telling them what a bunch of stupid fucks they are, we aren't helping the

          • by Lennie (16154)

            While I agree with you, it isn't as easy as you think it is.

            These malware will just be "I'm an anti-virus" so yes, the user just clicks yes to 'give access to whole system', as a virus-scanner would probably not work from a sandbox.

        • by mcrbids (148650)

          You could do what Google/Apple has done and take the user out of the equation. In both Google Market and iTunes, you can buy stuff with reasonable confidence that it won't hork your phone. By standardizing a place to safely get applications, and making it palatable for legitimate developers to submit to the implicit oversight, they've made it tough to end users to do stupid things like download an obvious scam/malware from an ad.

          Whatever the effect on our freedoms, the truth is that we can't reasonably expe

      • by Tom (822)

        I've just given a talk on this on tuesday. Technology is a sideshow in the full picture of this crap. Phishing, spam, etc. are not primary technological problems. Botnets are just the currently most effective technology underlying this crap. Before botnets, we had rooted servers pumping out spam by the millions. We made that more difficult, so spammers went to easier targets and began building botnets. If we push them ouf of there, they will find other ways. It's a game of whack-a-mole.

    • by Krishnoid (984597) *

      Scam 10 people out of $1000 each and you'll get a court case and jail time. Scam 1,000,000 people out of a cent each and nobody in law enforcement will care, even though the damage to society is the same.

      Not to nitpick, but more than half of those ten may be in trouble [] if they lost that kind of money. Is the impact proportional for losing a cent (pretty much anywhere in the world)? The gain is the same to the scammer, but how do you calculate 'damage' on a one-cent scale?

      • by Tom (822)

        Your argument is a moral, not a legal one. Legally, scamming a millionaire out of $1000 is the same as scamming a beggar out of his life savings of the same amount.

  • "takedowns of such a network will be extremely difficult because there is no one central source to attack."
  • by Progman3K (515744) on Saturday February 25, 2012 @01:05PM (#39158905)

    There is no one node controlling the ensemble, yet they still need to coordinate their operations. The nodes must have perform a sort of hopping from one control-frequency (for lack of a better analogy) to another so they can't be followed

  • by Anonymous Coward

    Now it's a collective, right?
    So, all we have to do is find the bot named "hugh"....

  • by couchslug (175151) on Saturday February 25, 2012 @06:16PM (#39160579)

    Never omit that salient point. It matters.

The relative importance of files depends on their cost in terms of the human effort needed to regenerate them. -- T.A. Dolotta