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A pair of free information security tools I wrote
Information security is a pretty big passion of mine; I don't think someone needs to have "something to hide" in order to make use of digital signing, encryption, etc. Another passion of mine is making things easier for other people to do. I've written a couple of tools that I think can be useful for the LW crowd.
Online PGP Signature: This is an online javascript-based tool which allows you to sign messages using your PGP private key. I love the idea of PGP-signed messages (I remember someone under the pseudonym "Professor Quirrell" handing out PGP-verified Quirrell points a few years back). The problem is, I had yet to find an easy way to do this that didn't involve downloading command-line based software. So I wrote this tool that uses open-sourced, javascript-based PGP libraries to let you easily sign messages in your browser.
The whole thing is client-side so your private key is never seen by me, but be smart about security. If you don't trust me, that's fine, just don't use the tool. But also remember that you could have a virus, your computer could be monitored, someone could be watching over your shoulder, etc. so please be smart about your security. But hopefully this can be helpful.
Decoy: an iPhone App: I wrote this after "The Fappening", where I was basically appalled at the terrible security practices that pretty much everyone uses when sending pictures back and forth. Decoy uses a combination of steganography and AES encryption to let you send images back and forth without having to sign up for an account or use some outside service that can be hacked or otherwise compromised.
You take the original picture, then you come up with a passphrase, then you take a "decoy" picture. The original picture is converted to base64 image data, which is then AES-encrypted using your passphrase. The resulting cipher text is then encoded into the pixels of the "decoy" picture, which is what gets saved on your phone and sent out. The "decoy" pictures are indistinguishable from any other picture on your or your recipients' camera rolls, and unless you have the passphrase, then the original image is thoroughly inaccessible.
If your phone is lost, hacked, stolen, or (more benignly) someone happens to be looking through pictures on your phone, all anyone will see are the "decoy" pictures. Without the password, those pictures are worthless. Although the app is primarily branded for, *ahem*, "personal use", there are plenty of other ways to use it. For example, my wife and I use it for things like sending pictures of sensitive physical documents like credit cards, birth certificates, social security cards, etc.
(full disclosure: although Decoy is free, it is ad-supported so I do financially benefit from people using the app. But on the bright side I'm an avowed rationalist and if I make a quajillion dollars with this app I will spend the vast majority of it on LW-friendly causes!)
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Comments (97)
For grins:
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256
Information security is a pretty big passion of mine; I don't think someone needs to have "something to hide" in order to make use of digital signing, encryption, etc. Another passion of mine is making things easier for other people to do. I've written a couple of tools that I think can be useful for the LW crowd.
Online PGP Signature: This is an online javascript-based tool which allows you to sign messages using your PGP private key. I love the idea of PGP-signed messages (I remember someone under the pseudonym "Professor Quirrell" handing out PGP-verified Quirrell points a few years back). The problem is, I had yet to find an easy way to do this that didn't involve downloading command-line based software. So I wrote this tool that uses open-sourced, javascript-based PGP libraries to let you easily sign messages in your browser.
The whole thing is client-side so your private key is never seen by me, but be smart about security. If you don't trust me, that's fine, just don't use the tool. But also remember that you could have a virus, your computer could be monitored, someone could be watching over your shoulder, etc. so please be smart about your security. But hopefully this can be helpful.
Decoy: an iPhone App: I wrote this after "The Fappening", where I was basically appalled at the terrible security practices that pretty much everyone uses when sending pictures back and forth. Decoy uses a combination of steganography and AES encryption to let you send images back and forth without having to sign up for an account or use some outside service that can be hacked or otherwise compromised.
You take the original picture, then you come up with a passphrase, then you take a "decoy" picture. The original picture is converted to base64 image data, which is then AES-encrypted using your passphrase. The resulting cipher text is then encoded into the pixels of the "decoy" picture, which is what gets saved on your phone and sent out. The "decoy" pictures are indistinguishable from any other picture on your or your recipients' camera rolls, and unless you have the passphrase, then the original image is thoroughly inaccessible.
If your phone is lost, hacked, stolen, or (more benignly) someone happens to be looking through pictures on your phone, all anyone will see are the "decoy" pictures. Without the password, those pictures are worthless. Although the app is primarily branded for, ahem, "personal use", there are plenty of other ways to use it. For example, my wife and I use it for things like sending pictures of sensitive physical documents like credit cards, birth certificates, social security cards, etc.
(full disclosure: although Decoy is free, it is ad-supported so I do financially benefit from people using the app. But on the bright side I'm an avowed rationalist and if I make a quajillion dollars with this app I will spend the vast majority of it on LW-friendly causes!) -----BEGIN PGP SIGNATURE----- Version: OpenPGP.js v1.0.1 Comment: http://openpgpjs.org
wsBcBAEBCAAQBQJVKahLCRBBM9OD/oMSegAAWY8H/1ffbDiZ2GsLlBdjCOk7 H6R5fhxq4szBSyByJdfj4p+iH1X4OCDlh5e7eF1gDFxm2p3IeoEBuJWv0Y8K 2/deTzKQ6ZpHF5L7fHKjneA1JXtsOEIcRX30UlPvPVxJ2xJZFTJumeF9G9tR jjrheAbV/IrmOB8tq8+OwF3I7csnD9GkFTZZnquzMpVpG0KKE15hh8QhrUIH Z1xgReB9NBdHHdsOx3CQmUkae3W+7YtlXVKuDD0uFOBtUkb17oTrH0nVy1tg o0ITrwm1LzRwZn4qiWrHwcJld43kU5WcifSj+/nlMMmMbATmhVoNOkWeIszy V4a+yYKSwSDxDRmlLyAFvmg= =QufC -----END PGP SIGNATURE-----
These sound like great tools. Thanks for making them available.
On a meta level I don't mind if members of the community promote their own work here if it's something that other community members will find useful. I'll also note that these seem lik tricky enouph things that they could also have been mentioned in the bragging thread when you finished them.
This is not a program I wrote but while we're posting things I have a guide to setting up Automator on a mac to send out an email on login on my blog.
Incidentally I also use Decoy as one method of PGP public key verification. The "decoy" picture is a screenshot of my public key. The photo hidden behind the decoy is a picture of me, holding up my drivers license and an index card with my username. The picture itself should prove sufficient in 99% of cases but in extreme circumstances I can give out the passcode which provides an additional two layers of verification (the validity of the password itself, and the photographic identity verification)
Of course that could still be spoofed if someone managed to replace all instances of my verification image, and then made a fake drivers license with my name on it and took a picture of that. But if that ever did happen I actually have a final layer of protection which I won't tell anyone about until I can figure out a way to re-tell it without rendering it worthless.
If anyone is interested in a more detailed security breakdown of Decoy, here it is:
The core goal is protecting the original image. The secondary goal is preventing the identification of the Decoy image as s decoy. The original inage goes through the following steps:
The presents the following possible vulnerabilities. (Vulnerabilties which assume malicious intent on the part of the app are ignored.)
Thus, any attack from someone other than the recipient requires either, 1. Physical access to the victim's phone, which is then compromised, and returned, undetected to the victim. Or 2. Access to the decoy image and passcode. As the first attack would represent a catastrophic breach of security far beyond a simple lost phone, which is not unique to this app, I won't spend a ton of time discussing it.
Thus, the primary attack type will involve an attacker firstly obtaining a set of images from the victim, secondly identifying a picture from that set as a Decoy, and thirdly, some means of obtaining the passcode. Means of acquiring a set of images include:
Once access to an imageset is acquired, the attacker must decide the most efficient way to narrow down which images are potential Decoys. Potential detection methods include:
Potential password attack methods include:
Thus the most attractive attack types will be: Fusking Attacks: This involves scraping a large number of images from publicly available sources with little regard for victim identity. Images are run through cursory detection, and superficial brute force attacks are applied to all images that pass the cursory detection. This will yield an extremely low success rate but requires very little time or effort. You can protect against this attack by using a strong password, which will resist superficial brute-force attempts.
Stolen-Phone Attacks: If a phone is lost or stolen, this significantly narrows down the size of the imageset. If Decoy is present on the phone in question, it significantly raises the probability that any given image is a decoy. Because of the small size of the imageset, detection will likely be unnecessary. The phone can be examined for clues to possible passwords (by looking through text message history, possible password managers on the phone, passwords sent in plaintext via email from -insert service here-, etc.) These passwords will be tried first on all images, then a cursory brute force will be attempted on all images. If an image can be conclusively determined as a decoy, (for example, if you leave a copy of it in your text message log), a fullscale brute force attack can be attempted. (This can be protected against by using a strong password, and not reusing passwords across multiple services)
Targeted Attacks: If you're a celebrity or public figure or someone that people have a reason to specifically single you out, the attacker will likely attempt to access some form of cloud-based image storage, such as your iCloud account. Once they gain access to this, they will use that same password on all images regardless of whether they are suspected Decoys or not, as the time it would take to do so is trivial. After that, a similar brute force using common variations on that password will be attempted. After that, a cursory brute force will be attempted. Finally, a fullscale brute-force will be launched on all images. (This can be protected against by using a strong password, and not reusing passwords across multiple services)
You're forgetting about rubberhose cryptanalysis.
Also your starting point should be the threat model which you skipped.
That made me smile. One of my favorite sayings is that "all security is security through obscurity", because all it really takes is a lead pipe and some duct tape to "de-obscure" the password. But, that said, I've always considered such "rubberhose cryptanalysis" to be a form of social engineering. Actually, that's a great doublespeak term for it. "Extreme Adversarial Social Engineering". It even has a good acronym: EASE.
When you say "the threat model which you skipped", what do you mean?
Which is why many contemporary secure systems do not rely on permanent passwords (e.g. OTR messaging).
The usual: who is your adversary and against which threats are you trying to protect yourself?
I have a superficial measure against this, which is having two user accounts, one of which is superficially similar to mine. If it is easy to send two images which unlock with different passwords then that could be an anti-rubberhose cryptanalysis measure?
Yes, some encryption programs (notably TrueCrypt) offer the ability to have two different encrypted areas, with different passwords, inside the same container (e.g. a file). You put, say, your pr0n collection into one, your sekrit revolutionary propaganda into the other, and in response to rubberhose unlock the appropriate one.
Of course now if I want to rubber hose you, I'll be sure to ask about your second account, too! Jokes aside, I think that's a good tool to keep in the belt.
I've always struggled with the idea of how to promote the practice without lessening its effectiveness since the more people that know about it, the more likely a rubber hoser is to ask about both.
Someone pointed out that the fact that TrueCrypt allows a hidden system gives an attacker the incentive to torture you until you reveal the secondary hidden one. And if you don't use the option, that's too bad for you -- then the attacker just tortures you until you die, since they don't believe you when you deny the existence of it.
One possible way around that, would be to allow a potentially infinite number of hidden systems each with their own passcode.
There are a few issues with this though:
With all that said, you could address these security concerns by only having the "hidden system" apply to the truly sensitive parts of the message. In other words, you would start with the main message that has been sanitized of any relevant information, and then encrypted. Then, for each piece of sanitized information, you'd provide several plausible answers, which are each encrypted with their own key.
So for example you would have is a master message: Master: "The plan to [1] the [2] is confirmed at [3]"
And then the possible answers: 1. attack, vandalize, have lunch at, prank call 2. enemy headquarters, subway platform, the local diner, your ex girlfriend 3. noon, midnight, 17:00, tonight
So the "full" password would basically be: Master Password + Password for Blank 1 + Password for Blank 2 + Password for Blank 3
So for this example there would be 64 different combinations of possible answers ranging from the correct one: "The plan to attack the enemy headquarters is confirmed at noon." to the incorrect but plausible "The plan to attack the enemy headquarters is confirmed at midnight" etc.
This would address the issues of #1 and #2. However it still would be possible for the attacker to guess based on the size of the message how many different combinations there may be. This can be circumvented one of several ways:
Of course this does present the logistical issue of how you exchange the correct password for such a message. Functionally the password exchange process would be no different than any other, this would simply require a longer password.
<shrug> If you're really paranoid you can implement a nesting-doll system with as many levels of depth as you want.
But that argument applies just as well to anything at all: the existence of flash drives (or, say, treasure chests filled with gold) gives the attacker an incentive to torture you until you reveal the location of your hidden data (or treasure).
Hah, the decoy account is trivially easy to determine to be not-mine, the idea is less "permanently trick someone into thinking it's my main account" and more "distract someone while I log into it so that it can send an automated email, then keep them from powering down my computer for 30 seconds while the program sends the email, because I can't get it to do that in the background just via Automator".
Also, in that sort of scenario there really isn't that much I have to hide. There are some computer of my computer usage that I would strongly prefer not to disclose, but at that point I wouldn't be concerned about "linking ilzolende to my real identity" or "what if my friends/parents/future employers know about my actions" or "what if something I did was actually intellectual property theft" or "what if I had to change all my passwords that would be really annoying".
If there was something I really didn't want to disclose I would probably do it from other people's computers using Tor Browser or a TAILS DVD with URLs I memorized. There isn't something I value my privacy for that much, so I don't do that. (Although I'm considering getting a TAILS USB for using with the school computers mostly to make the "the fact that this browser didn't tell me that Website X was not a reason I chose the browser, I use it for privacy, the fact that it apparently circumvents the filter is just a side effect, what am I supposed to do, check if the website is blocked from a different computer before I visit it?" claim.)
Honestly, a lot of my motives here are more "normalize security/privacy" and "make sure that if something goes wrong I can say that I took a ton of preventative measures" than "losing control of my data would be a complete disaster". If I were truly concerned about privacy, I wouldn't have participated in a study involving MRI scans and DNA analysis from a blood draw and whatnot for ~%100. I mostly don't like the state of affairs where people have more information about me than I do.
Given a private PGP key to a website that isn't even SSL encrypted is the antithesis of good encryption behavior.
Yeah... Ummmm..... There's a lot wrong with this.
Don't get me wrong. I appreciate the need for constant vigilance, but this type of knee-jerk reaction is what prevents the wider scale adoption of good crypto practices.
Edit- for posterity's sake: I accidentally down voted your post when I meant to upvote it. I wasn't just being snide when I said "I appreciate the need for constant vigilance", and it definitely resulted in a good discussion. I updated my vote.
Actually, there is still a small danger to executing this via a non-SSL encrypted web site, even if I trust that you have no malicious intent, your site has not been compromised and the script runs client-side. The danger is a man-in-the-middle attack, in which an attacker intercepts my http request for the script and replaces your script (in the response) with a version that captures my private key and sends it to a server controlled by the attacker.
I realize that most browsers won't let client-side javascript send requests to hosts other than the original host from which the javascript was loaded, but that fact won't solve the issue; the modified version of the script could send the private key to a URL apparently on your host, and the man-in-the middle daemon could intercept the request and send it to the attacker's host.
Wouldn't this be circumvented by performing Step 5 followed immediately by Step 4 before running the script? (Of course, the level of inspection necessary to determine what's going on in the script may be high enough that you may as well write your own script by that point.)
Yes, presumably this danger could be mitigated through a combination of code inspection and sandboxing. But, one of Nanashi's stated motivations for developing this was that "I had yet to find an easy way to do this that didn't involve downloading command-line based software". I doubt that anyone who is adverse to running a command-line program would be very excited about inspecting a javascript application for security vulnerabilities and/or sandboxing it each time he/she wants to sign a message.
As much as I dislike blindly following rules in most cases, I think that ChristianKI is correct that any security-related web application ought to be secured via SSL.
I think what I could have been more clear about was the use case here: this tool in its current form is sort of "pre-Alpha". Previously the script was just sitting locally on my computer and I thought it could be useful to others. If I ever try to deploy this on any sort of larger scale, absolutely it will be done on an SSL server. But for right now, it's just being shared amongst a few friends and the LW community.
If it turns out that people are using the tool frequently, I'll probably go ahead and pay to upgrade my hosting plan to SSL. But for something that's a free tool not meant for wide distribution, which takes about 7 seconds to right click and hit "Save As", I just didn't see it as necessary just yet.
Understood. Obtaining an SSL cert is a hassle (and an expense if you obtain it from a cert authority) that may not be warranted for a pre-alpha release. As long as your users use discretion before signing using their "real" private keys, I don't see any issue.
Thanks for making these available; the Decoy app sounds particularly innovative.
The scenario that everyone runs Step 5 and Step 4 everytime they run the script is unrealistic.
It wouldn't be every time you run the script; you would just need to vet it the first time. I expect that anyone using this for serious security purposes would just save the script locally. The point of this is that it's browser-based, not cloud-based. Saving an HTML + Javascript file with a (admittedly rudimentary) GUI is infinitely easier than downloading a command-line based program.
I'm not certain that it's impossible to hide a private PGP key in the the PGP signature. Are you?
I don't really understand the question. Why would someone want to hide their private PGP key in their public PGP signature?
You assume that the script can't leak the key if it's sandboxed.
For that to be true, it has to be impossible to hide the information from the private PGP key in the signature.
I did ask in security.stackexchange and according to it it's possible to steal the key.
5) doesn't guarantee security.
My own thinking on security is strongly influenced by the CCC hacker thinking. Seeing someone on stage holding a lecture on how he tracked Taiwanese money cards when he was for a few weeks there because the Taiwanese were just to stupid to implement proper security. There are a lot of cases where bad security failed and where the justification for thinking through security implications come from.
On the other hand you are right that the usability that comes out of that paradigm is lacking.
Now that I understand what you are asking, yes, it is all but impossible to hide a private PGP key in the PGP signature which would successfully verify.
The "answer" described in that Stack Exchange post doesn't work. If you attempted that, the signature would not verify.
How do you know?
A signed PGP message has three parts and thus only three places where additional information could be hidden. 1. The header 2. The message itself 3. The signature
The header is standardized. Any changes to the header itself (especially something as blatant as inserting a private key) would be enormously obvious, and would most likely result in a message that would fail to verify due to formatting issues.
The message itself can be verified by the author of the message. If anything shows up on this field that does not exactly match up with what he or she wrote, it will also be extremely obvious.
The signature itself, firstly, must be reproduced with 100% accuracy in order for the message to verify successfully. Any after-the-fact changes to either the message or the signature, will result in a message that does not verify successfully. (This is, of course, the entire purpose of a digital signature). Furthermore, the signature is generated algorithmically and cannot be manipulated by user input. The only way to change the signature would be to change the message prior to signing. However, as indicated above, this would be extremely obvious to the author.
"Algorithmically" doesn't mean that there exactly one way to create a valid signature. Hash functions quite often have collisions.
I'm downvoting this comment because it's misleading.
First of all, no one has ever found an SHA-2 hash collision yet. Second of all, the chances of two SHA-2 hashes colliding is about 1 in 1 quattuorvigintillion. It's so big I had to look up what the number name was. It's 1 with 77 zeroes after it. We're talking universe-goes-into-heat-death-before-it-happens type odds. Only under the most absurd definition of "quite often" could anyone ever reasonably claim that a cryptographic hash function like SHA-2 "quite often" has collisions.
https://tools.ietf.org/html/rfc4880#section-5.2.3.1 has a list of several subpackets that can be included in a signature. How many people check to make sure the order of preferred algorithms isn't tweaked to leak bits? Not to mention just repeating/fudging subpackets to blatantly leak binary data in subpackets that look "legitimate" to someone who hasn't read and understood the whole RFC.
Remember that I did not invent the PGP protocol. I wrote a tool that uses that protocol. So, I don't know if what you are suggesting is possible or not. But I can make an educated guess.
If what you are suggesting is possible, it would render the entire protocol (which has been around for something like 20 years) broken, invalid and insecure. It would undermine the integrity of vast untold quantities of data. Such a vulnerability would absolutely be newsworthy. And yet I've read no news about it. So of the possible explanations, what is most probable?
Such an obvious and easy to exploit vulnerability has existed for 20ish years, undiscovered/unexposed until one person on LW pointed it out?
The proposed security flaw sounds like maybe it might work, but doesnt.
I'd say #2 is more probable by several orders of magnitude
Don't set the bar lower; encourage competence. I'll quote this in order to further explain:
Why not use command line software? This is an important question I have a cached answer to, but I often find my own answer non-satisfactory. We should be living in a tell culture, so I'll tell you that in my experience, there's some sort of dichtomy between CLI and GUI and that gap is usually experienced people on one hand and inexperienced on the other.
I don't have anything to say against the experienced people, but I will say that the inexperienced ones, that seemingly always prefer the GUI also tend to suffer from learned helplessness, and more directly, baby duck syndrome.
That's not to say that they aren't right in a certain way of thought - they want things to be simple - but I often wonder if this is my own optimism about people in general, rather than the inexperienced people's refusal to learn and adapt a factually better way. Many a nerd/geek/enthusianist are baffled and infuriated when their attempts to actually better the world around them is simply bounced off despite their indisputable intent to help those around them. (This is how it feels like, by the way http://www.coding2learn.org/blog/2013/07/29/kids-cant-use-computers/)
But because they're inexperienced, wouldn't that mean that in due time they're going to run into problems? If you have a problem you did not know how to adequately solve, and someone is offering you a solution, would that mean your problem is solved? Not at all, unless you know both your problem good enough that you can say the solution will solve it. The inexperienced people are at the mercy of anyone who is going to give them a solution - and wouldn't you say it's a rationality failure to not correctly solve your problems?
I think that it was in the "shut up and multiply" page on the wiki that says that a whole human life is simply too significant in proximity to a certain fear, or a problem of unknown complexity. Or rather the opposite, fears and problems and other demotivating things that actively delay or even stunt growth are simply too insignificant in front of a whole human life (and that could be your life as well!) that stopping at those should simply not be an option, and is a negative consequence option no matter the situation.
Now, can you tell me why us, the people that actively try to improve our surroundings, the people that care about our environment, the people who are consistently shunned despite our unmistakable, undeniable, and unbelievable effort we put in, are putting in, and will put in, deserve to be completely, unforgivingly, and absolutely pushed aside, once again? Other that being just another plus one to the statistical curve; and secondly, why the implications that people pick up a few books, read a few articles here and there, and perhaps, more bottom line than those previous suggestions, learn some helpfulness, shut up and multiply, and grow up from their baby duck syndrome is such an horrendous thought, and worse, very seldom a suggestion?
And as for a good closing paragraph, I'd like to say that it's my belief that as an adult, you are responsible and indeed must work to self-improve yourself, and in extent, to anything and everyone around you. (Anyone here playing cops and robbers need not apply) I had some more to add but it slipped from my mind. Now, can I please have some answers?
(HAPPY WATCHLIST FOR ME)
If you are trying to trying to make the world a better place and find yourself pushed aside, shunned, bounced off, etc. you are doing it wrong. Stop blaming the people in the world for your inability to change them.
People can be stupid and stubborn. There are two ways around the problem. You can either convince them to stop being stupid and stubborn, in which case you are a salesperson. Or you can develop a solution that works around the problem, in which case you are an engineer. If you do neither and instead complain about how stubborn and stupid people are, then you are a whiner.
With this issue, the constraint is simple: people use GUIs, not CLIs. Doesn't matter which one is better. It matters which one people use. If you are taking the sales approach to the problem, you can try to convince people to use a CLI insetad of a GUI. If you are taking the engineering approach to the problem, you can try to build a better GUI. If you are taking the whiner's approach to the problem, you can tell people who build GUIs that CLIs are better.
What does "indistinguishable" mean in that sentence? Do you claim that a skilled attacker can't know that there metadata added?
Short answer is I don't know. The long answer will take a little background.
I haven't bothered to read through Decoy's internals, but this sort of steganography usually hides its secret data in the least significant bits of the decoy image. If that data is encrypted (assuming no headers or footers or obvious block divisions), then it will appear to an attacker like random bytes. Whether or not that's distinguishable from the original image depends on whether the low bits of the original image are observably nonrandom, and that's not something I know offhand -- although most images will be compressed in some fashion and a good compression scheme aims to maximize entropy, so that's something. And if it's mostly random but it does fit a known distribution, then with a little more cleverness it should be possible to write a reversible function that fits the encrypted data into that distribution.
It will definitely be different from the original image on the bit level, if you happen to have a copy of it. That could just mean the image was reencoded at some point, though, which is not unheard of -- though it'd be a little suspicious if only the low bits changed.
You're mostly correct. The data is encrypted, and then broken into a base-4 string. The least significant base-4 bit is dropped from each pixel leaving 98.4% fidelity, which is higher fidelity than the compression that gets applied. Thus in terms of image quality, the picture is indistinguishable from any other compressed image.
The encoding is deliberately reversible and also open-sourced. However, you can apply the same algorithm to any image, whether it's a decoy or not, and get a string of possibly-encrypted-data. The only confirmation that the data is meaningful would be a successful decryption which is only possible with the correct passphrase.
All that said, the fact that the picture is indistinguishable from other non-decoy images only adds a trivial amount of entropy to the encryption. An attacker who is determined to brute force their way into your pictures can simply attempt to crack every picture in your camera roll, decoy or no.
Does it change the low bits of white (0xFFFFFF) pixels? It would be a dead giveaway to find noise in overexposed areas of a photo, at least with the cameras I've used.
It does. Taking a picture of a solid white or black background will absolutely make it easier for an attacker with access to your data to be more confident that steganography is at work. That said there are some factors that mitigate this risk.
The iPhone's camera, combined with its JPG compression, inserts noise almost everywhere. This is far from exhaustive but in a series of 10 all-dark and 10 all-bright photos, the noise distribution of the untouched photos was comparable to the noise distribution of the decoy. Given that I don't control either of these, I'm not counting on this to hold up forever.
The app forces you to take a picture (and disables the flash) rather than use an existing one, lessening the chances that someone uses a noiseless picture. Again though, someone could still take a picture of a solid black wall.
Because of this, the visual decoy aspect of it is not meant as cryptographic protection. It's designed to lessen the chances that you will become a target. Any test designed to increase confidence in a tampered image requires access to your data which means the attacker has already targeted you in most cases. If that happens, there are other more efficient ways of determining what pictures would be worth attacking.
My original statement was that an attacker cannot confirm your image is a Decoy. They can raise their confidence that steganography is taking place. But unless a distinguishing attack against full AES exists, they can't say with certainty that the steganography at work is Decoy.
TL;DR: the decoy aspect of things is basically security through obscurity. The cryptographic protection comes from the AES encryption.
The fact that it distributes noise doesn't mean that the noise is uniformly distributed. It likely doesn't put the same noise in an area with is uniformly colored and an area that isn't uniformly colored.
I can't say with certainty either that the sun will rise tomorrow.
This seems like deliberate misinterpretation of Nanashi's point. You can't say with certainty that the Sun will rise tomorrow, but you can say so with extremely high probability. An attacker can't confirm that the image is a Decoy with a probability anywhere near as high.
Correct. I'd assign a probability of, say, 99.999999999999999999% that the sun will rise tomorrow.
If I were an attacker analyzing the noise distribution of an image, I could say with maybe 10% probability that an image has been tampered with. From there I have to further reduce the probability because there are hundreds of ways an image could have been tampered with that aren't Decoy.
For what it's worth, here is a sample of the noise distribution of the iPhone's JPEG compression vs. Decoy
(iPhone on left, Decoy on right)
http://i.cubeupload.com/ujKps6.png
(Note that these are not the same picture, because Decoy does not save or store the original version of either photo. It's two pictures where I held the iPhone very close against a wall. So there's a slight color variation)
That's pretty useless -- what you want is to look at some statistical measures of the empirical distributions of lower-order bits in these images. See e.g. this outdated page.
It's super-easy to spot in a histogram, so much so that there's ongoing research into making it less detectable.
Yes. Without the password, even a skilled attacker cannot confirm the presence of any metadata.
What do you mean with "confirm"? Can a attacker show that the image isn't of the type produced by the normal photo app?
I don't think that attack is practical, as long as Decoy leaves the metadata alone and works only on the image data. You'd need to reproduce the inputs to a particular implementation of the image encoding exactly, which is impossible unless you're snooping the raw data -- my phone camera produces images in JPEG format (high quality, but it's still lossy compression) and does the conversion before the raw image data even leaves RAM.
If you're dealing with images originating off the device, things get both easier and more difficult. Easier because there will typically be unchanged images in the wild to compare against; more difficult because there will typically be several different copies of an image floating around, and I don't think it's practical to reconstruct every possible chain of encodings. Many popular image-hosting sites, for example, reencode everything they get their grubby little paws on. Send an image as a text, that's another reencoding. And so forth.
As I've mentioned elsewhere, though, decoy images may be statistically distinguishable from an untouched JPEG even if you can't conclusively match it to an origin or e.g. validate against its EXIF tags -- though I could be proven wrong here with the right analysis, and I'd like to be.
If you would put a probability on it, how likely would you expect a proper security audit to prove you wrong?
.01%
How much money are you willing to bet on that?
If the amount is less than $50,000, I suggest you just offer it all as prize to whoever proves you wrong. The value to your reputation will be more than $5, and due to transaction costs people are unlikely to bet with you directly with less than $5 to gain.
I'd be willing to bet 50% of the market value of a feasible distinguishing-attack against AES. Under the condition that whoever proves me wrong discloses their method to me and only me.
In other words: a shitload. Such an attack would be far more valuable than any sum I'd possibly be able to offer.
Wrong on what count? I intended that sentence to refer only to the last paragraph of my post, and I'd expect that to be very implementation-dependent. Generally speaking, the higher the compression ratio the more perfectly random I'd expect the low bits to be -- but even at low ratios I'd expect them to be pretty noisy. I'm fairly confident that some JPEG implementations would leave distinguishable patterns when fed some inputs, but I don't have any good way of knowing how many or how easily distinguishable. To take a shot in the dark, I'm guessing there's maybe a 30% chance that an arbitrarily chosen implementation with arbitrarily chosen parameters would be easily checked in this way? That's mostly model uncertainty, though, so my error bars are pretty wide.
If we exclude that sort of statistical analysis, I'd estimate on the order of a 10 or 20% chance that Decoy images are distinguishable as such by examining metadata or other non-image traces -- but that comes almost entirely from the fact that I haven't read Nanashi's code, I'm not a JPEG expert, and security is hard. A properly done implementation should not be vulnerable to such an attack; I just don't know if this is properly done.
Your first paragraph nails it. Unless your phone is both jail broken and seriously compromised, there is no means of viewing the "original" version of either picture. Also re: the second paragraph. The app forces you to take a picture from your device to use as the "Decoy", it will not allow you to use an off-device image. (You CAN use an off-device image as the hidden picture).
As for the statistical analysis, it's mostly irrelevant. The encoding algorithm is both reversible and published. So you can extract "Decoy data" from ANY picture that you find, Decoy or no. The only thing that will confirm it one way or the other is a successful decryption. The best you could do is say, "Based on certain telltales, there's a 10% chance this image is a Decoy" or whatever the odds may be.
Such an attack has little to no value. If you are an attacker with a specific target, isolating which pictures are decoys removes a trivial amount of entropy from the equation, especially compared to the work of trying to brute-force an AES-encrypted ciphertext.
I understand that, and I understand that it should be impractical to decrypt the hidden image without its key given that strong attacks on AES have not yet been publicly found (key exchange difficulties, which are always considerable, aside). But I think you're being far too nonchalant about detection here. The fact that you can extract "decoy data" from any image is wholly irrelevant; it's the statistical properties of those bits that I'm interested in, and with maybe a million bits of data to play with, the bias per bit does not have to be very high for an attacker to be very confident that some kind of steganography's going on.
That does not, of course, prove that it's being used to hide anything interesting from an attacker's point of view; but that was never the point of this objection.
Well, my point has never been that it's impossible for an attacker to be confident that you're using steganography. Rather it's that an attacker cannot prove with certainty.
The "decoy picture" aspect of the protocol is intended to provide social protection and ensure plausible deniability can be maintained. It is not intended as cryptographic protection, that is what the AES is for.
"Confidence" is only useful to an attacker when it comes to determining a target. But an attacker has to already be confident in order to perform such a test in the first place. Which means you've already been selected as a target. Furthermore they would have to compromise enough of your security to access your image data. If that happens, then the benefit of gaining further confidence is marginal at best.
Incidentally, regarding the specific details of such a detection method:
We (and the attacker) already know that the distribution of base64 characters in an AES-encrypted ciphertext is approximately random and follows no discernible pattern. We also know that the ciphertext is encoded into the last 2 bits of each 8-bit pixel. So, we can, with X amount of confidence, show that an image is not a Decoy if we extract the last 2 bits of each pixel and discover the resulting data is non-randomly distributed.
However, because it is possible for normal, non-Decoy, compressed JPEGs to exhibit a random distribution of the data in the last 2 bits of each pixel, the presence of randomness does not confirm that an image is a Decoy.
The only viable attack here would be to pull images which are "visually similar" (a trivial task by simply using Google image search), reduce them to the same size, compress them heavily, and then examine the last 2 bits of each of their pixels. If there is a significant difference in the randomness of the control images vs. the randomness of the suspected image, you could then suggest with X% confidence that the suspected image has been tampered with.
However, because it is possible for an image to be tampered with and yet NOT be a Decoy image, even then you could still not, with any legitimate amount of confidence, use such a test to state that an image is a Decoy.
--moved to previous comment8
"Confirm" meaning an attacker cannot demonstrate with ~100% certainty that the image isn't of the type that could normally be found on the camera roll.
If you are passionate about computer security, you know the standard advice to NOT use unaudited tools from unknown sources.
"Just look at my code" is not a good answer since people who can usefully look at your code already have a large variety of well-known tools at their disposal. Your target is "normal" people and to them your code is gibberish anyway.
Actually my target audience isn't "normal" people since normal people don't have a PGP private key and probably don't even know what PGP is. I said this before somewhere in the vast miasma of comments that this is a tool I wrote for my own use that I wanted to share with the LW crowd. (Note that there is not a PGP key generator or signature validator included with the tool.)
As for its value? Well, there is at least one person capable of usefully looking at my code who finds value in this tool: me. If no one else finds it helpful, that's okay, the effort required to upload and share the app was pretty trivial. If other people DO find it helpful, then great, I've generated a bit of net utility for very little effort.
Value can be negative if people think something is secure when it isn't (that's not a claim that your software is insecure, just a general observation).
I'm not going to lose any sleep over that, at least with regard to the PGP tool.
Since someone seemed to take issue with this, I'll clarify: all the potential security flaws that we've discussed here operate under the assumption that I maliciously attempted to subvert the stated purpose of this tool. Since I am ~100% confident that I did not, in fact, do this, I am not too concerned.
This is not true at all. Security flaws of a crypto tool are bugs which make it less secure than expected and which the author is typically not aware of. Software with a hidden malicious load is just a trojan (or malware in general).
Note that I said "all the potential security flaws we've discussed here". Not, "any possible security flaw."
This is precisely why I've been annoyed by the direction this thread has taken. If someone wants to talk about potential flaws specific to this tool, I'm all ears. But instead it's mostly been a discussion about all the different ways I could possibly slip a Trojan into this tool.
I don't believe I ever said anything like that.
You didn't. Sorry, I should have clarified. When I said "this thread" I meant "the comments in general" and not your particular reply.