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SHA-1 Collisions for Meaningful Messages
mrogers writes "Following on the heels of last year's collision search attack against SHA-1, researchers at the Crypto 2006 conference have announced a new attack that allows the attacker to choose part of the colliding messages. "Using the new method, it is possible, for example, to produce two HTML documents with a long nonsense part after the closing </html> tag, which, despite slight differences in the HTML part, thanks to the adapted appendage have the same hash value." A similar attack against MD5 was announced last year."
Using the new method, it is possible, for example, to produce two HTML documents with a long nonsense part
To achieve this, the method uses material pulled from myspace.com.
Push Button, Receive Bacon
One thing is that cryptographic hash functions should be easier to make secure than ciphers. At leaste that is what many cryptogtaphers thought. The other is that up to now you could rely on SHA-1 to be collision resistant, no matter what. The argument that you have a large part of the message being "garbage" does not give any real security. Many, many applications can still be attacked, and they need not even be broken for that.
While expected since last year, selecting and using crypto-hashes just got a lot more difficult and error prone.
Most ACs are not even worth the keystrokes to insult them. Be generically insulted by this and ignored otherwise.
I'm not an expert so don't take me seriously about it, but: I think hash algorithms are very important when signing things. For example, SHA-1 is the default hash algorithm used in GPG to sign messages. When the first attacks were mentioned I changed that to use RIPEMD-160. If you download something that has a SHA-1 sum to check both correctness and autenticity, it's a problem. Someone could modify the original tarball, for example, introducing a trojan horse, and append some other not useful data to it so the sum matches. I don't know if that's technically feasible as I say, but I imagine the possibilities are not so far. And, furthermore, for me this is another important warning that we should move out of SHA-1 ASAP. BTW, if I recall correctly BitTorrent uses SHA-1 to verify the 256KB chunks. There, having a fixed-size chunk is an advantage for this case, but, as I said, I wouldn't trust SHA-1 much longer. A further step and people could build evil BitTorrent clients that, at least, could corrupt your downloads introducing noise chunks.
It's quite relevant for those using it as a way to verify executables are the way developer had intended. Like the attack last year they're saying it would be possible for someone craft an exe without a virus, generated a checksum for it, get it linked to from major websites (after passing a virus scan of course), and then drop a virus in the end of the file and not have the checksum change. That's the real-world relevance.
Moderation is not supposed to be used as an indicator of agreement.
The second reason to keep cool is just as important, if not even more so: hackers will have to execute a pre-image attack to manipulate, for instance, a contract that has been digitally signed. In other words, hackers will have to find a second, manipulated contract with the same hash value as the real contract. In principle, the number of operations needed is thus far greater (2160). Indeed, as far as we know all attacks to date have only concerned collisions, and Wang et al. does not change that. There are no known methods to reduce significantly the number of operations needed for pre-image attacks.
Don't you think you're flying off the meter here a bit... Just because a collision was found means truly little. So a garbage laced HTML page was created after the actual HTML closing tag... 1) No one will see what comes after that unless you like viewing the source of a webpage as opposed to an actual page. 2) You should read up on birthday paradoxes. If someone created two similar messages, it would take years for them to figure out how to compute a hash to match. Now in the field of sending out something so so so secure, what makes you think that even if a someone did re-computate a hash to match, that message would be worth anything years down the line. Someone would have to be able to accomplish a collision, re-computate the hash in their new message and send it all within minutes for it to truly be a threat.
Let's look at this scenario... A massive kernel update is made to say Linux... The information is hashed, posted, and everyone is now going to update their Linux boxes... Unless someone is so quick fast to intercept along this path, most are safe unless they choose to verify the hash years down the line (which by then would be worthless). So unless someone can exploit this within minutes (no more than I would guesstimate 36 hours), I see little reason to get all bent out of shape over this...
Infiltrated dot Net
Common web browsers (I just tried Opera, FF, and Lynx) will happily display everything after the closing tag. You would have to put it inside <!-- --> comment delimiters, but then it doesn't matter whether it is before or after the closing tag. Unless the attack doesn't work if the --> has to come at the end, but then you can just omit the closing tag. Only an XHTML-compliant browser would complain. From cursory scanning TFA it is not clear to me what the reason is for mentioning the closing </html.
Avantslash: low-bandwidth mobile slashdot.
Provide the following 3 pieces of data:
1) Message/file length
2) SHA1 hash
3) MD5 checksum or some other hash/checksum that's calculated way differently from SHA1.
Providing the length means that the person trying to change the data needs to keep it the same length which makes it more difficult.
Using 2 different hashing/checksumming methods means they have to be able to match both of them in order to be able to switch the data.
The more restrictrictsion we toss on the data, the harder it is to manipulate. I do think that using more than 2 or 3 hashing/checksumming methods would be overkill however.
The problem is that your old keys and the messages they encrypted are available for cracking now and forever. Most people only encrypt important messages, which are easy to look for in a mailbox, and at a later time could be easy to crack. There's probably even a good change the data in that mail could still be important.
Now, if all emails were encrypted, it would be harder to immediately see what messages in a mailbox deserve your attention. But then at a later date CPU speed may make that a negligible difference.
"Beware of he who would deny you access to information, for in his heart, he dreams himself your master."
I'm not quite sure what your comment means.
As the heise.de article points out; the twins are of equal length - the file size would be the same.
Finding hash twins whereby the chosen one is, oh, let's say 160 bits longer is a degree less sophisticated.
SCO employee? Check out the bounty
NO SHA-1 COLLISIONS HAVE EVER BEEN FOUND!
Ahem.
Sorry, my caps lock key got stuck there.
No SHA-1 collisions have ever been found. The first person or group to find one will achieve considerable fame. I say this as an attendee of both last week's Crypto conference and the immediately following hash function workshop.
The work factor estimated for a SHA-1 collision is something over 2^60 cycles. That would put it on par with the biggest calculations that have ever been done (publicly anyway). So far nobody has put together a sufficient effort to achieve a collision.
At the hash function workshop, cryptographer Antoine Joux published a set of recommendations for how such a hash collision effort should be mounted, in order to minimize the damaged from a published collision. The main goal is to make it difficult to take a published collision and use it to create harmful effects in various ways. Hopefully Joux's guidelines will be followed if and when a SHA-1 collision finding project gets started.
From the article:
Using the new method, it is possible, for example, to produce two HTML documents with a long nonsense part after the closing </html>tag, which, despite slight differences in the HTML part, thanks to the adapted appendage have the same hash value.
So it appears that both the original and the new messages need that appendage. This isn't just about adding an appendage to a known, appendage-less document.More data, damnit!
SHA-1 was proved to have insecurities years ago. Because of that SHA-2 ("SHA-256", "SHA-384", and "SHA-512") was released back in 2001 as a better version of SHA-1. SHA-2 was tested and no insecurities were found (yet). What's more, SHA-2 is now the official US standard.
Complaining that SHA-1 is insecure is like complaining that Windows 98 is insecure.
Oblig Wikipedia link: http://en.wikipedia.org/wiki/SHA_hash_functions
For now though from a theoretical viewpoint this is a major weakness, it still requires way too much processing power to be realistic. And the way git is designed, I don't think it is going to be any major problem switching to a new hash once cryptographers starts to agree which one should be considered secure in the future. Once they start using a new hash, you can actually still safely use old repositories based on SHA-1. Because once there is no longer being added new data based on SHA-1, a collision is no longer enough to perform an attack, rather you need a second preimage, something which there has not yet been demonstrated an efficient way to produce.
Do you care about the security of your wireless mouse?
Actually, hashes are difficult to secure for general communications purposes without putting a cap on the size of the transmission. In information-content terms, a collision proof hash is equivalent to a lossless compression algorithm.
A hash will either contain all of the non-redundant information in the original content, or some of the information gets lost during the hash. Non-redundant information being defined in an information-theory sense that a given bit is completely random/unpredictable based on the content of preceding bits.
In order for a hash to be completely collision proof, it has to contain all of the non-redundant information contained in the original file. Otherwise information in the orignal message is lost in the hash. And if information is lost from the original message, that creates a possibility of constructing a message that differs only in the information that is removed by the hash. Only if the original message is reconstructible from the hash (plus possible information contained in the hash algorithm itself) will it be collision-proof. You've either got the information-content, or you don't. And if you don't have the content, you can't validate it.
We are the 198 proof..
I think the key point is this:
No SHA1 collisions have ever been published
whether or not they have been found is a different matter entirely.