Aaron Griffin wrote:

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Anyone know if there's any way to decrypt the hashes created by the HashKnownHosts setting?

I think the whole point is that they *are* one way hashes. The only think I can think of is to find the algorithm they use (sha1?) and hash the hostnames that you know, then compare.

Alternatively, just remove those two and if you get the "(yes/no)" prompt you know that's the host 8)

That's kinda what I thought. Although, one of the hostname strings had an "=" at the end, so I was wondering if it might just have been something base64-oriented. Unfortunately I have no idea what machines they are. They're not machines that I use often (e.g., my home server, or my hosting isp), and since they're at the bottom of the known_hosts file (and have been hashed, which I think is a fairly recent setting change for Arch's openssh package) I assume they've been added recently. So I'm guessing they're machines here on our work network, and I'd like to know which ones. I can't think of any new machines I've SSH'ed into lately, though. Ah well. I deleted them from the known_hosts, and turned HashKnownHosts off for now, and I guess I'll just pay close attention the next time SSH asks me about a new host. Thanks, DR

On 5/20/08, Thomas Bächler wrote:

Aaron Griffin schrieb:

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On Tue, May 20, 2008 at 2:05 PM, David Rosenstrauch wrote:

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Problem is, though, since Arch recently turned on HashKnownHosts by default in ssh_config, those 2 lines in the known_hosts file are encrypted, and so I don't know which host machines that I've been ssh'ing into are affected by the problem.

I think the whole point is that they *are* one way hashes. The only think I can think of is to find the algorithm they use (sha1?) and hash the hostnames that you know, then compare.

I didn't find out about this change until much later - and it pissed me off. For no apparent reason, we changed the default configuration of openssh at one point and now I have an obfuscated known_hosts file. I don't see any security impact in having the hosts unhashed.

Just because you can't see it doesn't mean it doesn't exist. unhashed known_hosts *is* more unsecure. If someone gets access to your account, they would get a) your key b) a list of hosts that the key is valid for hey! great! Compund this with the fact that many people use keys without a passphrase (a bad practice), someone can 'harvest' known_host data, and worm out to other hosts.. here is the kicker ... in a way that is easily automated. http://www.google.com/search?q=known_hosts+harvesting

Hi, Was this change forwarded to the OpenSSH developers? I am sure that if it is indeed better security-wise to hash the known_hosts file, they would change the default configuration upstream. I'm also sure that they would give very good reasons for not wanting to do so. Thanks, Dimitris

On Wed, 2008-05-21 at 17:44 +0200, Xavier wrote:

On Wed, May 21, 2008 at 4:50 PM, Dimitrios Apostolou wrote:

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Hi,

Was this change forwarded to the OpenSSH developers? I am sure that if it is indeed better security-wise to hash the known_hosts file, they would change the default configuration upstream. I'm also sure that they would give very good reasons for not wanting to do so.

So I just went googling about this stuff. I saw this option got enabled years ago on Debian, and after that a few users complained about that change, but without any real reasons. (so a bit like what is happening here now :)) Anyway, there was a huge thread on debian mailing list, I finally found one mail which partially answers your question : http://lists.debian.org/debian-devel/2005/07/msg00041.html

Thanks but I had read that before posting, as I had also been reading the openssh source code to understand how known_hosts matching is being performed. I have also read the same answer in 2005 on the openssh-dev ML, "it will probably be the default really soon". Still it is 2008 and it is not the default, that's why I asked what I asked. Why isn't it the default in OpenSSH? I have some fears that it *might* be a security threat to have a hashed known_hosts file, however I didn't want to say it publicly since I'm not sure at all. The fact that fears me is that by hashing the known_hosts file information is lost, in particular the information that binds the IP address to the hostname. With unhashed known_hosts they are both recorded on the same line. With hashed known_hosts each one is independently recorded on different lines. I am only speculating here, I haven't fully understood how openssh does its checks. We should be careful about *security* choices we make. Especially these choices should be made on vast community majority, not on the personal decision of a package maintainer. I am talking about general arch policy here, I have tried to raise discussion on other security issues too but got the feeling of being ignored by the devs, and that discuss on their private list and mostly announce here. Lastly, FYI I don't consider debian an example of good security policy. They keep changing things to satisfy their project's bugzilla but they keep being bitten by the fact that upstream doesn't always agree with them. Dimitris

On Wed, 2008-05-21 at 16:12 -0500, Aaron Griffin wrote:

On Wed, May 21, 2008 at 3:52 PM, Dimitrios Apostolou wrote:

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The fact that fears me is that by hashing the known_hosts file information is lost, in particular the information that binds the IP address to the hostname.

You are more than welcome to change the config settings. In all my history with arch, we have always pointed fingers at the end user for not paying attention to their config files, and I seriously have no idea why this is a big deal now, a year after this change was made. Sure it could have been more visible, but we have always held that the config files are _your_ job.

Hey No Problem. I have changed most of my config files a year ago, I just grab the opportunity to discuss that now that it's hot. My main point is that it should have been hot a year ago. I'm also struggling to figure out why I disliked this change that much back then. :-s Dimitris

eliott schrieb:

Just because you can't see it doesn't mean it doesn't exist. unhashed known_hosts *is* more unsecure.

If someone gets access to your account, they would get a) your key b) a list of hosts that the key is valid for

hey! great!

Compund this with the fact that many people use keys without a passphrase (a bad practice), someone can 'harvest' known_host data, and worm out to other hosts.. here is the kicker ... in a way that is easily automated.

The point is, without any notice, we provided a different configuration file than the upstream configuration file. That's not how we do it, we always provide the upstream configuration file. If someone thinks that having unhased known_hosts is a security problem, then he/she can change this configuration option on his/her system, that is how Arch works. But now that hashed known_hosts silently became the default, I cannot revert back.

Thomas Bächler wrote:

I didn't find out about this change until much later - and it pissed me off. For no apparent reason, we changed the default configuration of openssh at one point and now I have an obfuscated known_hosts file.

I agree - it would have been better for there to have been a bit more noise made about this change. If it was reported anywhere I must have missed it.

I don't see any security impact in having the hosts unhashed.

Apparently there was a paper published by MIT researchers a couple of years back that described how an attacker could use the SSH known_hosts file to propagate a worm: http://lwn.net/Articles/135506/ In theory it makes sense, but I'm wondering how feasible a threat it is in practice. DR