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ktta

Here's a good blog about how SHA-1 works:

http://www.metamorphosite.com/one-way-hash-encryption-sha1-d....

The biggest risk I see with this is how torrents are affected:

https://en.wikipedia.org/wiki/Torrent_poisoning

There's also a problem with git, but I don't see it being that as susceptible as torrents:

http://stackoverflow.com/a/34599081/6448137

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What about git?

This makes it technically possible to get a backdoored linux repo with the same commit hash.

EDIT: this is wrong, it's not a second pre-image attack only a collision attack. That is, you can create 2 git repositories with the same commit hash, but not a git repo that matches an already existing repo. In other words, you can create 2 things with the same hash, but can't control what that hash actually is.

You might be able to fashion 2 commits to the linux repo with the same hash but a different modification though.

https://news.ycombinator.com/item?id=13713603

EDIT: My original comment was wrong. Git commit signing apparently only signs the commit hash itself, so it's only as trustworthy as the integrity of the hash. However, see also other comments which point out that this isn't a pre-image attack; you can't find a collision for a given thing (in this case, the linux kernel); rather, you can generate from scratch two pieces of data which happen to collide. Still, this is worrying.

ORIGINALLY (INCORRECT): Git commit hashes were at least never intended as a form of authentication. That's why git has commit signing. That said, they list GPG signatures as one of the things affected by SHA1 brokenness, so maybe even that's not enough? I don't know enough about how git commit signing or GPG works to tell.

>git has commit signing

What the commit actually signs? Is it the sha1 hash?

The commit object is signed; which contains a signature to both the tree and the parent. I don't see it as feasible to do a collision on both at once with the method outlined here, but maybe I'm missing something.

Tagging on the other hand is at risk, as it's just the hash that's signed.

Edit: You can check what is actually signed with `git cat-file -p $obj` where $obj is a commit or tag id.

You could also find a preimage for any tree or blob object referenced by that commit. And in large projects, you have a lot to choose from.

(As has been stated, I know this isn't a second preimage attack.)

Edit: See my answers in this thread: https://lists.gnu.org/archive/html/bug-guix/2016-06/msg00009...

It seems that commit object contains metadata and sha1 hash of the commit tree. In other words, signing comment object is not detecting if data is altered somewhere in the tree and creates identical sha1 hash.

The signature affirms the validity of the SHA1 hash, so ... the GPG is not more helpful than the underlying SHA1.

Yep, I saw that and edited as needed.

Isn't there still an attack where you create to different commits to the linux kernel with the same hash? (I presume here that a git commit hash is calculated over the diffs, not the end state of the repo).

If that is the case, you can still sneak code into the repo, though it would be easy to detect by seeing repos diverge.

When git signs a commit it pipes the entire commit, including the message at the end, and all headers to git. Then it goes back and adds the signature in a new gpgsig header.

I often see people getting this wrong in HN threads. Probably because that's how signed tags work.

Accidental collisions are still unlikely, it loses SHA1's "security" properties though. http://www.monotone.ca/docs/Hash-Integrity.html#Hash-Integri... goes in some detail for a different (older) DSCM, but same issue.

I don't think we're likely to see it used for torrent poisoning if it takes 110 GPUs 1 year to compute a collision - the electricity costs for such a rig would be enormous. 110 GPUs * about 300 W each = about 290 thousand kilowatt hours to break. Electricity costs of at least 10s of thousands of dollars. For a single torrent. You better be damn confident that those torrenters are going to go buy your product and that they won't just go to the next torrent down the list.

That said, attacks only get better, so it could happen at some point in the future and it's probably worth switching the hash function used sooner rather than later.

Malware infecting a few hundred beefy-gpu PCs isn't implausible.

"That said, attacks only get better"

In the case of MD5 collision research, initially it took a cluster of PS4 months to find one. But a few years later a laptop can find a collision instantaneously. So yeah, expect rapid improvement of SHA1 collision feasibility.

>torrents

Good, if something stronger and better comes out it'll still be a win.

Bittorrent is not a static/unchanged old protocol. It has constantly evolved to keep up with any attacks that could be mounted on it's distributed nature.

I'm sure a BEP will also come up, and speedtracked on the back of this news.

The importance of that update cannot be understated. Torrents are much more viable to being exploited than anything else out there.

I think everyone should be on alert for updates to their favorite torrent softwares.

What about l2tp/ipsec? It can use sha-1.

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