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Linus' reply on Git and SHA-1 collision

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Re: Linus' reply on Git and SHA-1 collision

#51

Post by dingo_bat »

stickfigure wrote: post #25

Several years ago I worked on a security product that used git as a sort of tripwire-type database. Since SHA1 was considered inadequate for Real Security, we had to hack jgit to use SHA256. It took a stupid amount of work - the 160-bit hash size was scattered all over the codebase in countless magic numbers. But it worked. The product was cancelled. I always wondered if the patch would be of any use to anyone.

> I always wondered if the patch would be of any use to anyone.

You think? I dunno why you wouldn't try to submit a pull request though.

Re: Linus' reply on Git and SHA-1 collision

#52

Post by somerandser99 »

paulddraper wrote: post #16

Linus has toned down a lot from a decade ago. > You are _literally_ arguing for the equivalent of "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected". > That's not engineering. That's five-year-olds discussing building their imaginary forts ("I want gun-turrets and a mechanical horse one mi…

Where are the technical arguments? Just false analogies and hyperboles. I wouldn't like to work with a person like this in my team.

Re: Linus' reply on Git and SHA-1 collision

#53

Post by astrodust »

DiabloD3 wrote: post #31
zkms wrote: post #21

Earlier quoted context omitted.

> "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected" I think this is a shockingly good example of how smart people get security questions utterly wrong. The right analogy when it comes to security has to involve some type of adversary, not just random, unmotivated natural phenomena -- as l…

The A-10 isn't a plane, it's a gun with a plane wrapped around it. And it hunts tanks.

It's also going to present zero obstacle to a meteorite travelling at six times the speed of sound. It'll rip through that armor like it's not even there.

Re: Linus' reply on Git and SHA-1 collision

#54

Post by aisofteng »

zkms wrote: post #21
paulddraper wrote: post #16

Linus has toned down a lot from a decade ago. > You are _literally_ arguing for the equivalent of "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected". > That's not engineering. That's five-year-olds discussing building their imaginary forts ("I want gun-turrets and a mechanical horse one mi…

> "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected" I think this is a shockingly good example of how smart people get security questions utterly wrong. The right analogy when it comes to security has to involve some type of adversary, not just random, unmotivated natural phenomena -- as l…

It was recently pointed out to me by a friend with a hobby for cryptography and mathematics that "secure" doesn't mean anything - something can only be secure with respect to a specific threat model.

It seems like a straightforward, almost painfully obvious definition in retrospect now, after having it articulated to me. I think education on security manners is poor and should be a standard topic.

Re: Linus' reply on Git and SHA-1 collision

#55

Post by astrodust »

protomyth wrote: post #38
DiabloD3 wrote: post #31

Earlier quoted context omitted.

The A-10 isn't a plane, it's a gun with a plane wrapped around it. And it hunts tanks.

I am told the gun is also quite good at shooting ground troops. A relative of mine had an A-10 stay with his group most of the night. Every time the enemy decided to go after them the A-10 came back and made them rethink their actions.

That zipper noise: To think that those are 30mm rounds being fired at that speed is just plain scary. http://www.xdtalk.com/attachments/img_7014-jpg.34438/

If one of those is lingering, I wouldn't even fight if I was the enemy.

It might not be able to kill a modern main battle tank, but I bet spraying it with rounds like that would fuck it up pretty seriously.

Re: Linus' reply on Git and SHA-1 collision

#56

Post by notfed »

Pertinent facts for the worried:

1) Git doesn't rely on SHA-1 for security. It relies on HTTPS, and a web of trust.

2) Even if git did rely on SHA-1, there's no imminent threat. What happened today was a SHA-1 collision, not a preimage attack. If a collision costs 2^n, a preimage attack costs 2^(2n).

3) Even if someone managed to pull off a preimage attack, creating a "poisonous" version of one your git repository's objects, they'd still have to convince you pull from their repo. This requires trust.

4) Even if you pulled it in, your git client would simply ignore their "poison" object, because it would say, "oh, no thanks, I already have that object". At worst, the code simply wouldn't work. No harm would be done.

When it comes to git, an attacker's time is better spent creating a secret buffer overflow than wasting millions of dollars on a SHA-1 collision.

Re: Linus' reply on Git and SHA-1 collision

#57

Post by slavik81 »

zkms wrote: post #21
paulddraper wrote: post #16

Linus has toned down a lot from a decade ago. > You are _literally_ arguing for the equivalent of "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected". > That's not engineering. That's five-year-olds discussing building their imaginary forts ("I want gun-turrets and a mechanical horse one mi…

> "what if a meteorite hit my plane while it was in flight - maybe I should add three inches of high-tension armored steel around the plane, so that my passengers would be protected" I think this is a shockingly good example of how smart people get security questions utterly wrong. The right analogy when it comes to security has to involve some type of adversary, not just random, unmotivated natural phenomena -- as l…

I would interpret it more charitably than that. He was responding to a claim that malicious collisions could be passed off as being pure chance:

> it could be incredible bad luck that caused that good-looking patch to be mistakenly matching a dangerous object

Re: Linus' reply on Git and SHA-1 collision

#58

Post by flukus »

tanderson92 wrote: post #34
jessaustin wrote: post #32

Earlier quoted context omitted.

Haha you reminded me of the old riddle about how British engineers decided where to place armor on their bombers, because, "if they put it everywhere the plane wouldn't get off the ground!"

Specifically, during the war the engineers took a Bayesian approach and put armor on the bombers in locations where bombers which came back did not have damage. Reason being that hits in those sections of the planes were more likely than not resulting in fatalities. Smart thinking.

AA hits planes on the bottom, presumably they weren't adding all the armor to the top?

Re: Linus' reply on Git and SHA-1 collision

#59

Post by dlubarov »

> That usually tends to make collision attacks much harder, because you either have to make the resulting size the same too, or you have to be able to also edit the size field in the header.

> pdf's don't have that issue, they have a fixed header and you can fairly arbitrarily add silent data to the middle that just doesn't get shown.

This doesn't seem like much of an obstacle, since you can add silent data to all kinds of files, like

- With HTML, JS, etc. you can just add whitespace.

- Some formats like GIF89a have variable-length comments.

- With any media format that uses palettes, you can add extra, unused colors.

- Just about any compression algorithm can be tuned to manipulate the compressed size. E.g. with DEFLATE (which is used by PNG in addition to some archive formats), you can use a suboptimal static coding rather than the correct Huffman tree.

- With most human-readable document formats, you can add zero-width spaces or something similar.

Re: Linus' reply on Git and SHA-1 collision

#60

Post by paulddraper »

astrodust wrote: post #53
DiabloD3 wrote: post #31

Earlier quoted context omitted.

The A-10 isn't a plane, it's a gun with a plane wrapped around it. And it hunts tanks.

It's also going to present zero obstacle to a meteorite travelling at six times the speed of sound. It'll rip through that armor like it's not even there.

Mach 6 is a heck of a slow meteorite.

But the if a small meteorite (only a few grams) were traveling that slow, 1.5 inches of titanium would do quite a bit.

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