Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Casync – A tool for distributing file system images

0pointer.net

Post reply on HN
Print view View original

31–36 of 36 posts

Re: Casync – A tool for distributing file system images

#31

Post by thinkMOAR »

poettering wrote: post #28
thinkMOAR wrote: post #23

Earlier quoted context omitted.

Please elaborate on 'need to be protected against offline modification'?

Think of cell towers or wind power turbines: they both are primary hacking targets in today's world, and they are placed in the wild, in uncontrolled and unprotected locations. This means more or less anybody can just walk by, temporarily cut the power source, take the harddisk out, plug it into their hacking laptop, install an OS trojan on it, place it back into the original device and restore the power. From the Po…

But physical access == game over? Whatever software layer you add imho.

Wouldn't it be easier to simply dunk the whole device in some epoxy preventing access to the hardware with some anti-tamper deadman switch?

Re: Casync – A tool for distributing file system images

#32

Post by poettering »

thinkMOAR wrote: post #31
poettering wrote: post #28

Earlier quoted context omitted.

Think of cell towers or wind power turbines: they both are primary hacking targets in today's world, and they are placed in the wild, in uncontrolled and unprotected locations. This means more or less anybody can just walk by, temporarily cut the power source, take the harddisk out, plug it into their hacking laptop, install an OS trojan on it, place it back into the original device and restore the power. From the Po…

But physical access == game over? Whatever software layer you add imho. Wouldn't it be easier to simply dunk the whole device in some epoxy preventing access to the hardware with some anti-tamper deadman switch?

trusted boot and TPMs with remote attestation exist precisely to ensure that physical access does not mean game over. It's all there, people just need to make use of it in their systems. And yes, trusted boot and TPM has issues, but without all this the attack surface is massive, and I think needlessly so.

Re: Casync – A tool for distributing file system images

#33

Post by dchest »

poettering wrote: post #29
dom0 wrote: post #11

Earlier quoted context omitted.

Low software performance

what would you suggest instead? numbers?

BLAKE2b mentioned above would be more than twice faster on 64-bit CPUs. But I think we'll soon see SHA-256 CPU instructions on most processors (ARM and lower-cost Intel and latest AMD already ship them -https://neosmart.net/blog/2017/will-amds-ryzen-finally-bring...), so I guess it's not important. For numbers, see blake2.net or bench.cr.yp.to.

For IoT devices, hashes that work on 32-bit words, like SHA-256, actually make more sense and will be faster, so BLAKE2s would work well.

What I'd like to hear from the above commenter is about a faster replacement for buzhash, which I'm also interested in.

Re: Casync – A tool for distributing file system images

#34

Post by dchest »

dom0 wrote: post #30
dchest wrote: post #22

Earlier quoted context omitted.

Considering that it uses xz for compression, does the performance of SHA-256 matter? (Well, using faster hash function can speed up finding duplicate blocks, which were already packed.) I'm more interested to hear about buzhash, though.

I assume™ that xz won't stay the only choice. I think it's important to understand that in deduplication, you'll pass all data through your hashes one to two times. Regarding buzhash, it can break with byte granularity, and it has a dependency chain that prohibits parallelization. You'll likely never see it go faster than 700-750 MB/s on a desktop CPU (~3.8 GHz Haswell) and it won't profit from non-clock improvements…

Thanks for explanation. Do you know of any implementations?

Re: Casync – A tool for distributing file system images

#35

Post by ralphmender »

poettering wrote: post #15
tkfu wrote: post #12

I'm not sure I buy the embedded/IoT use case; OSTree is a really good model there and is more featureful. The "well, if your filesystem image delta happens to be in the form of a lot of very small files it's not so great for CDNs" doesn't strike me as a terribly good reason to give up everything OSTree gives you (especially with stuff like the meta-updater [1] Yocto integration). [1] https://github.com/advancedtelema…

Well, I am pretty sure IoT devices should be designed with security in mind, and that means that they need to be protected against offline modification. And that's something OSTree can't really deliver, but dm-crypt can. And casync works pretty well for delivering dm-crypt enabled disk images. I think OSTree is great — but for embedded devices that are installed in the wild, humm, uh, I don't think so? I am pretty su…

I'm with the open source project Mender.io (OTA for embedded Linux) and we think Casync is a very interesting building block and may look into this and evaluate whether it makes sense to incorporate it into our project.

We had looked into OSTree before but given the use case of embedded devices in the wild, we concluded it was too risky as OSTree relies on the filesystem to protect from power failures. And rollback was not built-in and is quite challenging to implement reliably.

Re: Casync – A tool for distributing file system images

#36

Post by thinkMOAR »

poettering wrote: post #32
thinkMOAR wrote: post #31

Earlier quoted context omitted.

But physical access == game over? Whatever software layer you add imho. Wouldn't it be easier to simply dunk the whole device in some epoxy preventing access to the hardware with some anti-tamper deadman switch?

trusted boot and TPMs with remote attestation exist precisely to ensure that physical access does not mean game over. It's all there, people just need to make use of it in their systems. And yes, trusted boot and TPM has issues, but without all this the attack surface is massive, and I think needlessly so.

(trusted boot and TPM are afaik already compromised albeit you need to bring a near rocket scientist)

I will always think physical access is game over whatever 'rocket science' or re-invented old principles people come up with software wise and i'm not sure, but hardware probably too but software is easier to mangle.

And indeed yes, security is layers, layers that make it more difficult, and having many options for layers to choose from that is great.

Also didn't hear about OStree before really, reading up on both for some future project.

Post reply on HN