Insecurity in the Jungle (disk)

http://www.daemonology.net/blog/2011-06-03-insecurity-in-the-jungle.html

by cperciva • 15 years ago

92 75 15 years ago

I continue to not understand how people imagine these services working (de-dupe, block level updates, etc) without access to the unsecured version of the data. As for the claims about what Amazon could do to your data... there's even less sinister options. S3 is not 100% safe storage. There's a chance for bit rot and that may occur. If you don't check the file yourself, you won't know. Again, that seems a bit inevitable, no?

edit: left out a 'not'

cperciva16• 15 years ago

There's a sucker born every minute. Jungle Disk claims to be secure, and most people believe them -- most people have no way to assess whether they're doing things right or not.

gst2• 15 years ago

De-dupe: Wuala encrypts the file with a key derived from the file itself. This key is then encrypted with the user's key and both (the file and the encrypted key) are uploaded to the cloud. Disadvantage: If the file is known to an attacker (i.e., a copyright holder) the attacker can possibly find out which users have access to this file. Advantage: Allows for de-duplication, but is more secure than Dropbox.

Block-level updates: I don't see a problem with this. Partition the file into blocks on the client (before the encryption). The server doesn't need access to the data for this.

tptacek19• 15 years ago

As Steve Weis pointed out in an earlier thread about schemes like this, deriving keys from the contents of files breaks semantic security. Lay engineers reason about this problem the way you just did: "the RIAA can tell I have Lady Gaga MP3s". But practitioners are worried about much more subtle and devastating flaws, particularly in cases where attackers may exercise some control over the blocks being encrypted.

Any scheme that derives passwords from file contents gives me the willies.

drivebyacct22• 15 years ago

Most file-level encryption that I'm aware destroys benefits of blocked data. For example, changing a few bytes in an encrypted file will cause MANY bytes to change in the actual file on the disk... at least with ecryptfs and truecrypt. If there is an encryption scheme that works well with striping, I'd really super appreciate you pointing me in that direction - it would greatly help with a problem I'm currently trying to solve.

PonyGumbo• 15 years ago

Given the available options, what's the best option for automated backups?

cperciva16• 15 years ago

I use Tarsnap. ;-)

acqq• 15 years ago

I took a look https://www.tarsnap.com/gettingstarted.html and I don't understand why "If you have multiple machines, you almost certainly want to create a separate key file for each machine."? Can you explain why, if I'd like to access the same data from the different machines? Or is the main assumption that every machine has its own, nor shareable, backup? Isn't the main advantage of an online service to have the same data accessible from more machines?

dchest2• 15 years ago

Apart from Tarsnap, maybe Duplicity http://duplicity.nongnu.org?

cperciva16• 15 years ago

Agreed. There are some things I dislike about duplicity (e.g., its reliance upon GPG) but it's probably what I would use if I couldn't use Tarsnap.

chirayuk• 15 years ago

SpiderOak? https://spideroak.com/

If TarSnap had iPhone, etc. versions, I would use it for everything. (the permission specific keys are just awesome.)

gst2• 15 years ago

Depends.

On my server I use Tarsnap.

On my client I use Wuala's "backup folder" feature.

masnick2• 15 years ago

What about Arq for Mac? http://www.haystacksoftware.com/arq/

It is/was used by Chris Wanstrath (of github), among others. http://chris.wanstrath.usesthis.com

tptacek19• 15 years ago

Arq appears to have similar flaws to JungleDisk, judging from its file format specification.

sreitshamer3• 15 years ago

I'm in the process of getting an app review done by a security expert. Then I can answer that question (hopefully) definitively. (I'm the author of Arq)

imajes• 15 years ago

@cperciva: Thanks for this; now i'll convert my 8char ascii system password to a 10char one. Do you have any data showing how large a password needs to be to make it ridiculously expensive for a TLA (gency) to commit a large amount of hardware to cracking? i.e. how much time past the 10chars does it consume ?

cperciva16• 15 years ago

It depends on your KDF. MD5 is ridiculously weak; the standard MD5-crypt is 1000 times stronger; bcrypt is better yet; and scrypt is vastly stronger.

The best source for this my scrypt paper, really.

dchest2• 15 years ago

Link: http://www.tarsnap.com/scrypt.html

SoftwareMaven2• 15 years ago

What license is the scrypt code released under?

inklesspen• 15 years ago

http://xkcd.com/538/

Splines• 15 years ago

True enough, if you're targeted it's not going to help very much. However, like outrunning a bear, you only need to be harder to catch than the guy behind you.

pilom• 15 years ago

I wish I could find a link but,US military spec for secure passwords is 14 characters with capitals and special chars. And they have to be changed every 30 days.

tptacek19• 15 years ago

Summary: Jungledisk doesn't protect the integrity of encrypted data, and doesn't securely derive keys and is thus vulnerable to fast offline attacks. The thing Jungledisk right is to use the same block cipher mode as Tarsnap (and, incidentally, virtually every mainstream encrypted storage system).

The impact of using unauthenticated encryption to store data is that your backup provider could end up owning your machine. Attackers can carefully choose which data to corrupt. They can exploit the randomization of corrupted decryption to set up conditions for memory corruption exploits, and, in more sophisticated but totally realistic attacks, exploit guesses about known plaintext to produce attacker-controlled nonrandom plaintexts. A backup provider with client-authenticated crypto can't do that, because the keys that encrypt the data also ensure it's integrity.

The password storage issue is no different than any other password storage problem; again, direct your attention to http://codahale.com/how-to-safely-store-a-password/, mentally substituting "storage of password hash" to "derivation of AES key".

To my mind, the key derivation is the real problem here. A surprisingly large number of secure encryption storage products don't ensure data integrity. Realistic attacks against that vulnerability are feasible but difficult: you'd have to be targeted.

If you're going to write an article about how a competitor's encryption is inferior to yours and cast it as a vulnerability report, I'd suggest not recommending your own encryption scheme as the replacement. The scrypt recommendation in this article sticks out like a sore thumb. Virtually nothing uses scrypt.

We can nerd out on CTR mode vs. CBC mode; I'm starting to come around to Colin's take on CTR because of ciphertext indistinguishability as I see more practical vulnerabilities that take advantage of it. I think the padding issue is a red herring. CBC padding is easier to get right than absolute rock solid reliable generation of CTR nonces and absolute rock solid management of CTR counters, which are things I see people get wrong regularly. Distinguishability is the real problem with CBC.

cperciva16• 15 years ago

I think the lack of integrity is more important than you're making it sound. There's a lot of situations where a lack of integrity can be exploited to create a lack of privacy too.

But the main reason I mentioned the lack of integrity first is that I needed to mention the lack of HMAC to explain why they had the ridiculous "salted key hash" construct.

I think you're misstating what I wrote a bit. I said that scrypt is the state of the art in the field -- which it is -- and that given that Jungle Disk was around before I developed scrypt, they should have used PBKDF2 or bcrypt.

tptacek19• 15 years ago

I'd rather geek out about CTR v CBC than harp on the scrypt recommendation. Consider the scrypt thing a friendly style note. You wrote an article about a competitor's insecurities. When you do that, don't recommend they adopt your own cryptosystem unless (like CRI had to do with DPA countermeasures) they have to. Here, it just made you look unnecessarily petty.

What privacy attacks were you thinking of? Call some of them out.

sunchild6• 15 years ago

Setting aside the data integrity issues:

I think the author's point about privacy is valid, and a little silly. If I understand correctly (the article is very confusingly worded in some places), he is saying that weak passwords are weak. Anyone who cares about privacy should already be choosing long, complex, strong passwords for this kind of application.

Also, I'm confused about one feature of JD. When I signed up years ago, they allowed me to hold my key privately and it never left the client. I had the option to upload that key to the server, if I wanted to, or not. I understand from the article that the client might misbehave and, for example, share my key in ways I don't want it to. Am I getting this right?

When I looked into secure cloud-based storage two years ago, I found that JD was the best mix of privacy and convenience, if for no other reason than it could be deployed on a mix of Windows, Mac and Linux boxes. It was clear even then that data integrity was the weak link/trade off.

I'm interested in hearing about the latest, best solutions for easy, cross-platform, secure backups to cloud services that offer better data integrity.

tptacek19• 15 years ago

This article points out two flaws. Neither of them are silly.

First, there's no integrity protection on data stored on Jungledisk. Jungledisk can own up your machine. That's not a good property for a secure backup system to have.

Second, the key derivation scheme it uses makes every passphrase, no matter how carefully chosen, drastically weaker.

I'm glad you like Jungledisk and I don't think you need to read stories like this as an indictment of your choice or a demand to change services. But it doesn't help to downplay them.

tptacek19• 15 years ago

Missed the word "right" in "The thing Jungledisk (does) right is". Sorry!

rarrrrrr2• 15 years ago

My understanding is that SpiderOak, Tarsnap, and Wuala all do this correctly (using one of PBKDF2, bcrypt, or scrypt.)

Colin - Perhaps the companies in the backup space that put effort into handling this carefully should work together and create a PSA style website with a matrix chart of how the varies providers handle "encrypted" data. Make it a separate domain and do our best to be elaborately objective about it. Any interest?

tptacek19• 15 years ago

What block cipher mode does SpiderOak use, and how does it verify the authenticity of its data? Tarsnap goes through a lot of extra trouble to MAC its data; few other providers do. You'd hate to see everyone treat key derivation as a shorthand for "doing all of encryption right".

I looked on the SpiderOak site, saw a lot of material on how keys are derived and not stored on SpiderOak servers (great!), but didn't see a lot of details about the mechanics of actually encrypting and checking data.

rarrrrrr2• 15 years ago

Thanks for asking. If you're interested, would be very happy to discuss SpiderOak's crypto strategy in depth with you the next time I'm in Chicago. Could share source code, etc. IMO, most interesting parts are the key scoping, which allow users to selectively publish ("share") portions of stored data by publishing the appropriately scoped keys.

SpiderOak uses AES256 in CFB mode with authentication via HMAC. The code is careful about unique nonce/counter usage, crypto code is confined to specific modules that rarely change, and reviewed by cryptographers outside SpiderOak. Client and server have minimal trust relationship.

Being paranoid about data integrity (not only because of crypto issues, but also because bitrot happens routinely at petabyte scale) the data authentication happens repetitively at a few different layers. From all end user devices, we see about one bit error per 4.2tb of upload transactions.

cperciva16• 15 years ago

Any interest?

Not really. I've seen far too many "best practices" and "standards" bodies go nowhere to think that a committee can put together a useful website like this.

jasondavies• 15 years ago

A matrix chart sounds like a great idea. Anyone know if Backblaze handles this correctly? Here is a post describing their setup: http://blog.backblaze.com/2008/11/12/how-to-make-strong-encr...

euroclydon3• 15 years ago

If, like me, you're wondering: "Does 1Password use all the stuff?"

http://agilebits.com/products/1Password/user_guide

jmtulloss• 15 years ago

More details here: http://help.agilebits.com/1Password3/agile_keychain_design.h...

euroclydon3• 15 years ago

Thanks, that's the page I was on -- stupid AJAX navigation!

Anyway, the document seems *nix specific. Does Windows have a /dev/random?

kevindication• 15 years ago

Stupid question: Why is the 34 character password easier to crack than the 8 character password?

(Upon re-reading I think I may have missed the assumption that the long password only contains english text, no punctuation, numerics, etc.)

cperciva16• 15 years ago

Yes, the 34-character is English text.

Insecurity in the Jungle (disk)

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