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Comment by esutton | Hacker News Reader

I'm confused, they mentioned at techcrunch disrupt that they are able to offer infinite storage because they are deduplicating data. Fine. But they also said that they will be doing client side encryption. Contradiction - encrypted data cannot be deduplicated.

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What if in the header of the file they put an md5sum that is not encrypted?

I don't think that would represent a data leak, and you could then dedupe based on md5sums...

But only one of the two users could decrypt the block - useless.

downvoters: If I encrypt a plaintext, and you encrypt the same plaintext, we'll have different ciphertexts. If we detect that we encrypted the same plaintext, how do we deduplicate? (Also, I would consider that a leak.)

I encourage everyone in this thread to read up on convergent encryption.

http://research.microsoft.com/apps/pubs/default.aspx?id=7421... http://www.ssrc.ucsc.edu/Papers/storer-storagess08.pdf

I encourage anyone who is thinking of implementing convergent encryption to read "Convergent Encryption Reconsidered" from the tahoe guys: http://www.mail-archive.com/cryptography@metzdowd.com/msg089...

(tldr; there are serious security/privacy vulnerabilities with convergent encryption)

Client-side encryption does not exclude server-side decryption, ie: only encrypt it on the wire, not the storage device.

Like Dropbox.

from their faq: Your Data Is Secure Bitcasa encrypts your data before it is sent to the cloud. It is actually impossible for Bitcasa to access any of your data for any reason.

"Client-side encryption" generally means that only the client can decrypt the data. Unfortunately, if you forget your key, you can't reset your password to recover your data.

> Contradiction - encrypted data cannot be deduplicated.

That's correct; there might be a middle ground that's yet to be explored and proved secure.

Check previous discussion here: http://news.ycombinator.com/item?id=2570538

wouldn't deduplicating encrypted data still work but your hit rate would just be a lot lower. you would have to work on a block instead of a file level but in theory you could dedupe file blocks that had been encrypted, because either way in the end its just bits.

Or is the assumption that with encrypted data you are so incredibly unlikely to see hits that its not worth doing...

if a file is encrypted correctly, you should not be able to compress it or match it to the same file encrypted a second time.

I understand that, but couldn't two different files encrypted with two different keys theoretically share some identical blocks. Obviously this is way less likely than two unencrypted files sharing blocks but still it could happen.

The hell it can't.

Block-level de-duplication can easily work for encrypted data.

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