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Comment by ectopod | Hacker News Reader

It makes some kind of sense for them to keep the private key. Most customers are more concerned about data loss from losing the key than snooping.

Except, they say your data is unrecoverable if you lose your public key (which presumably serves as proof of your ownership of the data). Of course, it is not technically unrecoverable, they just won't recover it for you. In other words, this negates the one good reason for them storing the private key. I'm confused.

(Also, why is the private key generated on the client and then transmitted to their server?)

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The OpenSSL RSA private key contains all the elements needed to recreate the public key from it. [0] Explains it in more detail.

You create the private key on the client side so that you can encrypt it with a custom password before sending it to Backblaze. Doing this makes they key recoverable from you - as long as you remember the passphrase to decrypt it - and unusable from Backblaze.

[0] https://stackoverflow.com/a/26048132/2080636

I think the GP is referring to this part:

> The priv key is NEVER written to disk on your system,it is then thrown away after transmission. The public key can be found on your local system here on a Windows PC (ask if you want to know on a Macintosh): C:\Program Files\Backblaze\userPub.pem This is a standard "OpenSSL PEM File" as decribed here: http://www.openssl.org/docs/crypto/pem.html#PEM_ENCRYPTION_F....

> You can read these files with Notepad or Wordpad, but be very very careful not to write out any changes or it will effectively destroy your backup -> make it unrecoverable/unreadable.

Cheers, I was, I should have quoted it.

I think that quote is saying that if you modify your public key locally then the data you encrypt with it will no longer be decryptable by the private key you previously sent backblaze.

I understand that losing the public key doesn't matter cryptographically, which is why I said "it is not technically unrecoverable".

Also, the encryption happens on the server. If you protected the private key with a passphrase from the moment it was created then your data couldn't be encrypted because backblaze would never have access to the key.

> You create the private key on the client side so that you can encrypt it with a custom password before sending it to Backblaze.

Their client is not designed this way. Their installer generates and sends the private key before the user has an option to set a passphrase. I replied to a different comment about this with more detail[0].

[0] https://news.ycombinator.com/item?id=28350404

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