2 ms·

You upload the file with computer A, and you want to download it with computer B. Computer B has to be able to decrypt the file that was encrypted on computer A

You upload the file with computer A, and you want to download it with computer B. Computer B has to be able to decrypt the file that was encrypted on computer A. If you want this to be doable without keys being installed computer B ahead of time (for example, you want to be able to go over to your friend's house and download the file from his computer, using just the web browser), then the server has to know the encryption key. For my own personal use, I prefer something more like Tarsnap's or Mozy's design, but the benefits of ZumoDrive's design definitely would be appealing for many users.

evgen 17y ago
No, computer B just needs some way to access the encrypted key for the files from computer A. You can either have computer A create a master key that gets encrypted with a user-supplied password and then handed out to any client that passes a "does user A approve of this transfer" check, or you can use fine-grained ACLs or capabilities similar to what is done in something like Tahoe-LAFS. The server does not need to see unencrypted data in either case.
- shrughes 17y ago
  The hard part here is for computer A to be sure it's handing the information it thinks it's handing it to. No matter how you do it, computer A needs to be sure it's giving permission to computer B, and not somebody claiming to be computer B, and that involves either personally meeting beforehand to trade some kind of keys or trusting a third party.
  - evgen 17y ago
    Thus the additional re-encryption of the master key. If this re-encryption key is secure then you don't need to care who the master key is handed out to as only an authorized party can decrypt it. There are other, more complicated, crypto schemes that can make this process easier or more secure, but something like this has KISS appeal. An external channel is required here (to transmit the re-encryption key) but that can be as simple as a phone call or email; anything that does not need to pass through the third-party holding the data files.
JoachimSchipper 17y ago
Either use a key on a USB device or enter a (long!) password. There are much more clever solutions (two-factor authentication involving sending an SMS to the mobile phone of the account holder, "delegate" keys that are valid for a limited amount of time, etc), but even the basic stuff works reasonably well.

evgen 17y ago

No, computer B just needs some way to access the encrypted key for the files from computer A. You can either have computer A create a master key that gets encrypted with a user-supplied password and then handed out to any client that passes a "does user A approve of this transfer" check, or you can use fine-grained ACLs or capabilities similar to what is done in something like Tahoe-LAFS. The server does not need to see unencrypted data in either case.

shrughes 17y ago
The hard part here is for computer A to be sure it's handing the information it thinks it's handing it to. No matter how you do it, computer A needs to be sure it's giving permission to computer B, and not somebody claiming to be computer B, and that involves either personally meeting beforehand to trade some kind of keys or trusting a third party.
- evgen 17y ago
  Thus the additional re-encryption of the master key. If this re-encryption key is secure then you don't need to care who the master key is handed out to as only an authorized party can decrypt it. There are other, more complicated, crypto schemes that can make this process easier or more secure, but something like this has KISS appeal. An external channel is required here (to transmit the re-encryption key) but that can be as simple as a phone call or email; anything that does not need to pass through the third-party holding the data files.

JoachimSchipper 17y ago

Either use a key on a USB device or enter a (long!) password. There are much more clever solutions (two-factor authentication involving sending an SMS to the mobile phone of the account holder, "delegate" keys that are valid for a limited amount of time, etc), but even the basic stuff works reasonably well.