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[[!tag obnam-wishlist]]
Normally, GPG can encrypt to a key using only the public key, without needing the private key available. I'd like to have the same model for obnam encrypted backups: perform a backup to a key using only the public key, without having the private key available. That way, I could perform backups of various systems using public keys whose corresponding private keys I keep entirely offline and secured.
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Unfortunately, Obnam requires the secret in order to do backups. It needs to download, and decrypt,
some metadata from the backup repository in order to add new backups there. See the [[obnam/ondisk]]
and [[obnam/encryption]] pages for details. --liw
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I've read those pages, but as far as I can tell, the ability to decrypt part of the repository doesn't seem like an inherent requirement to meet obnam's goals, just an implementation detail. And as far as I can tell, without some change in this area, obnam does not support letting a client perform backups without giving that client full access to previous backups.
Use case: I want to back up to a storage system that I trust for availability but not for privacy; thus, I need encryption, and I can't do "pull" backups since I don't trust the storage server with access to the system that needs backing up. However, I also want the backups to protect me from security breaches, by giving me the ability to follow the standard advice for how to deal with a system break-in: nuke everything and restore from the last-known good backup. How can I have a "last-known good backup" if the backed-up system can access the old backups?
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I understand the desire to do what you want to do, but I haven't found a technical solution to do it. If you can
come up with one, then write it up and send me a pointer. --liw
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It'd be nice if Obnam could do this, but in the meantime, I think Duplicity can work this way. --AP
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If Obnam had a local metadata cache, could this be implemented? If Obnam had the metadata locally, could it do a quick verification that the metadata is the same as that on the remote repository, and then backup to the remote repository "blindly", using only the local cache as a guide? --AP
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A concrete existence proof: tarsnap does this. You can do tarsnap backups with only a restricted public key, and that key need not also provide access to decrypt old backups. -- Josh
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I've re-opened this bug since there's ongoing discussion. I'll have a think and respond later. --liw
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Having thought about this...
* Obnam will probably eventually get a local cache for stuff from the repository.
However, this is not enough: we can't assume the cache is complete, since
other clients will be making changes to the repository as well.
* Obnam will thus need to read, and decrypt, files from the repository.
* Having access to the PGP private key is thus necessary for Obnam.
I haven't looked at how duplicity and tarsnap do things. If someone comes up
with a design for Obnam that can do this, without sacrificing things such as
de-duplication across clients, please e-mail the Obnam mailing list with
suggestions. Thanks!
--liw
[[done]]
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