How do you trust two different cloud provider servers?

Question

I've found a question with an answer here on Security StackExchange or on Unix StackExchange, but I can't find it anymore apparently :( If you find this answer already, help would be appreciated, I already searched both places extensively.

What I want to achieve is to have two different cloud providers and have the encryption key locally on my machine and have the file system of the other server mounted ENCRYPTED and for it to be secure. I know there's been an answer for this somewhere but I can't find it anymore. It contained something with mounting with encfs, cryfs or similar. The only requirements are for it to be secure and that the other server gets mounted with an encryption layer (encrypted on transit & encrypted at rest).

• What would be the most secure method for this?

• As an alternative: what would be the most secure method if one of the cloud servers is trusted and the other one is just used for encrypted-only storage? By that I mean only encrypted data goes in and comes out of the storage server.

I found a good sshfs guide here: https://unix.stackexchange.com/a/62682

But I need to somehow to combine it with cryfs, encfs or similar (the best option)

Answer 1

In short, you mount the remote server, and then an encryption layer in top of that (such as encfs or cryfs).

For example, using encfs:

mkdir /mnt/server-A
mkdir /mnt/server-A/insecure
mkdir /mnt/server-A/secure
sshfs serverA.example.com:/storage/folder /mnt/server-A/insecure
ENCFS6_CONFIG=/mnt/server-A/encfs6.xml encfs /mnt/server-A/insecure /mnt/server-A/secure
cp -v /home/SirMuffington/secret_file.txt /mnt/server-A/secure

encfs can be replaced with cryfs as a more modern alternative that fixes some of its shortcomings.

The folders are used this way:

/mnt/server-A/insecure provides the files in server-A, mounted locally. These are encrypted, so someone at server A would have no access to the underlying files, as they are encrypted by encfs/cryfs.

/mnt/server-A/secure provides the plaintext view of the files. They are actually saved (once encrypted) on /mnt/server-A/insecure which really is storing the files on folder /storage/folder of serverA.example.com.

/mnt/server-A/encfs6.xml Is a configuration file for encfs. It is usually stored on the encrypted folder, but here I am storing it outside.

And that's it. Your local computer is presumed to be trusted. Since all encryption takes place locally, there is quite little trust placed on the server (the are are some things it could do, most importantly, deleting all the files or reverting them to the state they were in some months ago).

Note that you don't need a second server, as you are not trusting the server to begin with.