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Re: [Duplicity-talk] Questions regarding symmetric encryption/signing

From: Eponymous -
Subject: Re: [Duplicity-talk] Questions regarding symmetric encryption/signing
Date: Thu, 25 Jun 2015 14:04:57 +0100

Thanks for taking the time to help :)

On Thu, Jun 25, 2015 at 2:01 PM,  <address@hidden> wrote:
> On 25.06.2015 14:33, Eponymous - wrote:
>>>> duplicity utilizes the command line binary gpg which only supports piping 
>>>> on passphrase. so both of the above is true.
>> if no key is given the passphrase is used for symmetric de/encryption.
>> if one is givenit's used to unlock the keys for decryption or signing.
>> I still don't see how in the symmetric role of operation this marries
>> up with how GPG works: "Both PGP and GnuPG use hybrid ciphers. The
>> session key, encrypted using the public-key cipher, and the message
>> being sent, encrypted with the symmetric cipher, are automatically
>> combined in one package. The recipient uses his private-key to decrypt
>> the session key and the session key is then used to decrypt the
>> message." [1]
> my above was speaking with duplicity in mind.
>> Or are you saying that if we don't give a key, then we don't use GPG
>> at all and instead the data is raw encrypted using the PASSPHRASE as a
>> key to some internal encryption engine in Duplicity?
> no. for all encryption in duplicity gpg is utilized.
>>>> let me point you to "a Note on Symmetric Encryption and Signing" 
>>>> http://duplicity.nongnu.org/duplicity.1.html#sect24 which seems to have 
>>>> slipped your man page reading ;)
>> I did  read this over a few times before asking and it still didn't
>> make sense to me. For example, it states:
> then you should point out the unclear parts ;)
>> "3. The used PASSPHRASE for symmetric encryption and the passphrase of
>> the signing key are identical. "
>> What does this mean? Is it referring to the PASSPHRASE env variable
>> used to protect the GPG keys? How does the SIGN_PASSPHRASE env
>> variable play into this?
> for symmetric encryption _no_ keys are used. for signed symmetric encrytion 
> you need a passphrase and a secret key. you can however only pipe _one_ 
> passphrase into gpg, hence both passphrases need to be identical.
> SIGN_PASSPHRASE was introduce to mitigate the issue above for signed key 
> encryption. especially if you choose a different key for signing than for 
> encryption.
> for servers i would suggest you to use gpg-agent , which is a little bit more 
> complicated to set up, but the most secure setup avail. of course you will 
> have to do manual backup everytime the machine is rebooted for the 
> passphrases to be read into memory again.
>>>> how about using http://duply.net which takes care to generate the proper 
>>>> command lines for you?
>> I think it's more that I think some of the references to
>> command-lines/env variables are confusing.
> well, if you find ways to improve duplicity documentation your more than 
> welcome.
>> For example:
>> "--encrypt-key key-id When backing up, encrypt to the given public
>> key, instead of using symmetric (traditional) encryption. Can be
>> specified multiple times. The key-id can be given in any of the
>> formats supported by GnuPG; see gpg(1) , section "HOW TO SPECIFY A
>> USER ID" for details. "
>> I'd say that's not a helpful choice of name there :) From reading:
>> "--encrypt-key" it's not obvious that this only applies to
>> public/private keypair encryption.
> well it says _key_ in the name and you are the first to complain. you cannot 
> do symmetric encryption against keys. also it clearly says what happens if 
> this parameter is not set.
> sorry, i don't see the issue here ;(
>> Again, sorry if these seem like obvious questions but Duplicity looks
>> to be perfect for my needs seems like it's worth the effort of getting
>> a deep understanding :)
> it's a nifty piece of software. agreed. and exactly because it is difficult 
> to get the parameters right i co-developed ftplicity now duply.
> ..ede/duply.net

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