Best mail encryption solution for per-user

[Aki Tuomi]

> On 11 February 2018 at 09:17 tai74 at vfemail.net wrote:
> 
> 
> 
> Quoting tai74 at vfemail.net:
> 
> > Aki really thanks for reply,, I hope for continue the conversation,
> >
> >>> Do you have advice about Dovecot plugins for mail encryption:
> >>>
> >>> https://wiki2.dovecot.org/Plugins/MailCrypt
> >>> https://0xacab.org/riseuplabs/trees
> >>>
> >>> I like NaCL based encryption but the MailCrypt plugin is better
> >>> because it's maintained by Dovecot developers (is this correct?)
> >>>
> >>> Hard to understand MailCrypt docs so may I ask, may I provide per user
> >>> encryption? I don't like global because I want every user to have
> >>> different encryption based on their password. But I don't see the
> >>> useful of per-folder encryption. Docs for TREES are easier to
> >>> understand it will work on a per-user encryption using the user
> >>> password to decrypt the needed keys.
> >>>
> >>> How about indexing? Does native fast Dovecot indexing must be turned
> >>> off for one of these or both? Is this a big performance hit? And how
> >>> to turn off indexing on a per-user basis?
> >>>
> >> Mail crypt plugin in dovecot core is indeed developed and  
> >> maintained by us. Turning on encryption is simple, but you'll have  
> >> to consider key management closely.
> >>
> >> The simple instructions are
> >>
> >> mail_plugins = $mail_plugins mail_crypt
> >> mail_attribute_dict = file:%h/Maildir/dovecot-attributes
> >>
> >> plugin {
> >>  mail_crypt_curve = <some curve you like>
> >>  mail_crypt_save_version = 2
> >> }
> >
> > With no other settings, I understand this to auto-generate keys.
> > It's putting them in mail_attribute_dict? How is that related to
> > mail_crypt_private_key,, which is for what then?
> >

mail_crypt_private_key is *NOT* same as mail_crypt_*global*_private_key.

mail_crypt_global_private_key means it will use this private key to decrypt all mails. mail_crypt_global_public_key means this key will be used to encrypt all mails.

"mail_crypt_private_key - Private key to decrypt user's master key, can be base64 encoded"

in other words, this is used to encrypt the user key.

All generated keys are stored in mail attribute dict.

> >> This will turn on automatic key creation for folders and users.
> >
> > I guess it's saying "folders" mode because each folder has a keypair
> > but I don't understand why a single keypair isn't enough? Because of
> > possibility to have shared folders? Anywayz I'm confused how it relating
> > against global mode and where user mode fits (there is no user mode
> > say the docs).
> >

In managed mode, where no global key(s) are provided, dovecot will generate a user-key and folder keys. The folders keys are encrypted using user key. User key is optionally encrypted with either password or some other key (mail_crypt_private_password / mail_crypt_private_key).

Having per-folder key makes key sharing easier and doable in ACL situation, and also reduces problems if single folder key is for some inexplicable reason exposed.

> >> NOTE that the keys won't be encrypted in this scenario.
> >>
> >> To encrypt keys you have few choices. You can use a static  
> >> password, static ECC key, per-user password, per-user ECC key,  
> >> per-user password protected ECC key.
> >
> > Sorry newb question, though how is this choice made (on different values
> > of mail_crypt_require_encrypted_user_key?)
> >

No. This is only for key sharing with ACLs.

> >> With the per-user model, the easiest, if you don't let users change  
> >> password could be adding into userdb or passdb,  
> >> mail_crypt_private_password = %{sha512:password}
> >>
> >> If you want users to be able to change passwords, you either need  
> >> to manage the encryption passwords in user database, or call  
> >> doveadm mailbox cryptokey password during password change.
> >
> > I have custom userdb lookup so I don't mind to manage key encryption
> > passwords in database, but how do I do it? Is it by to include
> > mail_crypt_private_password in the userdb lookup?
> >
> > Moreso,, if I want password same to be as regular IMAP password, is
> > there security problem? I think it's possible to return plaintext
> > password for userdb lookup with %w right?
> >

You can do that, too.

> > Moreso,, I don't see that to work because maybe pwd changes in userdb
> > but encryption keys (stored where?) still be encrypted with old pwd
> > no? In such case, what is to be done? Your indication was that doveadm
> > can be used but that there is another soultion which doesn't become
> > full evident to me yet.
> >

If you have a custom user database, you can generate random encryption keys or passwords for user key encryption and provide those. 

As I said before, the single most important thing is key management.

> > Is literally solution now I must also store keys in database and make
> > custom pwd change module decrypt to re-encrypt them with new pwd?
> >

Yes, or use doveadm mailbox cryptokey password to re-encrypt the user key using new password.

> >> Also we do not currently support DR keys, so if the per-user key is  
> >> lost, you will not be able to recover the mails.
> >
> > Fair!
> >
> >> Forgot to add that currently it's not possible to encrypt indexes due
> >> to how lib-index is designed.
> >
> > To infer your statement, this means indexing will work anyway ok even
> > with encryption turning on? With caveat of have unencrypted metadata in
> > index files? So that we must choose, unprotected metadata or remove
> > indexing? May indexing be removed per-user, maybe from userdb lookup?
> 

No. It means that dovecot.index* and dovecot.cache* will be unencrypted on your disk. Any access to email requires a key to be able to decrypt them.

> Adding questions: will encryption work equally with maildir, dbox, mdbox
> et al?

Yes.

> Will encryption work with zlib compression?

Yes.

> I want to test encryption but only on a test account not all accounts,
> possible to enable for just a coulple testing users?

Set mail_save_version=2 in userdb. 0 is "read-only mode".

> 
> Regards,Tai
> 

Regards, Aki

> 
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