Best mail encryption solution for per-user

[Aki Tuomi]

> On 19 May 2018 at 16:40 mail at sjemm.net wrote:
> 
> 
> May 18, 2018 10:01 PM, "Aki Tuomi" <aki.tuomi at dovecot.fi> wrote:
> >> On 18 May 2018 at 21:44 mail at sjemm.net wrote:
> >> 
> >> May 18, 2018 4:43 PM, "Aki Tuomi" <aki.tuomi at dovecot.fi> wrote:
> >> On 18 May 2018 at 17:38 mail at sjemm.net wrote:
> >> 
> >> May 18, 2018 4:05 PM, "Aki Tuomi" <aki.tuomi at dovecot.fi> wrote:
> >> On 18 May 2018 at 16:43 mail at sjemm.net wrote:
> >> 
> >> Hi Tai74 and Aki,
> >> I followed your conversation with interest on how to setup per user encryption in dovecot.
> >> I have setup my dovecot with the following in a conf file:
> >> 
> >> ==============
> >> 
> >> mail_attribute_dict = file:%h/Maildir/dovecot-attributes
> >> mail_plugins = $mail_plugins mail_crypt
> >> plugin {
> >> 
> >> mail_crypt_curve = secp521r1
> >> 
> >> mail_crypt_save_version = 2
> >> 
> >> }
> >> 
> >> ==============
> >> 
> >> This works nice, all emails are being encrypted and every user/folder has keys.
> >> But as I understood from your conversation these keys are not protected. And I want them to be
> >> protected by the users password used by imap.
> >> 
> >> Those passwords are stored in a mysql DB file. ( I used a guide from workaround [dot] org to set up
> >> the DB and postfix/dovecot)
> >> 
> >> but how would i set it so, that the users password from the DB is used to encrypt the keys?
> >> 
> >> should i use mail_crypt_private_password = ?
> >> how do i point it to the mysql db then?
> >> im unsure about this
> >> 
> >> Do you have any hints on this?
> >> 
> >> Kind regards,
> >> Zjemm
> >> 
> >> The passwords in your MySQL database are, hopefully, not in plaintext. If you want to secure your
> >> user's keys using user's login password, you must have a TOOL that manages this.
> >> 
> >> You can use mail_crypt_private_password = %w in (mysql) passdb fields to provide the user's login
> >> password as private password. You might want to run it thru some hash, so %{sha1:password} might be
> >> a good option.
> >> 
> >> You can change the key password using 'doveadm mailbox cryptokey', this needs to be done every time
> >> user changes his password.
> >> 
> >> Also note that if you go down this road, and the user forgets his password, you will not be able to
> >> recover the emails without backup copy of the private key.
> >> 
> >> Aki
> >> 
> >> Hi Aki
> >> 
> >> I used the following command:
> >> dovecot pw -s SHA256-CRYPT
> >> 
> >> the output on the chosen password looks like: {SHA256-CRYPT}$5$Rokc06a7In4SF3bO$OQpGQWqg........
> >> 
> >> This output is used to store in the password fields in the database. So no plain text passwords no
> >> :)
> >> 
> >> You can use mail_crypt_private_password = %w in (mysql) passdb fields to provide the user's login
> >> password as private password.
> >> 
> >> can you explain this a bit more for me?
> >> 
> >> for now i have in the 10-auth.conf file the following:
> >> ==============
> >> passdb {
> >> driver = sql
> >> 
> >> # Path for SQL configuration file, see example-config/dovecot-sql.conf.ext
> >> args = /etc/dovecot/dovecot-sql.conf.ext
> >> }
> >> 
> >> and:
> >> 
> >> userdb {
> >> driver = static
> >> args = uid=vmail gid=vmail home=/var/vmail/%d/%n
> >> }
> >> ==============
> >> 
> >> then i have in dovecot-sql.conf.ext
> >> ==============
> >> driver = mysql
> >> connect = host=x.x.x.x dbname=mailserver user=mailuser password=mailpasswordexample
> >> default_pass_scheme = SHA256-CRYPT
> >> password_query = SELECT email as user, password FROM virtual_users WHERE email='%u';
> >> ==============
> >> Where do i need to set : mail_crypt_private_password = %w ?
> >> 
> >> password as private password. You might want to run it thru some hash, so %{sha1:password} might be
> >> a good option.
> >> 
> >> the passwords are allready hashed in the DB using: dovecot pw -s SHA256-CRYPT to genereate the has.
> >> so this step isnt nesesary anymore am i right?
> >> 
> >> Thank you for your quick response, very helpfull
> >> 
> >> Zjemm
> >> 
> >> You misunderstood a bit. The idea is to use the *plaintext* password as the password for the
> >> private key. Otherwise anyone could just decrypt it by looking at your database where the hashed
> >> password is..
> >> 
> >> So:
> >> 
> >> password_query = SELECT email as user, password, '%w' AS userdb_mail_crypt_private_password FROM
> >> virtual_users WHERE email='%u'
> >> 
> >> Aki
> >> 
> >> Hi Aki,
> >> 
> >> Thank you very much for your help, i realy appreciate that.
> >> 
> >> Ok so if i understand it correctly i'll have to use:
> >> 
> >> password_query = SELECT email as user, password, '%w' AS userdb_mail_crypt_private_password FROM
> >> virtual_users WHERE email='%u'
> >> 
> >> in my dovecot-sql.conf.ext file
> >> 
> >> This query selects the user, the password, and %w
> >> 
> >> if i run a little query myself:
> >> MariaDB [mailserver]> SELECT email as user, password, '%w' AS userdb_mail_crypt_private_password
> >> FROM virtual_users;
> >> +------------------+----------------------------------+------------------------------------+
> >> | user | password | userdb_mail_crypt_private_password |
> >> +------------------+----------------------------------+------------------------------------+
> >> | john at example.org | {SHA256-CRYPT}$5$M/GWzmtjsLroRWI | %w |
> >> +------------------+----------------------------------+------------------------------------+
> >> 
> >> %w is a dovecot variable, and stands for the plaintext password, but the password is not stored as
> >> plaintext in the DB, %w get filled with the actual plaintext password by dovecot upon the user that
> >> is typing in the password when authenticating.
> >> 
> >> is this correct?
> > 
> > yes.
> > 
> >> so then i have the username the hashed password en the plaintext password as a result of the query.
> > 
> > yes
> > 
> >> now userdb_mail_crypt_private_password = the plaintext password
> >> do i need to reference it somewhere? or is userdb_mail_crypt_private_password autmatically used by
> >> the dovecot mail_crypt plugin to encrypt the keys? or should it be mail_crypt_private_password?
> > 
> > It gets injected into the mail process as 'mail_crypt_private_password', as if it was set in plugin
> > {} section.
> > 
> >> if i have this setup working i'm going to write a blog post on this topic to share this knowledge
> >> 
> >> Thanks again and have a great weekend.
> >> 
> >> Zjemm
> > 
> > Aki
> 
> Hi Aki,
> 
> Cool i'm testing it right now.
> I have set up a new mailserver (life is great with lxc containers :) )
> 
> postfix and dovecot are working like normal
> 
> next i enable mail_crypt
> 
> i did create a file: /etc/dovecot/conf.d/10-mailcrypt.conf
> ==========================
> mail_attribute_dict = file:%h/Maildir/dovecot-attributes
> 
> mail_plugins = $mail_plugins mail_crypt
> 
> plugin {
>     mail_crypt_curve = secp521r1
>     mail_crypt_save_version = 2
> }
> ==========================
> 
> and then i changed the file: /etc/dovecot/dovecot-sql.conf.ext
> 
> so the query is now the new query:
> password_query = SELECT email as user, password, '%w' AS userdb_mail_crypt_private_password FROM virtual_users WHERE email='%u';
> 
> then i restarted dovecot and postfix and send a test email to the one and only testuser that is in there.
> 
> when i open the mailbox with the tool mutt, i can see the new email, and when openening the email the mutt client drops the connection.
> 
> in the log i can see:
> 
> May 19 13:34:48 mailserver1.example.local dovecot[600]: imap-login: Login: user=<john at example.org>, method=PLAIN, rip=::1, lip=::1, mpid=713, TLS, session=<E3PnIY9sNM4AAAAAAAAAAAAAAAAAAAAB>
> May 19 13:34:49 mailserver1.example.local dovecot[600]: imap(john at example.org): Error: read() failed: read(/var/vmail/example.org/john/Maildir/cur/1526736378.M161472P641.mailserver1.example.local,S=559,W=571:2,) failed: Private key not available: Cannot decrypt key bfc5bb25b1bf64290eea6dc14b516c6a0a25b64551b6e4f0f8677ba7274887cb: error:03070068:bignum routines:BN_mpi2bn:encoding error (FETCH BODY[] for mailbox INBOX UID 8)
> 
> 
> i think i missed a step, but witch one?
> 
> the userpassword hasnt been changed (that would be the next step in the testing process)
> 
> should i've use doveadm first to encrypt the key with that userpassword? i thought it would do that on the fly, because the initial keys where only just created when enabling the mail_crypt plugin
> 
> please let me know your thougts
> Zjemm

I noticed you replied directly to me, and not to the list, too... fixed that for you. 

mail_crypt_private_password is used when key is created, but if you have created it before using password, you'll need to encrypt it before turning the setting on.

Aki