Secure remailer

Secure remailer
founder:	ruza
depends on:
interested:	sachy
software license:	-
hardware license:	-

~~META: status = active &relation firstimage = :project:gnupg.png ~~

We have an contact email where people can send sensitive info. We dont want to transfer such info unsecurely nor have stored anywhere where can be leaked later. This email addres is actually an alias whose membership is changing overtime. Only actual final recipients (ie brmlab council members) should be able to decrypt such email.

Explore existing solutions like Ciphermail or similar.

An email client of the sender. Since we are expecting an heterogenous environment that consist of different email clients “public subkey distribution” evolves to be a known requirement or we could say a problem to be solved. An email client generates an email content in the following forms:

• GnuPG encrypted email - encrypted against currently valid subkey
• an unencrypted email - such email is not protected on its transfer

Email/SMTP server is receiving emails from email clients and encrypts them eventually (depending on the form of an email received). Following inputs are assumed:

• unencrypted email
  • email needs to be encrypted against currently valid public subkey and delivered to the recipients
  • email is stored in the mailing list archive untouched (unencrypted)
• email was already encrypted by a sender using subkey currently valid
  • email is resend to recipients untouched
  • email is decrypted and stored in the mailing list archive
• email was already encrypted by a sender using subkey expired
  • email needs to be decrypted using expired subkey, reencrypted against currently valid public subkey and send to recipients
  • email is decrypted and stored in the mailing list archive
• email was already encrypted by a sender using an unknown key
  • email should be dropped, rejected or delivered (and stored in the archive) untouched

Emails are stored in the email archive in a decrypted form. Since there is no requirement to reencrypt emails to the specific recepients an email archive is the only reason to hold private subkey on the receiving server.

Access to the email archive is possible via an HTTPS protected web interface (data transfer security) and managed by assigning user rights in such web interface (confidentiality).

Authentication and autorization to the data archived on the filesystem level are realized using operating system user rights assignment and SSH remote access capabilities. Also possible access to the raw data by the virtual infrastructure provider should not be forgotten.

Recipient group consist of an organization board members or another small similarly defined user group. For the managing simplicity we didnt choose the way to reencrypt each incoming email to specific list of recipients instead of simply distribute private subkey to the recipients.

Recipient always receives an mailing list processed emails in the form of GnuPG encrypted email and using private subkey to decrypt such email.

As we know in our situation (brmlab council mailing list) consist of:

• council members
• brmlab members interested in council activities

Master keypair is not used in a day-to-day operation. Its purpose is to be a source of trust in case new subkey is released. Limited number of persons (“master key holders/owners”) should be established (probably a couple). His/her role and responsibility is to hold whole keypair in a secure way, issue and distribute newly created subkey to the recipient group at the at the right time and do the appropriate steps in case some subkeys should be matter of a revocation.

• public subkey distribution on email sender site - since subkey is generated with one year validity (technically 12-15 months) an email clients needs to refresh to currently valid keyring state at least once a year.
• private subkey distribution on email recipients site - each time new subkey generated (Usually related to the list of newly elected board members) this new subkey needs to be distributed to board members
• Using GnuPG expert mode there is possibility to generate “encrypt only” subkey (key without signing and authentication capabilities) howto. Since we have an requirement to decrypt email content for the purpose of storing it in the mailing list archive an receiving email server needs to hold a private subkey currently used. The question whether processing server should also keep expired subkeys could be a matter of discussion. RSA Signing is Not RSA Decryption

Sender encrypts an email against Brmlab public GPG key. Email is delivered to an brmlab SMTP server, reencrypted by GPG-remailer to an up-to-date list of recipients (that is changed more frequently than contact email) and resent to them. Only actual and specific members can decrypt such message with his/her own private key.

All email aliases that should be reencrypted are directed to one OS user:

/etc/aliases

confidential: secmail
join:           secmail

Per user delivery using procmail:

~secmail/.forward

|/usr/bin/procmail

Each email alias has an own configuration file for GPG-remailer.

~secmail/.gpg-remailer-secmail

logfile: remailer.log
loglevel: 0
debug: true
nomail: false 
replyto: <rada@brmlab.cz>
member: member01@somewhe.re
member: member02@elsewhe.re
member: member02@nowhe.re
clear-text: rejected       
signature:  absent
recipient: members

Corŕect configuration file is chosen by procmail parsing mechanism. Here we are also generating reject messages if messages seems to be not encrypted.

~secmail/.procmailrc

SHELL=/bin/sh
LOCKFILE=$HOME/.lockfile.lock
LOGFILE=$HOME/procmail.log
LOGABSTRACT=yes
UMASK=007
VERBOSE=yes
SUBJECT=`formail -x Subject:`
TO=`formail -x To: `
FROM=`formail -x From:`
 
# copy of all incoming emails for debugging purposes
:0c
{
        :0Bfw
        * !(-----(BEGIN|END) PGP MESSAGE-----)
        | formail -I "Subject: [unencrypted and not delivered to $TO]$SUBJECT"
 
        :0
        ! ruza@ruza.eu
}
 
# rejects all non-encrypted emails
:0B
        * !(-----(BEGIN|END) PGP MESSAGE-----)
        | ( formail -r -a "X-Brm: Rejected from $TO";\
        echo "This is an auto-generated reply.";\
        echo " ";\
        echo "Your e-mail";\
        echo "To: $TO";\
        echo "Subject:$SUBJECT ";\
        echo " ";\
        echo "was REJECTED and NOT DELIVERED because";\
        echo "this address accepts GnuPG ENCRYPTED emails only.";\
        echo " ";\
        echo "SEND your email AGAIN:";\
        echo " 1) encrypted by GPG/PGP (https://www.gnupg.org/) to $TO or";\
        echo " 2) unencrypted to [email protected]";\
        echo " ";\
        echo "Or You can ask for help at [email protected]";\
        echo ""; ) | /usr/sbin/sendmail -t -oi -f "$TO"
 
# gpg-remailer drops original From, so its added by this filter to Subject
:0 cfw
        | formail -I "Subject: [$FROM] $SUBJECT"
 
# re-encrypt email
:0
        * ^(To|Cc|Bcc):.*secmail@brmlab.cz.*
        | /usr/bin/gpg-remailer /var/lib/secmail/.gpg-remailer-secmail
 
:0
        * ^(To|Cc|Bcc):.*join@brmlab.cz.*
        | /usr/bin/gpg-remailer /var/lib/secmail/.gpg-remailer-secmail