kuvert - Automatically sign and/or encrypt emails based on the
kuvert [-d] [-o] [-r|-k]
Kuvert is a tool to protect the integrity and secrecy of your outgoing
email independent of your mail client and with minimal user
It reads mails from its queue (or accepts SMTP submissions), analyzes
the recipients and decides to whom it should encrypt and/or sign the
mail. The resulting mail is coerced into the PGP-MIME framework defined
in RFC3156 and finally sent to your outbound mail server. Kuvert uses
GnuPG for all cryptographic tasks and is designed to interface cleanly
with external secret caching tools.
After startup kuvert periodically scans its queue directory and
processes mails from there; depending on your GnuPG passphrase setup
kuvert may daemonize itself. In either case, kuvert runs forever until
Kuvert’s behaviour is configured primarily using a configuration file,
with exception of the following commandline options:
-d Enables debugging mode: extra debugging information is written to
STDERR. (This is independent of normal logging.)
-o Enables one-shot mode: kuvert does not loop forever but processes
only the current queue contents and then exits. Kuvert does also
not start an SMTP listener in this mode.
-r Tells a running kuvert daemon to reload the configuration file and
the gpg keyring. This is equivalent to sending a SIGUSR1 to the
-k Tells a running kuvert daemon to terminate cleanly. This is
equivalent to sending a SIGTERM to the respective process.
At startup kuvert reads its configuration file and your gnugp keyring
and remembers the association of email addresses to keys.
Kuvert then works as a wrapper around your mail transfer agent (MTA):
you author your emails like always but instead of sending them out
directly you submit them to kuvert.
Periodically kuvert scans its queue and processes any email therein.
If your keyring contains a key for a recipient, kuvert will encrypt and
sign the email to that recipient. If no key is available, kuvert will
only (clear/detached-)sign the email. Subsequently, the email is sent
onwards using your MTA program or SMTP.
Emails to be processed can have any valid MIME structure; kuvert
unpacks the MIME structure losslessly and repacks the
(encrypted/signed) mail into a PGP/MIME object as described in RFC3156.
The mail’s structure is preserved. Signature and encryption cover all
of the mail content with the exception of the top-level headers: for
example the "Subject" header will be passed in clear, whereas any body
or attached MIME object will be signed/encrypted.
The encrypt-or-sign decision can be overridden on a per-address basis
using the configuration file or, even more fine-grainedly, by using
directives in the actual email. Kuvert can also be told not to modify
an email at all.
Submitting Emails to Kuvert
Kuvert primarily relies on mails being dumped into its queue directory.
Kuvert operates on files with numeric file names only. Anything that
you store in its queue directory with such a filename will be treated
as containing a single RFC2822-formatted email.
However, no mainstream MUA supports such a drop-your-files-somewhere
scheme, and therefore kuvert comes with a helper program called
kuvert_submit (see kuvert_submit(1)) which mimics sendmail’s mail
submission behaviour but feeds to the kuvert queue. If your MUA can be
instructed to run a program for mail submission, kuvert_submit can be
Alternatively, you can send your email to kuvert via SMTP. Kuvert comes
with a built-in receive-only mail server, which feeds to the queue
directory. As allowing others to submit emails for your signature
would be silly and dangerous, kuvert’s mail server only listens on the
localhost IP address and requires that your MUA uses SMTP
Authentication to ensure that only your submissions are accepted. If
your MUA supports SMTP AUTH PLAIN or LOGIN and can be told to use
localhost and a specific port for outbound email, then you can use this
Transporting Emails Onwards
Kuvert can send outbound emails either by running a local MTA program
or by speaking SMTP to some (fixed) outbound mail server of your
Recipients, Identities and the SMTP Envelope
In general kuvert identifies recipients using the To, Cc, Bcc and
Resent-To headers of the queued email. If the mechanism you used to
submit the mail to kuvert did explicitely set recipients, then these
override the headers within the email.
This is the case if kuvert_submit is called with a list of recipients
and no -t option and for SMTP submission.
If kuvert enqueues email via inbound SMTP, the SMTP envelope overrides
the email headers: recipients that are present in the envelope but not
the headers are treated as Bcc’d, and recipients listed in the headers
but not the envelope are ignored. Any Resent-To header is ignored for
Only if no overriding recipients are given, kuvert checks the mail for
a Resent-To header. If present, the email is sent out immediately to
the Resent-To addresses without further processing. (This is the
standard "bounce" behaviour for MUAs that don’t pass recipients on to
an MSP/MTA directly.)
When sending outbound email, kuvert usually uses the From header from
the queued email as identity. If the email was queued via SMTP, the
envelope again overrides the mail headers.
Note that kuvert sets the envelope sender using "-f" if sending email
via a local MTA program; if you are not sufficiently trusted by your
MTA to do such, your mail may get an X-Authentication-Warning header
tacked on that indicates your username and the fact that the envelope
was set explicitely.
Kuvert does not handle your precious keys’ passphrases. You can either
elect to use gpg-agent as an (on-demand or caching) passphrase store,
or you can tell kuvert what program it should run to query for a
passphrase when required. Such a query program will be run in a
pipeline to GnuPG, and kuvert will not access, store or cache the
passphrases themselves: there are better programs available for secret
caching, eg. quintuple-agent or the Linux in-kernel keystorage
(keyctl(1)). Kuvert interfaces cleanly with these.
How Kuvert Decides What (Not) To Do
For each recipient, kuvert can be told to apply one of four different
The email is sent as-is (except for configuration directive
The email is (clear/detached-) signed.
The email is encrypted and signed if there is a key available for
this recipient or only signed.
The email is encrypted and signed if keys are available for all
recipients, or only signed otherwise. Recipients whose action is
set to "none" and Bcc’d recipients are not affected by this action.
The fallback-all action is an "all-or-nothing" action as far as
encryption is concerned and ensures that no mix of encrypted or
unencrypted versions of this email are sent out: if we can we use
encryption for everybody, or otherwise everybody gets it signed (or
even unsigned). (Bcc’d recipients are the exception.)
Kuvert uses four sources for action specifications: directives in the
individual email addresses, action directives in the configuration
file, an X-Kuvert header in your email, and finally the default action
given in the configuration file.
1. First kuvert looks for action directives in your configuration
file. Such directives are given as action plus regular expression
to be matched against an address, and the first matching directive
2. If no matching directive is found, the default action given in the
configuration file is applied.
3. Kuvert now checks for the presence of an X-Kuvert header: its
content must be an action keyword, which is applied to all
recipients of this email except the ones whose action at this stage
is "none". (In other words: if you specify "no encryption/signing"
for some addresses, then this cannot be overridden in a blanket
4. Kuvert then analyzes each recipient email address. If an address
has the format
Some Text "action=someaction" <email@example.com>", kuvert strips the
quoted part and overrides the addressee’s action with someaction.
5. Finally kuvert checks if any recipient has action "fallback-all".
If so, kuvert
a) checks if any recipients (except Bcc’d) have action "signonly"
or "none". If this is the case, all "fallback" and "fallback-
all" actions are downgraded to "signonly".
b) checks if keys for all recipients (except Bcc’d) are available.
If not, all "fallback" and "fallback-all" actions are
downgraded to "signonly".
6. Recipients which are given in a Bcc: header are always treated
independently and separately from all others: any "fallback-all"
action is downgraded to "fallback" for Bcc’d addresses, and if
encryption is used, the email is encrypted separately so that no
record of the Bcc’d recipient is visible in the email as sent out
to the "normal" recipients. Also, any Bcc: header is removed before
sending an email onwards.
Kuvert depends on the order of keys in your keyring to determine which
key (of potentially many) with a given address should be used for
encryption. By default kuvert uses the last key that it encounters for
a given address. For people who have multiple keys for a single
address this can cause problems, and therefore kuvert has override
mechanisms for encryption key selection: You can specify a key to
encrypt to for an address in the configuration file (see below), or you
can override the key selection for and within a single mail:
If the recipient address is given in the format
Some Name "key=keyid" <firstname.lastname@example.org>
Kuvert will strip the double-quoted part and use this particular key
for this recipient and for this single email. The keyid must be given
as the hex key identifier. This mechanism overrides whatever
associations your keyring contains and should be used with caution.
Note that both key and action overrides can be given concurrently as a
single comma-separated entry like this:
Some Name "action=fallback,key=0x12345" <email@example.com>
The signing key can be overridden in a similar fashion: if the From
address contains a "key=keyid" stanza, kuvert will use this key for
signing this single email.
The kuvert configuration file is plain text, blank lines and lines that
start with "#" are ignored.
The configuration has of two categories: options and address/action
Address and Action
Address+action specifications are given one per line. Such lines must
start with some whitespace, followed by an address regexp, followed by
some whitespace and the action keyword. For actions "fallback" and
"fallback-all" kuvert also allows you to specify a single key
identifier like this: "fallback,0x42BD645D". The remainder of the line
The address regexp is a full Perl regular expression and will be
applied to the raw SMTP address (i.e. not to the comment or name in the
email address), case-insensitively. The regular expression may need to
be anchored with ^ and $; kuvert does not do that for you. You must
give just the core of the regexp (no m// or //), like in this example:
# don't confuse mailing list robots
The action keyword must be one of "none", "signonly", "fallback" or
"fallback-all"; see section "How Kuvert Decides What (Not) To Do" for
semantics. Order of action specifications in the config file is
significant: the search terminates on first match.
Options are given one per line, and option lines must start with the
option name followed by some whitespace. All options are case-
sensitive. Depending on the option content, some or all of the
remainder of the option line will be assigned as option value. Inline
comments are not supported.
In the following list of options angle brackets denote required
arguments like this:
Options that have boolean arguments recognize "1", "on" and "t" as true
and "0", "off", "f" as false (plus their upper-case versions). Other
options have more restricted argument types; kuvert generally sanity-
checks options at startup.
syslog <syslog facility or blank>
Whether kuvert should use syslog for logging, and if so, what
facility to use. Default: nothing. This is independent of the
logfile option below.
logfile <path or blank>
Whether kuvert should write log messages to a file, appending to
it. Default: not set. This is independent of the syslog option
mail-on-error <email address or blank>
If kuvert encounters serious or fatal errors, an email is sent back
to this address if set. Default: undef. This email is sent in
addition to the normal logging via syslog or logfile.
Where kuvert and its helper programs store mails to be processed.
Default: ~/.kuvert_queue. The directory is created if necessary.
The directory must be owned by the user running kuvert and have
Where kuvert stores temporary files. Default: a directory called
kuvert.<username>.<pid> in $TMPDIR or /tmp. The directory is
created if necessary, and must be owned by the user running kuvert
and have mode 0700. This directory is completely emptied after
processing an email.
Whether kuvert should add an X-Mailer header to outbound emails.
Default: false. The X-Mailer header consists of the program name
This sets the queue checking interval in seconds. Default: 60
Mail Submission Server for outbound email. Default: unset. If this
is set, kuvert will use SMTP to send outbound emails. If not set,
kuvert uses the mail submission program on the local machine. See
The TCP port on which the Mail Submission Server listens. Default:
587. Ignored if msserver is not set.
msp <program-path and args>
Defines the program kuvert should use to deliver email. Default:
"/usr/sbin/sendmail -om -oi -oem" Ths is ignored if msserver is
set. The argument must include the full path to the program, and
the program must accept the common mail transfer agent arguments as
defined in the Linux Standards Base (see
Indicates to kuvert that it can background itself on startup,
detaching from the terminal. Default: false. This is possible only
if you either delegate passphrase handling to gpg-agent, or if your
secret-query program does not require interaction via the original
terminal (e.g. if it is an X11 program with its own window).
Kuvert can accept email for processing via SMTP. This option sets
the TCP port kuvert listens on (localhost only). Default: 2587.
Ignored if ma-user and ma-pass are not both set. If you want to use
this mechanism, tell your mail program to use localhost or
127.0.0.1 as outgoing mail server and enable SMTP Authentication
This option sets the required SMTP authentication username for
accepting mails via SMTP. Default: undef. Kuvert does not listen
for SMTP submissions unless both ma-user and ma-pass are set.
Kuvert does not accept emails for processing via SMTP unless you
prove your identity with SMTP Authentication (or anybody on your
local machine could use kuvert to send emails signed by you!).
Kuvert currently supports only AUTH PLAIN and LOGIN (which is not a
major problem as we listen on the loopback interface only). This
option sets the username kuvert recognizes as yours. This can be
anything and doesn’t have to be a real account name.
This option sets the password your mail user agent must use for
SMTP Authentication if submitting mails via SMTP. Default: unset.
Kuvert does not listen for SMTP submissions unless both ma-user and
ma-pass are set. This password does not have to be (actually
shouldn’t be) your real account’s password. Note that using SMTP
submission requires that you protect your kuvert configuration file
with strict permissions (0600 is suggested).
Specifies a default key to use as signing key. Default: unset,
which means GnuPG gets to choose (usually the first available
secret key). Can be overridden in the From: address, see section
Which action is to be taken if no overrides are found for a
recipient. Default: none. See section "How Kuvert Decides What
(Not) To Do" for recognized actions.
Whether gpg should be told to trust all keys for encryption or not.
Whether kuvert should delegate all passphrase handling to the gpg-
agent and call gpg with appropriate options. Default: false. If
not set, kuvert will ask the user (or some nominated passphrase
store) for passphrases on demand.
query-secret <program-path and args with %s>
Tells kuvert which program to use for passphrase retrieval.
Default: "/bin/sh -c ’stty -echo; read -p \"Passphrase %s: \" X; \
stty echo; echo $X’" Ignored if use-agent is set. Kuvert does not
store passphrases internally but rather runs the indicated program
in a pipeline with gpg when signing. If you use a passphrase store
(like the Linux-kernel keyutils or secret-agent or the like), enter
your retrieval program here. The program is run with kuvert’s
environment, the first %s in the argument spec is replaced with the
hex keyid and the passphrase is expected on stdout. The exit code
is ignored. If can-detach is not set, the program has access to
kuvert’s terminal. Note that the default query program prohibits
kuvert from backgrounding itself.
flush-secret <program-path and args with %s>
This program is called to invalidate an external passphrase cache
if kuvert is notified by gpg of the passphrase being invalid.
Default: undef. Ignored if use-agent is set. The program is run
with kuvert’s environment and with the first %s of its argument
spec being replaced by the hex keyid in question. Its exit code is
ignored. If can-detach is not set, the program has access to
Kuvert usually logs informational messages to syslog and/or its own
logfile, both of which can be disabled and adjusted.
If kuvert detects a fault that makes successful processing of a
particular email impossible, kuvert will report that on STDERR (if not
detached) and also email an error report if the option mail-on-error is
enabled. Such partially or completely unprocessed mails are left in the
queue but are renamed (the name is prefixed with "failed."); it is up
to you to either remove such leftovers or rename them to something all-
numeric once the problem has been resolved.
The behaviour is similar if fatal problems are encountered; after
alerting kuvert will terminate with exit code 1.
ENVIRONMENT AND SIGNALS
Kuvert itself uses only on environment variable: $TMPDIR provides the
fallback location for kuvert’s temporary directory.
Kuvert passes its complete environment to child processes, namely gpg
and any passphrase-query programs.
On reception of SIGUSR1, kuvert reloads its configuration file and
keyring. Any one of SIGHUP, SIGINT, SIGQUIT and SIGTERM causes kuvert
to terminate cleanly, invalidating the passphrases if a query program
is used. All other signals are ignored.
The configuration file read by kuvert and kuvert_submit.
The default queue directory.
holds the pid of a running kuvert daemon.
gpg(1), kuvert_submit(1), RFC3156, RFC2440, RFC2015
Alexander Zangerl <firstname.lastname@example.org>
COPYRIGHT AND LICENCE
copyright 1999-2008 Alexander Zangerl <email@example.com>
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License version 2 as
published by the Free Software Foundation.