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GnuPG support in SqWebMail
GnuPG support in SqWebMail
Table of contents:
This is experimental beta of GnuPG support in SqWebMail. The
following functionality is available:
- Create new keypairs
- List public/private keys
- Delete keys
- Sign keys
- Export keys (attach them to the current message)
- Import keys from messages
- Sign outgoing messages
- Encrypt outgoing messages
- Display and verify signed messages
- Decrypt encrypted messages
- Reply and forward encrypted messages. Correctly handle
multipart/alternative and multipart/related.
SqWebMail creates and manages its own keyrings.
$HOME/.gnupg is not used. This allows GnuPG functions to work
for virtual accounts that have no home directory.
This implementation requires GnuPG, which is used for all of the heavy
lifting. No encryption code exists in SqWebMail itself. The
configuration script searches the current PATH for the gpg
binary. The final SqWebMail binary gets the hardcoded absolute
path to the gpg binary. If gpg is not found in the
current PATH, /usr/bin/gpg will be used. Therefore, if GnuPG is
locally installed in a non-default location, make sure that gpg
can be found in the current PATH.
SqWebMailgpg automatically to
handle all the actual tasks involved in encrypting and decrypting messages.
make install also installs two more binaries:
reformime and mimegpg. Those binaries do some other
things, and nothing really needs to be done about them, or to them, except to
know that they're there.
The webgpg script
make install automatically installs a shell script,
webgpg. Encryption/decryption capabilities for all mail accounts
are disabled by default. The
webgpg script takes one argument -
the complete pathname to a Maildir.
webgpg creates a new
subdirectory,
Maildir/gpg, and initializes it. Once that
subdirectory is created, encryption/decryption in
SqWebMail will
become available for that account. Not every mail account has to have
encryption/decryption enabled. This functionality can be selectively enabled
and disabled for individual accounts.
NOTE: webgpg script MUST be executed by the same userid and
groupid that owns the Maildir directory.
NOTE: The current implementation consists of the bare minimum required to
get the overall functionality. As a result, a number of shortcuts are taken
with respect to avoiding some hoops that GnuPG occasionally requires to be
jumped through. For example, by default GnuPG requires that a public key
must be signed before it can be used for encryption. SqWebMail
will automatically provide the "always-trust" option to bypass the check.
Basically, if you have the public key in your keyring, you can use it.
Issues with adding or activating encryption for existing accounts
Obviously, encryption/decryption takes additional server CPU cycles. There
is no hard and fast rule for how much additional load is needed. Fortunately,
this is not an "all or none" deal. Encryption support can be activated in a
controlled manner and phased in gradually, in steps, allowing the impact on
the web/mail server to be closely monitored, and controlled.
Issues with adding or activating encryption for new accounts
The webgpg script needs to be used to initialize encryption
support for newly-created accounts. Many systems use the
/etc/skel directory as a template for setting up the initial
contents of new accounts. In that case, use run "webgpg
/etc/skel/Maildir", and all new accounts will have encryption
automatically activated.
As of now, SqWebMail's encryption/decryption is experimental.
There's not been much interoperability testing with other encryption-capable
mail software. It is very much possible that there are some interoperability
issues that will have to be worked out.
The directory Maildir/gpg does pretty much what
$HOME/.gnupg does for the command-line gpg tool.
SqWebMail does not use $HOME/.gnupg, it keeps its
keyrings and other GnuPG-related stuff in the Maildir/gpg
directory. That's because SqWebMail can be configured to use
both physical system accounts and virtual mail accounts, and after
authenticating, there is no difference whatsoever between the two. Therefore,
SqWebMail has no concept of the $HOME directory. Its entire
universe consists of the Maildir directory, and its contents.
SqWebMail is completely unaware of the existence of
$HOME/.gnupg, and will never be aware of it. Any keypairs in
$HOME/.gnupg will have to be imported into
Maildir/gpg.
Importing keys
Since $HOME/.gnupg is not used, any existing keys will have
to be imported. If shell access is available, the pubring and
secring files can simply be copied to Maildir/gpg.
The following procedure can be used to import keys in all other
circumstances. The following example imports keys from mail account M to
SqWebMail mail account W.
- Create a temporary keypair in W.
- Create a new message, addressed to M, attach the new public key, and
send it.
- In M, take the new public key and import it.
- Export M's secret key. With GnuPG, use the following command:
gpg --export-secret-key --armor [fingerprint]
The output from this command looks something like this:
-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: GnuPG v1.0.4 (GNU/Linux)
Comment: For info see http://www.gnupg.org
+M3OSONvootZCzKXP6VT29Vf+lZLQkjSyuY28PpfflaFKi8YpeCEKo4dDhynxCbV
NAYk2I6wXguwl4MOT0ebkEWM1WS9lTsto7cCzz0ovSg3xe82PozA/4s6E5UUgl1B
...
RQj/UASJxoPEEDHAhfZ0FYPsDFbo/P///4nDiTQk6Pz///+J2I1l+FteXcOJ9lWJ
RSBQ/3Uc/3UY/3UU/3UQ/3UM/3UI6GsAAACDxBz/dSxqBf91KGoAicP/dST8McCJ
-----END PGP PRIVATE KEY BLOCK-----
- Wrap this inside a MIME-formatted mail message. The end result should
look something like this:
Mime-Version: 1.0
Content-Type: multipart/mixed; boundary="xxx"
--xxx
Content-Type: text/plain
Here's the key
--xxx
Content-Type: application/pgp-keys
-----BEGIN PGP PRIVATE KEY BLOCK-----
Version: GnuPG v1.0.4 (GNU/Linux)
Comment: For info see http://www.gnupg.org
+M3OSONvootZCzKXP6VT29Vf+lZLQkjSyuY28PpfflaFKi8YpeCEKo4dDhynxCbV
NAYk2I6wXguwl4MOT0ebkEWM1WS9lTsto7cCzz0ovSg3xe82PozA/4s6E5UUgl1B
...
RQj/UASJxoPEEDHAhfZ0FYPsDFbo/P///4nDiTQk6Pz///+J2I1l+FteXcOJ9lWJ
RSBQ/3Uc/3UY/3UU/3UQ/3UM/3UI6GsAAACDxBz/dSxqBf91KGoAicP/dST8McCJ
-----END PGP PRIVATE KEY BLOCK-----
--xxx
- Encrypt the whole message using W's public key. The end result should
look something like this:
Mime-version: 1.0
Content-Type: multipart/encrypted;
boundary="yyy";
protocol="application/pgp-encrypted"
--yyy
Content-Type: application/pgp-encrypted
Content-Transfer-Encoding: 7bit
Version: 1
--yyy
Content-Type: application/octet-stream
Content-Transfer-Encoding: 7bit
-----BEGIN PGP MESSAGE-----
Version: GnuPG v1.0.4 (GNU/Linux)
Comment: For info see http://www.gnupg.org
hQIOA8nMaZ+WFjCiEAf+IzymijY0K0ELubJnDuTo0OZg6n8EApLTdwXVZ7sjoWvU
YbEjfvwBXp+wc7X4lvw3S/AGYe4/mmshgTnRjuDPKsN3zHViYGdGF0UaB40sjUp3
...
U9Qi2kvxRvV0b8nBN1qeFUSSRF8Ncwug1sr/rFVNNWA0yavb5WDFrqRE6WFJU3z3
p0vPPRbdWCOTP7WGOygpHdRMLQSCxG4m+L9uG3roG5Uu7A==
=GutI
-----END PGP MESSAGE-----
--yyy--
- Mail this message to W. Repeat the procedure with M's public key
(
gpg --export-key --armor). If you're handy with composing MIME
messages, both keys can be sent in a single encrypted message.
- In
SqWebMail, you should be able to decrypt the message
containing the keys, and import them.
SqWebMail can handle passphrase-protected keys only when SSL is being
used. This is an artificial restriction that prevents passphrases from going
over the network, in the clear. When SSL is used, SqWebMail will
automatically prompt for a passphrase. Leave the passphrase field empty if
the secret key is not protected by a passphrase.
- Although
SqWebMail contains mappings for many charsets,
GnuPG versions prior to 1.0.6 know only about iso-8859-1 and
iso-8859-2. All messages displayed by SqWebMail
from GnuPG 1.0.5 or earlier will come out in iso-8859-1.
- Currently, there are no command line options in GnuPG for removing key
signatures (is that even possible?). You can sign keys, but can't
"unsign" them.
- There has not been any/much interoperability testing with other
GnuPG/PGP mail software. It is entirely possible that
SqWebMail's implementation is currently not interoperable,
and may require some tinkering as time goes by.
- GnuPG will always blather "Warning: secret key NOT protected" when
using keys not protected by a passphrase. This whinge is harmless, and
can be ignored.
- The error message from GnuPG for a missing passphrase is not very
helpful: "Unable to get tty input". This is because SqWebMail runs GnuPG
with the --no-tty flag, and provides a passphrase separately, when it's
entered. When SqWebMail does not provide a passphrase, GnuPG complains
that tty input is disabled.
If GnuPG is compiled on a system that has a /dev/random
device, GnuPG will use the /dev/random device as a source for
random entropy. The /dev/random device driver in Linux, and in
many other implementations, creates the entropy pool from system device
driver background activity. Unfortunately, it's is not a bottomless pit of
random entropy. When the random device driver entropy pool is empty,
/dev/random halts until device drivers create more noise.
That means that when GnuPG is used on Linux, GnuPG may pause for an
excessive period of time, and issue its famous "please do something" prompt.
Since GnuPG is invoked in automatic mode, there is no other alternative,
except to wait until more device driver noise becomes available.
This is more likely to happen when GnuPG is used to create new keypairs,
especially ones that are 1024 bits, or longer. Before deploying, extensive
testing is recommended to make sure that even an idle system generates
sufficient background noise so that the additional wait is not excessive.
There are several possible solutions that can also be used:
- GnuPG can be optionally configured to use a pseudo-random generator
that does not use
/dev/random, but instead uses several
alternative sources of random junk. Consult GnuPG's documentation for
more information.
- Many systems also have a
/dev/urandom device, which
automatically generates pseudo-random noise if /dev/random
runs out of entropy. This makes it possible to simply remove the
/dev/random device and create a link from
/dev/random to /dev/urandom.
Although - technically - both approaches can theoretically result in
slightly less secure keypairs, practically the difference is probably
academic in nature.
