Tag Archives: gnu

Privilege separation of GSS-API credentials for Apache

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To protect web resources with Kerberos you may use Apache HTTPD with mod_auth_gssapi — however, all web scripts (e.g., PHP) run under Apache will have access to the Kerberos long-term symmetric secret credential (keytab). If someone can get it, they can impersonate your server, which is bad.

The gssproxy project makes it possible to introduce privilege separation to reduce the attack surface. There is a tutorial for RPM-based distributions (Fedora, RHEL, AlmaLinux, etc), but I wanted to get this to work on a DPKG-based distribution (Debian, Ubuntu, Trisquel, PureOS, etc) and found it worthwhile to document the process. I’m using Ubuntu 22.04 below, but have tested it on Debian 11 as well. I have adopted the gssproxy package in Debian, and testing this setup is part of the scripted autopkgtest/debci regression testing.

First install the required packages:

root@foo:~# apt-get update
root@foo:~# apt-get install -y apache2 libapache2-mod-auth-gssapi gssproxy curl

This should give you a working and running web server. Verify it is operational under the proper hostname, I’ll use foo.sjd.se in this writeup.

root@foo:~# curl --head http://foo.sjd.se/
HTTP/1.1 200 OK
…

The next step is to create a keytab containing the Kerberos V5 secrets for your host, the exact steps depends on your environment (usually kadmin ktadd or ipa-getkeytab), but use the string “HTTP/foo.sjd.se” and then confirm using something like the following.

root@foo:~# ls -la /etc/gssproxy/httpd.keytab
-rw------- 1 root root 176 Sep 18 06:44 /etc/gssproxy/httpd.keytab
root@foo:~# klist -k /etc/gssproxy/httpd.keytab -e
Keytab name: FILE:/etc/gssproxy/httpd.keytab
KVNO Principal
---- --------------------------------------------------------------------------
   2 HTTP/foo.sjd.se@GSSPROXY.EXAMPLE.ORG (aes256-cts-hmac-sha1-96) 
   2 HTTP/foo.sjd.se@GSSPROXY.EXAMPLE.ORG (aes128-cts-hmac-sha1-96) 
root@foo:~#

The file should be owned by root and not be in the default /etc/krb5.keytab location, so Apache’s libapache2-mod-auth-gssapi will have to use gssproxy to use it.

Then configure gssproxy to find the credential and use it with Apache.

root@foo:~# cat<<EOF > /etc/gssproxy/80-httpd.conf
[service/HTTP]
mechs = krb5
cred_store = keytab:/etc/gssproxy/httpd.keytab
cred_store = ccache:/var/lib/gssproxy/clients/krb5cc_%U
euid = www-data
process = /usr/sbin/apache2
EOF

For debugging, it may be useful to enable more gssproxy logging:

root@foo:~# cat<<EOF > /etc/gssproxy/gssproxy.conf
[gssproxy]
debug_level = 1
EOF
root@foo:~#

Restart gssproxy so it finds the new configuration, and monitor syslog as follows:

root@foo:~# tail -F /var/log/syslog &
root@foo:~# systemctl restart gssproxy

You should see something like this in the log file:

Sep 18 07:03:15 foo gssproxy[4076]: [2022/09/18 05:03:15]: Exiting after receiving a signal
Sep 18 07:03:15 foo systemd[1]: Stopping GSSAPI Proxy Daemon…
Sep 18 07:03:15 foo systemd[1]: gssproxy.service: Deactivated successfully.
Sep 18 07:03:15 foo systemd[1]: Stopped GSSAPI Proxy Daemon.
Sep 18 07:03:15 foo gssproxy[4092]: [2022/09/18 05:03:15]: Debug Enabled (level: 1)
Sep 18 07:03:15 foo systemd[1]: Starting GSSAPI Proxy Daemon…
Sep 18 07:03:15 foo gssproxy[4093]: [2022/09/18 05:03:15]: Kernel doesn't support GSS-Proxy (can't open /proc/net/rpc/use-gss-proxy: 2 (No such file or directory))
Sep 18 07:03:15 foo gssproxy[4093]: [2022/09/18 05:03:15]: Problem with kernel communication! NFS server will not work
Sep 18 07:03:15 foo systemd[1]: Started GSSAPI Proxy Daemon.
Sep 18 07:03:15 foo gssproxy[4093]: [2022/09/18 05:03:15]: Initialization complete.

The NFS-related errors is due to a default gssproxy configuration file, it is harmless and if you don’t use NFS with GSS-API you can silence it like this:

root@foo:~# rm /etc/gssproxy/24-nfs-server.conf
root@foo:~# systemctl try-reload-or-restart gssproxy

The log should now indicate that it loaded the keytab:

Sep 18 07:18:59 foo systemd[1]: Reloading GSSAPI Proxy Daemon…
Sep 18 07:18:59 foo gssproxy[4182]: [2022/09/18 05:18:59]: Received SIGHUP; re-reading config.
Sep 18 07:18:59 foo gssproxy[4182]: [2022/09/18 05:18:59]: Service: HTTP, Keytab: /etc/gssproxy/httpd.keytab, Enctype: 18
Sep 18 07:18:59 foo gssproxy[4182]: [2022/09/18 05:18:59]: New config loaded successfully.
Sep 18 07:18:59 foo systemd[1]: Reloaded GSSAPI Proxy Daemon.

To instruct Apache — or actually, the MIT Kerberos V5 GSS-API library used by mod_auth_gssap loaded by Apache — to use gssproxy instead of using /etc/krb5.keytab as usual, Apache needs to be started in an environment that has GSS_USE_PROXY=1 set. The background is covered by the gssproxy-mech(8) man page and explained by the gssproxy README.

When systemd is used the following can be used to set the environment variable, note the final command to reload systemd.

root@foo:~# mkdir -p /etc/systemd/system/apache2.service.d
root@foo:~# cat<<EOF > /etc/systemd/system/apache2.service.d/gssproxy.conf
[Service]
Environment=GSS_USE_PROXY=1
EOF
root@foo:~# systemctl daemon-reload

The next step is to configure a GSS-API protected Apache resource:

root@foo:~# cat<<EOF > /etc/apache2/conf-available/private.conf
<Location /private>
  AuthType GSSAPI
  AuthName "GSSAPI Login"
  Require valid-user
</Location>

Enable the configuration and restart Apache — the suggested use of reload is not sufficient, because then it won’t be restarted with the newly introduced GSS_USE_PROXY variable. This just applies to the first time, after the first restart you may use reload again.

root@foo:~# a2enconf private
Enabling conf private.
To activate the new configuration, you need to run:
systemctl reload apache2
root@foo:~# systemctl restart apache2

When you have debug messages enabled, the log may look like this:

Sep 18 07:32:23 foo systemd[1]: Stopping The Apache HTTP Server…
Sep 18 07:32:23 foo gssproxy[4182]: [2022/09/18 05:32:23]: Client [2022/09/18 05:32:23]: (/usr/sbin/apache2) [2022/09/18 05:32:23]: connected (fd = 10)[2022/09/18 05:32:23]: (pid = 4651) (uid = 0) (gid = 0)[2022/09/18 05:32:23]:
Sep 18 07:32:23 foo gssproxy[4182]: message repeated 4 times: [ [2022/09/18 05:32:23]: Client [2022/09/18 05:32:23]: (/usr/sbin/apache2) [2022/09/18 05:32:23]: connected (fd = 10)[2022/09/18 05:32:23]: (pid = 4651) (uid = 0) (gid = 0)[2022/09/18 05:32:23]:]
Sep 18 07:32:23 foo systemd[1]: apache2.service: Deactivated successfully.
Sep 18 07:32:23 foo systemd[1]: Stopped The Apache HTTP Server.
Sep 18 07:32:23 foo systemd[1]: Starting The Apache HTTP Server…
Sep 18 07:32:23 foo gssproxy[4182]: [2022/09/18 05:32:23]: Client [2022/09/18 05:32:23]: (/usr/sbin/apache2) [2022/09/18 05:32:23]: connected (fd = 10)[2022/09/18 05:32:23]: (pid = 4657) (uid = 0) (gid = 0)[2022/09/18 05:32:23]:
root@foo:~# Sep 18 07:32:23 foo gssproxy[4182]: message repeated 8 times: [ [2022/09/18 05:32:23]: Client [2022/09/18 05:32:23]: (/usr/sbin/apache2) [2022/09/18 05:32:23]: connected (fd = 10)[2022/09/18 05:32:23]: (pid = 4657) (uid = 0) (gid = 0)[2022/09/18 05:32:23]:]
Sep 18 07:32:23 foo systemd[1]: Started The Apache HTTP Server.

Finally, set up a dummy test page on the server:

root@foo:~# echo OK > /var/www/html/private

To verify that the server is working properly you may acquire tickets locally and then use curl to retrieve the GSS-API protected resource. The "--negotiate" enables SPNEGO and "--user :" asks curl to use username from the environment.

root@foo:~# klist
Ticket cache: FILE:/tmp/krb5cc_0
Default principal: jas@GSSPROXY.EXAMPLE.ORG

Valid starting Expires Service principal
09/18/22 07:40:37 09/19/22 07:40:37 krbtgt/GSSPROXY.EXAMPLE.ORG@GSSPROXY.EXAMPLE.ORG
root@foo:~# curl --negotiate --user : http://foo.sjd.se/private
OK
root@foo:~#

The log should contain something like this:

Sep 18 07:56:00 foo gssproxy[4872]: [2022/09/18 05:56:00]: Client [2022/09/18 05:56:00]: (/usr/sbin/apache2) [2022/09/18 05:56:00]: connected (fd = 10)[2022/09/18 05:56:00]: (pid = 5042) (uid = 33) (gid = 33)[2022/09/18 05:56:00]:
Sep 18 07:56:00 foo gssproxy[4872]: [CID 10][2022/09/18 05:56:00]: gp_rpc_execute: executing 6 (GSSX_ACQUIRE_CRED) for service "HTTP", euid: 33,socket: (null)
Sep 18 07:56:00 foo gssproxy[4872]: [CID 10][2022/09/18 05:56:00]: gp_rpc_execute: executing 6 (GSSX_ACQUIRE_CRED) for service "HTTP", euid: 33,socket: (null)
Sep 18 07:56:00 foo gssproxy[4872]: [CID 10][2022/09/18 05:56:00]: gp_rpc_execute: executing 1 (GSSX_INDICATE_MECHS) for service "HTTP", euid: 33,socket: (null)
Sep 18 07:56:00 foo gssproxy[4872]: [CID 10][2022/09/18 05:56:00]: gp_rpc_execute: executing 6 (GSSX_ACQUIRE_CRED) for service "HTTP", euid: 33,socket: (null)
Sep 18 07:56:00 foo gssproxy[4872]: [CID 10][2022/09/18 05:56:00]: gp_rpc_execute: executing 9 (GSSX_ACCEPT_SEC_CONTEXT) for service "HTTP", euid: 33,socket: (null)

The Apache log will look like this, notice the authenticated username shown.

127.0.0.1 - jas@GSSPROXY.EXAMPLE.ORG [18/Sep/2022:07:56:00 +0200] "GET /private HTTP/1.1" 200 481 "-" "curl/7.81.0"

Congratulations, and happy hacking!

Towards pluggable GSS-API modules

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GSS-API is a standardized framework that is used by applications to, primarily, support Kerberos V5 authentication. GSS-API is standardized by IETF and supported by protocols like SSH, SMTP, IMAP and HTTP, and implemented by software projects such as OpenSSH, Exim, Dovecot and Apache httpd (via mod_auth_gssapi). The implementations of Kerberos V5 and GSS-API that are packaged for common GNU/Linux distributions, such as Debian, include MIT Kerberos, Heimdal and (less popular) GNU Shishi/GSS.

When an application or library is packaged for a GNU/Linux distribution, a choice is made which GSS-API library to link with. I believe this leads to two problematic consequences: 1) it is difficult for end-users to chose between Kerberos implementation, and 2) dependency bloat for non-Kerberos users. Let’s discuss these separately.

  1. No system admin or end-user choice over the GSS-API/Kerberos implementation used

    There are differences in the bug/feature set of MIT Kerberos and that of Heimdal’s, and definitely that of GNU Shishi. This can lead to a situation where an application (say, Curl) is linked to MIT Kerberos, and someone discovers a Kerberos related problem that would have been working if Heimdal was used, or vice versa. Sometimes it is possible to locally rebuild a package using another set of dependencies. However doing so has a high maintenance cost to track security fixes in future releases. It is an unsatisfying solution for the distribution to flip flop between which library to link to, depending on which users complain the most. To resolve this, a package could be built in two variants: one for MIT Kerberos and one for Heimdal. Both can be shipped. This can help solve the problem, but the question of which variant to install by default leads to similar concerns, and will also eventually leads to dependency conflicts. Consider an application linked to libraries (possible in several steps) where one library only supports MIT Kerberos and one library only supports Heimdal.

    The fact remains that there will continue to be multiple Kerberos implementations. Distributions will continue to support them, and will be faced with the dilemma of which one to link to by default. Distributions and the people who package software will have little guidance on which implementation to chose from their upstream, since most upstream support both implementations. The result is that system administrators and end-users are not given a simple way to have flexibility about which implementation to use.
  2. Dependency bloat for non-Kerberos use-cases.

    Compared to the number of users of GNU/Linux systems out there, the number of Kerberos users on GNU/Linux systems is smaller. Here distributions face another dilemma. Should they enable GSS-API for all applications, to satisfy the Kerberos community, or should they be conservative with adding dependencies to reduce attacker surface for the non-Kerberos users? This is a dilemma with no clear answer, and one approach has been to ship two versions of a package: one with Kerberos support and one without. Another option here is for upstream to support loadable modules, for example Dovecot implement this and Debian ship with a separate ‘dovecot-gssapi’ package that extend the core Dovecot seamlessly. Few except some larger projects appear to be willing to carry that maintenance cost upstream, so most only support build-time linking of the GSS-API library.

    There are a number of real-world situations to consider, but perhaps the easiest one to understand for most GNU/Linux users is OpenSSH. The SSH protocol supports Kerberos via GSS-API, and OpenSSH implement this feature, and most GNU/Linux distributions ship a SSH client and SSH server linked to a GSS-API library. Someone made the choice of linking it to a GSS-API library, for the arguable smaller set of people interested in it, and also the choice which library to link to. Rebuilding OpenSSH locally without Kerberos support comes with a high maintenance cost. Many people will not need or use the Kerberos features of the SSH client or SSH server, and having it enabled by default comes with a security cost. Having a vulnerability in OpenSSH is critical for many systems, and therefor its dependencies are a reasonable concern. Wouldn’t it be nice if OpenSSH was built in a way that didn’t force you to install MIT Kerberos or Heimdal? While still making it easy for Kerberos users to use it, of course.

Hopefully I have made the problem statement clear above, and that I managed to convince you that the state of affairs is in need of improving. I learned of the problems from my personal experience with maintaining GNU SASL in Debian, and for many years I ignored this problem.

Let me introduce Libgssglue!

Matryoshka Dolls
Matryoshka Dolls – photo CC-4.0-BY-NC by PngAll

Libgssglue is a library written by Kevin W. Coffman based on historical GSS-API code, the initial release was in 2004 (using the name libgssapi) and the last release was in 2012. Libgssglue provides a minimal GSS-API library and header file, so that any application can link to it instead of directly to MIT Kerberos or Heimdal (or GNU GSS). The administrator or end-user can select during run-time which GSS-API library to use, through a global /etc/gssapi_mech.conf file or even a local GSSAPI_MECH_CONF environment variable. Libgssglue is written in C, has no external dependencies, and is BSD-style licensed. It was developed for the CITI NFSv4 project but libgssglue ended up not being used.

I have added support to build GNU SASL with libgssglue — the changes required were only ./configure.ac-related since GSS-API is a standardized framework. I have written a fairly involved CI/CD check that builds GNU SASL with MIT Kerberos, Heimdal, libgssglue and GNU GSS, sets ups a local Kerberos KDC and verify successful GSS-API and GS2-KRB5 authentications. The ‘gsasl’ command line tool connects to a local example SMTP server, also based on GNU SASL (linked to all variants of GSS-API libraries), and to a system-installed Dovecot IMAP server that use the MIT Kerberos GSS-API library. This is on Debian but I expect it to be easily adaptable to other GNU/Linux distributions. The check triggered some (expected) Shishi/GSS-related missing features, and triggered one problem related to authorization identities that may be a bug in GNU SASL. However, testing shows that it is possible to link GNU SASL with libgssglue and have it be operational with any choice of GSS-API library that is shipped with Debian. See GitLab CI/CD code and its CI/CD output.

This experiment worked so well that I contacted Kevin to learn that he didn’t have any future plans for the project. I have adopted libgssglue and put up a Libgssglue GitLab project page, and pushed out a libgssglue 0.5 release fixing only some minor build-related issues. There are still some missing newly introduced GSS-API interfaces that could be added, but I haven’t been able to find any critical issues with it. Amazing that an untouched 10 year old project works so well!

My current next steps are:

  • Release GNU SASL with support for Libgssglue and encourage its use in documentation.
  • Make GNU SASL link to Libgssglue in Debian, to avoid a hard dependency on MIT Kerberos, but still allowing a default out-of-the-box Kerberos experience with GNU SASL.
  • Maintain libgssglue upstream and implement self-checks, CI/CD testing, new GSS-API interfaces that have been defined, and generally fix bugs and improve the project. Help appreciated!
  • Maintain the libgssglue package in Debian.
  • Look into if there are applications in Debian that link to a GSS-API library that could instead be linked to libgssglue to allow flexibility for the end-user and reduce dependency bloat.

What do you think? Happy Hacking!

Automatic Replicant Backup over USB using rsync

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I have been using Replicant on the Samsung SIII I9300 for over two years. I have written before on taking a backup of the phone using rsync but recently I automated my setup as described below. This work was prompted by a screen accident with my phone that caused it to die, and I noticed that I hadn’t taken regular backups. I did not lose any data this time, since typically all content I create on the device is immediately synchronized to my clouds. Photos are uploaded by the ownCloud app, SMS Backup+ saves SMS and call logs to my IMAP server, and I use DAVDroid for synchronizing contacts, calendar and task lists with my instance of ownCloud. Still, I strongly believe in regular backups of everything, so it was time to automate this.

For my use-case, taking backups of the phone whenever I connect it to one of my laptops is sufficient. I typically connect it to my laptops for charging at least every other day. My laptops are all running Debian, but this should be applicable to most modern GNU/Linux system. This is not Replicant-specific, although you need a rooted phone. I thought that automating this would be simple, but I got to learn the ins and outs of systemd and udev in the process and this ended up taking the better part of an evening.

I started out adding an udev rule and a small script, thinking I could invoke the backup process from the udev rule. However rsync would magically die after running a few seconds. After an embarrassing long debugging session, finally I found someone with a similar problem which led me to a nice writeup on the topic of running long-running services on udev events. I created a file /etc/udev/rules.d/99-android-backup.rules with the following content:

ACTION=="add", SUBSYSTEMS=="usb", ENV{ID_SERIAL_SHORT}=="323048a5ae82918b", TAG+="systemd", ENV{SYSTEMD_WANTS}+="android-backup@$env{ID_SERIAL_SHORT}.service"
ACTION=="add", SUBSYSTEMS=="usb", ENV{ID_SERIAL_SHORT}=="4df9e09c25e75f63", TAG+="systemd", ENV{SYSTEMD_WANTS}+="android-backup@$env{ID_SERIAL_SHORT}.service"

The serial numbers correspond to the device serial numbers of the two devices I wish to backup. The adb devices command will print them for you, and you need to replace my values with the values from your phones. Next I created a systemd service to describe a oneshot service. The file /etc/systemd/system/android-backup@.service have the following content:

[Service]
Type=oneshot
ExecStart=/usr/local/sbin/android-backup %I

The at-sign (“@”) in the service filename signal that this is a service that takes a parameter. I’m not enough of an udev/systemd person to explain these two files using the proper terminology, but at least you can pattern-match and follow the basic idea of them: the udev rule matches the devices that I’m interested in (I don’t want this to happen to all random Android devices I attach, hence matching against known serial numbers), and it causes a systemd service with a parameter to be started. The systemd service file describe the script to run, and passes on the parameter.

Now for the juicy part, the script. I have /usr/local/sbin/android-backup with the following content.

#!/bin/bash

DIRBASE=/var/backups/android
export ANDROID_SERIAL="$1"

exec 2>&1 | logger

if ! test -d "$DIRBASE-$ANDROID_SERIAL"; then
    echo "could not find directory: $DIRBASE-$ANDROID_SERIAL"
    exit 1
fi

set -x

adb wait-for-device
adb root
adb wait-for-device
adb shell printf "address 127.0.0.1\nuid = root\ngid = root\n[root]\n\tpath = /\n" \> /mnt/secure/rsyncd.conf
adb shell rsync --daemon --no-detach --config=/mnt/secure/rsyncd.conf &
adb forward tcp:6010 tcp:873
sleep 2
rsync -av --delete --exclude /dev --exclude /acct --exclude /sys --exclude /proc rsync://localhost:6010/root/ $DIRBASE-$ANDROID_SERIAL/
: rc $?
adb forward --remove tcp:6010
adb shell rm -f /mnt/secure/rsyncd.conf

This script warrant more detailed explanation. Backups are placed under, e.g., /var/backups/android-323048a5ae82918b/ for later off-site backup (you do backup your laptop, right?). You have to manually create this directory, as a safety catch to not wildly rsync data into non-existing directories. The script logs everything using syslog, so run a tail -F /var/log/syslog& when setting this up. You may want to reduce verbosity of rsync if you prefer (replace rsync -av with rsync -a). The script runs adb wait-for-device which you rightly guessed will wait for the device to settle. Next adb root is invoked to get root on the device (reading all files from the system naturally requires root). It takes some time to switch, so another wait-for-device call is needed. Next a small rsyncd configuration file is created in /mnt/secure/rsyncd.conf on the phone. The file tells rsync do listen on localhost, run as root, and use / as the path. By default, rsyncd is read-only so the host will not be able to upload any data over rsync, just read data out. Next rsync is started on the phone. The adb forward command forwards port 6010 on the laptop to port 873 on the phone (873 is the default rsyncd port). Unfortunately, setting up the TCP forward appears to take some time, and adb wait-for-device will not wait for that to complete, hence an ugly sleep 2 at this point. Next is the rsync invocation itself, which just pulls in everything from the phone to the laptop, excluding some usual suspects. The somewhat cryptic : rc $? merely logs the exit code of the rsync process into syslog. Finally we clean up the TCP forward and remove the rsyncd.conf file that was temporarily created.

This setup appears stable to me. I can plug in a phone and a backup will be taken. I can even plug in both my devices at the same time, and they will run at the same time. If I unplug a device, the script or rsync will error out and systemd cleans up.

If anyone has ideas on how to avoid the ugly temporary rsyncd.conf file or the ugly sleep 2, I’m interested. It would also be nice to not have to do the ‘adb root’ dance, and instead have the phone start the rsync daemon when connecting to my laptop somehow. TCP forwarding might be troublesome on a multi-user system, but my laptops aren’t. Killing rsync on the phone is probably a good idea too. If you have ideas on how to fix any of this, other feedback, or questions, please let me know!

Scrypt in IETF

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Colin Percival and I have worked on an internet-draft on scrypt for some time. I realize now that the -00 draft was published over two years ago, turning this effort today somewhat into archeology rather than rocket science. Still, having a published RFC that is easy to refer to from other Internet protocols will hopefully help to establish the point that PBKDF2 alone no longer provides state-of-the-art protection for password hashing.

I have written about password hashing before where I give a quick introduction to the basic concepts in the context of the well-known PBKDF2 algorithm. The novelty in scrypt is that it is designed to combat brute force and hardware accelerated attacks on hashed password databases. Briefly, scrypt expands the password and salt (using PBKDF2 as a component) and then uses that to create a large array (typically tens or hundreds of megabytes) using the Salsa20 core hash function and then de-references that large array in a random and sequential pattern. There are three parameters to the scrypt function: a CPU/Memory cost parameter N (varies, typical values are 16384 or 1048576), a blocksize parameter r (typically 8), and a parallelization parameter p (typically a low number like 1 or 16). The process is described in the draft, and there are further discussions in Colin’s original scrypt paper.

The document has been stable for some time, and we are now asking for it to be published. Thus now is good time to provide us with feedback on the document. The live document on gitlab is available if you want to send us a patch.

Creating a small JPEG photo for your OpenPGP key

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I’m in the process of moving to a new OpenPGP key, and I want to include a small JPEG image of myself in it. The OpenPGP specification describes, in section 5.12.1 of RFC 4880, how an OpenPGP packet can contain an JPEG image. Unfortunately the document does not require or suggest any properties of images, nor does it warn about excessively large images. The GnuPG manual helpfully asserts that “Note that a very large JPEG will make for a very large key.”.

Researching this further, it seems that proprietary PGP program suggests 120×144 as the maximum size, although I haven’t found an authoritative source of that information. Looking at the GnuPG code, you can see that it suggests around 240×288 in a string saying “Keeping the image close to 240×288 is a good size to use”. Further, there is a warning displayed if the image is above 6144 bytes saying that “This JPEG is really large”.

I think the 6kb warning point is on the low side today, however without any more researched recommendation of image size, I’m inclined to go for a 6kb 240×288 image. Achieving this was not trivial, I ended up using GIMP to crop an image, resize it to 240×288, and then export it to JPEG. Chosing the relevant parameters during export is the tricky part. First, make sure to select ‘Show preview in image window’ so that you get a file size estimate and a preview of how the photo will look. I found the following settings useful for reducing size:

  • Disable “Save EXIF data”
  • Disable “Save thumbnail”
  • Disable “Save XMP data”
  • Change “Subsampling” from the default “4:4:4 (best quality)” to “4:2:0 (chroma quartered)”.
  • Try enabling only one of “Optimize” and “Progressive”. Sometimes I get best results disabling one and keeping the other enabled, and sometimes the other way around. I have not seen smaller size with both enabled, nor with both disabled.
  • Smooth the picture a bit to reduce pixel effects and size.
  • Change quality setting, I had to reduce it to around 25%.

See screenshot below of the settings windows.

GnuPG photo GIMP settings window

Eventually, I managed to get a photo that I was reasonable happy with. It is 240×288 and is 6048 bytes large.

GnuPG photo for Simon

If anyone has further information, or opinions, on what image sizes makes sense for OpenPGP photos, let me know. Ideas on how to reduce size of JPEG images further without reducing quality as much would be welcome.

Portable Symmetric Key Container (PSKC) Library

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For the past weeks I have been working on implementing RFC 6030, also known as Portable Symmetric Key Container (PSKC). So what is PSKC? The Portable Symmetric Key Container (PSKC) format is used to transport and provision symmetric keys to cryptographic devices or software.

My PSKC Library allows you to parse, validate and generate PSKC data. The PSKC Library is written in C, uses LibXML, and is licensed under LGPLv2+. In practice, PSKC is most commonly used to transport secret keys for OATH HOTP/TOTP devices (and other OTP devices) between the personalization machine and the OTP validation server. Yesterday I released version 2.0.0 of OATH Toolkit with the new PSKC Library. See my earlier introduction to OATH Toolkit for background. OATH Toolkit is packaged for Debian/Ubuntu and I hope to refresh the package to include libpskc/pskctool soon.

To get a feeling for the PSKC data format, consider the most minimal valid PSKC data:

<?xml version="1.0"?>
<KeyContainer xmlns="urn:ietf:params:xml:ns:keyprov:pskc" Version="1.0">
  <KeyPackage/>
</KeyContainer>

The library can easily be used to export PSKC data into a comma-separated value (CSV) format, in fact the PSKC library tutorial concludes with that as an example. There is complete API documentation for the library. The command line tool is more useful for end-users and allows you to parse and inspect PSKC data. Below is an illustration of how you would use it to parse some PSKC data, first we show the content of a file “pskc-figure2.xml”:

<?xml version="1.0" encoding="UTF-8"?>
<KeyContainer Version="1.0"
	      Id="exampleID1"
	      xmlns="urn:ietf:params:xml:ns:keyprov:pskc">
  <KeyPackage>
    <Key Id="12345678"
         Algorithm="urn:ietf:params:xml:ns:keyprov:pskc:hotp">
      <Issuer>Issuer-A</Issuer>
      <Data>
        <Secret>
          <PlainValue>MTIzNA==
          </PlainValue>
        </Secret>
      </Data>
    </Key>
  </KeyPackage>
</KeyContainer>

Here is how you would parse and pretty print that PSKC data:

jas@latte:~$ pskctool -c pskc-figure2.xml 
Portable Symmetric Key Container (PSKC):
	Version: 1.0
	Id: exampleID1
	KeyPackage 0:
		DeviceInfo:
		Key:
			Id: 12345678
			Issuer: Issuer-A
			Algorithm: urn:ietf:params:xml:ns:keyprov:pskc:hotp
			Key Secret (base64): MTIzNA==

jas@latte:~$

For more information, see the OATH Toolkit website and the PSKC Library Manual.

Unattended SSH with Smartcard

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I have several backup servers that run the excellent rsnapshot software, which uses Secure Shell (SSH) for remote access. The SSH private key of the backup server can be a weak link in the overall security. To see how it can be a problem, consider if someone breaks into your backup server and manages to copy your SSH private key, they will now have the ability to login to all machines that you take backups off (and that should be all of your machines, right?).

The traditional way to mitigate SSH private key theft is by password protecting the private key. This works poorly in an unattended server environment because either the decryption password needs to be stored in disk (where the attacker can read it) or the decrypted private key has to be available in decrypted form in memory (where attacker can read it).

A better way to deal with the problem is to move the SSH private key to a smartcard. The idea is that the private key cannot be copied by an attacker who roots your backup server. (Careful readers may have spotted a flaw here, and I need to explain one weakness with my solution: an attacker will still be able to login to all your systems by going through your backup server, however it will require an open inbound network connection to your backup server and the attacker will never know what your private key is. What this does is to allow you to more easily do damage control by removing the smartcard from the backup server.)

In this writeup, I’ll explain how to accomplish all this on a Debian/Ubuntu-system using a OpenPGP smartcard, a Gemalto USB Shell Token v2 with gpg-agent/scdaemon from GnuPG together with OpenSSH.

Continue reading Unattended SSH with Smartcard→

Introducing the OATH Toolkit

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I am happy to announce a project that I have been working quietly on for about a year: the OATH Toolkit. OATH stands for Open AuTHentication and is an organization that specify standards around authentication. That is a pretty broad focus, but practically it has translated into work on specifying standards around deploying and using electronic token based user authentication such as the YubiKey.

YubiKey

OATH’s most visible specification has been the HOTP algorithm which is a way to generate event-based one-time passwords from a shared secret using HMAC-SHA1. HOTP has been published through the IETF as RFC 4226. Built on top of HOTP is the time-based variant called TOTP, which requires a clock in the token. OATH do some other work too, like specifying a data format for transferring the token configuration data (e.g., serial number and shared secret) called PSKC.
Continue reading Introducing the OATH Toolkit→

GNU SASL with SCRAM-SHA-1-PLUS

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I have finished the SCRAM implementation in GNU SASL. The remaining feature to be added were support for the “enhanced” SCRAM-SHA-1-PLUS variant instead of just the normal SCRAM-SHA-1 mechanism. The difference is that the latter supports channel bindings to TLS, which makes it possible to detect man-in-the-middle attacks even if TLS is not used with server authentication. In GnuTLS we recently added an API for applications to extract channel bindings, which you will need to use in order to use SCRAM-SHA-1-PLUS. I announced the experimental version 1.5.4 release together with a writeup on how to test it. With this, our support for SCRAM should be complete.

GS2-KRB5 using GNU SASL and MIT Kerberos for Windows

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I have blogged about GNU SASL and GS2-KRB5 with the native Kerberos on Mac OS X before, so the next logical step has been to support GS2-KRB5 on Windows through MIT Kerberos for Windows (KfW). With the latest release of GNU SASL 1.5.2 I have added support for the KfW GSS-API library. There were several issues in completing this due to problems with KfW, but I won’t bore you with those details.

What is important is to demonstrate how GNU SASL can now talk IMAP authenticated with GS2-KRB5 using KfW on native Windows. Continue reading GS2-KRB5 using GNU SASL and MIT Kerberos for Windows→