Let’s Encrypt Clients

As many others, I have been following the launch of Let’s Encrypt. Let’s Encrypt is a new zero-cost X.509 Certificate Authority that supports the Automated Certificate Management Environment (ACME) protocol. ACME allow you to automate creation and retrieval of HTTPS server certificates. As anyone who has maintained a number of HTTPS servers can attest, this process has unfortunately been manual, error-prone and differ between CAs.

On some of my personal domains, such as this blog.josefsson.org, I have been using the CACert authority to sign the HTTPS server certificate. The problem with CACert is that the CACert trust anchors aren’t shipped with sufficient many operating systems and web browsers. The user experience is similar to reaching a self-signed server certificate. For organization-internal servers that you don’t want to trust external parties for, I continue to believe that running your own CA and distributing it to your users is better than using a public CA (compare my XMPP server certificate setup). But for public servers, availability without prior configuration is more important. Therefor I decided that my public HTTPS servers should use a CA/Browser Forum-approved CA with support for ACME, and as long as Let’s Encrypt is trustworthy and zero-cost, they are a good choice.

I was in need of a free software ACME client, and set out to research what’s out there. Unfortunately, I did not find any web pages that listed the available options and compared them. The Let’s Encrypt CA points to the “official” Let’s Encrypt client, written by Jakub Warmuz, James Kasten, Peter Eckersley and several others. The manual contain pointers to two other clients in a seamingly unrelated section. Those clients are letsencrypt-nosudo by Daniel Roesler et al, and simp_le by (again!) Jakub Warmuz. From the letsencrypt.org’s client-dev mailing list I also found letsencrypt.sh by Gerhard Heift and LetsEncryptShell by Jan Mojžíš. Is anyone aware of other ACME clients?

By comparing these clients, I learned what I did not like in them. I wanted something small so that I can audit it. I want something that doesn’t require root access. Preferably, it should be able to run on my laptop, since I wasn’t ready to run something on the servers. Generally, it has to be Secure, which implies something about how it approaches private key handling. The letsencrypt official client can do everything, and has plugin for various server software to automate the ACME negotiation. All the cryptographic operations appear to be hidden inside the client, which usually means it is not flexible. I really did not like how it was designed, it looks like your typical monolithic proof-of-concept design. The simp_le client looked much cleaner, and gave me a good feeling. The letsencrypt.sh client is simple and written in /bin/sh shell script, but it appeared a bit too simplistic. The LetsEncryptShell looked decent, but I wanted something more automated.

What all of these clients did not have, and that letsencrypt-nosudo client had, was the ability to let me do the private-key operations. All the operations are done interactively on the command-line using OpenSSL. This would allow me to put the ACME user private key, and the HTTPS private key, on a YubiKey, using its PIV applet and techniques similar to what I used to create my SSH host CA. While the HTTPS private key has to be available on the HTTPS server (used to setup TLS connections), I wouldn’t want the ACME user private key to be available there. Similarily, I wouldn’t want to have the ACME or the HTTPS private key on my laptop. The letsencrypt-nosudo tool is otherwise more rough around the edges than the more cleaner simp_le client. However the private key handling aspect was the deciding matter for me.

After fixing some hard-coded limitations on RSA key sizes, getting the cert was as simple as following the letsencrypt-nosudo instructions. I’ll follow up with a later post describing how to put the ACME user private key and the HTTPS server certificate private key on a YubiKey and how to use that with letsencrypt-nosudo.

So you can now enjoy browsing my blog over HTTPS! Thank you Let’s Encrypt!

Automatic Replicant Backup over USB using rsync

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!

Combining Dnsmasq and Unbound

For my home office network I have been using Dnsmasq for some time. Dnsmasq provides me with DNS, DHCP, DHCPv6, and IPv6 Router Advertisement. I run dnsmasq on a Debian Jessie server, but it works similar with OpenWRT if you want to use a smaller device. My entire /etc/dnsmasq.d/local configuration used to look like this:

dhcp-authoritative
interface=eth1
read-ethers
dhcp-range=192.168.1.100,192.168.1.150,12h
dhcp-range=2001:9b0:104:42::100,2001:9b0:104:42::1500
dhcp-option=option6:dns-server,[::]
enable-ra

Here dhcp-authoritative enable DHCP. interface=eth1 says to listen on eth1 only, which is my internal (IPv4 NAT) network. I try to keep track of the MAC address of all my devices in a /etc/ethers file, so I use read-ethers to have dnsmasq give stable IP addresses for them. The dhcp-range is used to enable DHCP and DHCPv6 on my internal network. The dhcp-option=option6:dns-server,[::] statement is needed to inform the DHCP clients of the DNS resolver’s IPv6 address, otherwise they would only get the IPv4 DNS server address. The enable-ra parameter enables IPv6 router advertisement on the internal network, thereby removing the need to run radvd too — useful since I prefer to use copyleft software.

Recently I had a desire to use DNSSEC, and enabled it in Dnsmasq using the following statements:

dnssec
trust-anchor=.,19036,8,2,49AAC11D7B6F6446702E54A1607371607A1A41855200FD2CE1CDDE32F24E8FB5
dnssec-check-unsigned

The dnssec keyword enable DNSSEC validation in dnsmasq, using the indicated trust-anchor (get the root-anchors from IANA). The dnssec-check-unsigned deserves some more discussion. The dnsmasq manpage describes it as follows:

As a default, dnsmasq does not check that unsigned DNS replies are legitimate: they are assumed to be valid and passed on (without the “authentic data” bit set, of course). This does not protect against an attacker forging unsigned replies for signed DNS zones, but it is fast. If this flag is set, dnsmasq will check the zones of unsigned replies, to ensure that unsigned replies are allowed in those zones. The cost of this is more upstream queries and slower performance.

For example, this means that dnsmasq’s DNSSEC functionality is not secure against active man-in-the-middle attacks between dnsmasq and the DNS server it is using. Even if example.org used DNSSEC properly, an attacker could fake that it was unsigned to dnsmasq, and I would get potentially incorrect values in return. We all know that the Internet is not a secure place, and your threat model should include active attackers. I believe this mode should be the default in dnsmasq, and users should have to configure dnsmasq to not be in that mode if they really want to (with the obvious security warning).

Running with this enabled for a couple of days resulted in frustration about not being able to reach a couple of domains. The behaviour was that my clients would hang indefinitely or get a SERVFAIL, both resulting in lack of service. You can enable query logging in dnsmasq with log-queries and enabling this I noticed three distinct form of error patterns:

jow13gw dnsmasq 460 - -  forwarded www.fritidsresor.se to 213.80.101.3
jow13gw dnsmasq 460 - -  validation result is BOGUS
jow13gw dnsmasq 547 - -  reply cloudflare-dnssec.net is BOGUS DNSKEY
jow13gw dnsmasq 547 - -  validation result is BOGUS
jow13gw dnsmasq 547 - -  reply linux.conf.au is BOGUS DS
jow13gw dnsmasq 547 - -  validation result is BOGUS

The first only happened intermittently, the second did not cause any noticeable problem, and the final one was reproducible. To be fair, I only found the last example after starting to search for problem reports (see post confirming bug).

At this point, I had a confirmed bug in dnsmasq that affect my use-case. I want to use official packages from Debian on this machine, so installing newer versions manually is not an option. So I started to look into alternatives for DNS resolving, and quickly found Unbound. Installing it was easy:

apt-get install unbound
unbound-control-setup 

I created a local configuration file in /etc/unbound/unbound.conf.d/local.conf as follows:

server:
	interface: 127.0.0.1
	interface: ::1
	interface: 192.168.1.2
	interface: 2001:9b0:104:42::2
	access-control: 127.0.0.1 allow
	access-control: ::1 allow
	access-control: 192.168.1.2/24 allow
	access-control: 2001:9b0:104:42::2/64 allow
	outgoing-interface: 155.4.17.2
	outgoing-interface: 2001:9b0:1:1a04::2
#	log-queries: yes
#	verbosity: 2

The interface keyword determine which IP addresses to listen on, here I used the loopback interface and the local address of the physical network interface for my internal network. The access-control allows recursive DNS resolving from those networks. And outgoing-interface specify my external Internet-connected interface. log-queries and/or verbosity are useful for debugging.

To make things work, dnsmasq has to stop providing DNS services. This can be achieved with the port=0 keyword, however that will also disable informing DHCP clients about the DNS server to use. So this has to be added in manually. I ended up adding the two following lines to /etc/dnsmasq.d/local:

port=0
dhcp-option=option:dns-server,192.168.1.2

Restarting unbound and dnsmasq now leads to working (and secure) internal DNSSEC-aware name resolution over both IPv4 and IPv6. I can verify that resolution works, and that Unbound verify signatures and reject bad domains properly with dig as below, or use online DNSSEC resolver test page although I’m not sure how confident you can be in the result from that page.

$ host linux.conf.au
linux.conf.au has address 192.55.98.190
linux.conf.au mail is handled by 1 linux.org.au.
$ host sigfail.verteiltesysteme.net
;; connection timed out; no servers could be reached
$ 

I use Munin to monitor my services, and I was happy to find a nice Unbound Munin plugin. I installed the file in /usr/share/munin/plugins/ and created a Munin plugin configuration file /etc/munin/plugin-conf.d/unbound as follows:

[unbound*]
user root
env.statefile /var/lib/munin-node/plugin-state/root/unbound.state
env.unbound_conf /etc/unbound/unbound.conf
env.unbound_control /usr/sbin/unbound-control
env.spoof_warn 1000
env.spoof_crit 100000

I run munin-node-configure --shell|sh to enable it. To work unbound has to be configured as well, so I create a /etc/unbound/unbound.conf.d/munin.conf as follows.

server:
	extended-statistics: yes
	statistics-cumulative: no
	statistics-interval: 0
remote-control:
	control-enable: yes

The graphs may be viewed at my munin instance.

Laptop decision fatigue

I admit defeat. I have made some effort into researching recent laptop models (see first and second post). Last week I asked myself what the biggest problem with my current 4+ year old X201 is. I couldn’t articulate any significant concern. So I have bought another second-hand X201 for semi-permanent use at my second office. At ~225 USD/EUR, including another docking station, it is an amazing value. I considered the X220-X240 but they have a different docking station, and were roughly twice the price — the latter allowed for a Samsung 850 PRO SSD purchase. Thanks everyone for your advice, anyway!

Laptop indecision

I wrote last month about buying a new laptop and I still haven’t made a decision. One reason for this is because Dell doesn’t seem to be shipping the E7250. Some online shops claim to be able to deliver it, but aren’t clear on what configuration it has – and I really don’t want to end up with Dell Wifi.

Another issue has been the graphic issues with the Broadwell GPU (see the comment section of my last post). It seems unlikely that this will be fixed in time for Debian Jessie. I really want a stable OS on this machine, as it will be a work-horse and not a toy machine. I haven’t made up my mind whether the graphics issue is a deal-breaker for me.

Meanwhile, a couple of more sub-1.5kg (sub-3.3lbs) Broadwell i7’s have hit the market. Some of these models were suggested in comments to my last post. I have decided that the 5500U CPU would also be acceptable to me, because some newer laptops doesn’t come with the 5600U. The difference is that the 5500U is a bit slower (say 5-10%) and lacks vPro, which I have no need for and mostly consider a security risk. I’m not aware of any other feature differences.

Since the last round, I have tightened my weight requirement to be sub-1.4kg (sub-3lbs), which excludes some recently introduced models, and actually excludes most of the models I looked at before (X250, X1 Carbon, HP 1040/810). Since I’m leaning towards the E7250, with the X250 as a “reliable” fallback option, I wanted to cut down on the number of further models to consider. Weigth is a simple distinguisher. The 1.4-1.5kg (3-3.3lbs) models I am aware that of that is excluded are the Asus Zenbook UX303LN, the HP Spectre X360, and the Acer TravelMate P645.

The Acer Aspire S7-393 (1.3kg) and Toshiba Kira-107 (1.26kg) would have been options if they had RJ45 ports. They may be interesting to consider for others.

The new models I am aware of are below. I’m including the E7250 and X250 for comparison, since they are my preferred choices from the first round. A column for maximum RAM is added too, since this may be a deciding factor for me. Higher weigth is with touch screens.

Toshiba Z30-B 1.2-1.34kg 16GB 13.3″ 1920×1080
Fujitsu Lifebook S935 1.24-1.36kg 12GB 13.3″ 1920×1080
HP EliteBook 820 G2 1.34-1.52kg 16GB 12.5″ 1920×1080
Dell Latitude E7250 1.25kg 8/16GB? 12.5″ 1366×768
Lenovo X250 1.42kg 8GB 12.5″ 1366×768

It appears unclear whether the E7250 is memory upgradeable, some sites say max 8GB some say max 16GB. The X250 and 820 has DisplayPort, the S935 and Z30-B has HDMI, and the E7250 has both DisplayPort/HDMI. The E7250 does not have VGA which the rest has. All of them have 3 USB 3.0 ports except for X250 that only has 2 ports. The E7250 and 820 claims NFC support, but Debian support is not given. Interestingly, all of them have a smartcard reader. All support SDXC memory cards.

The S935 has an interesting modular bay which can actually fit a CD reader or an additional battery. There is a detailed QuickSpec PDF for the HP 820 G2, haven’t found similar detailed information for the other models. It mentions support for Ubuntu, which is nice.

Comparing these laptops is really just academic until I have decided what to think about the Broadwell GPU issues. It may be that I’ll go back to a fourth-gen i7 laptop, and then I’ll probably pick a cheap reliable machine such as the X240.

Laptop Buying Advice?

My current Lenovo X201 laptop has been with me for over four years. I’ve been looking at new laptop models over the years thinking that I should upgrade. Every time, after checking performance numbers, I’ve always reached the conclusion that it is not worth it. The most performant Intel Broadwell processor is the the Core i7 5600U and it is only about 1.5 times the performance of my current Intel Core i7 620M. Meanwhile disk performance has increased more rapidly, but changing the disk on a laptop is usually simple. Two years ago I upgraded to the Samsung 840 Pro 256GB disk, and this year I swapped that for the Samsung 850 Pro 1TB, and both have been good investments.

Recently my laptop usage patterns have changed slightly, and instead of carrying one laptop around, I have decided to aim for multiple semi-permanent laptops at different locations, coupled with a mobile device that right now is just my phone. The X201 will remain one of my normal work machines.

What remains is to decide on a new laptop, and there begins the fun. My requirements are relatively easy to summarize. The laptop will run a GNU/Linux distribution like Debian, so it has to work well with it. I’ve decided that my preferred CPU is the Intel Core i7 5600U. The screen size, keyboard and mouse is mostly irrelevant as I never work longer periods of time directly on the laptop. Even though the laptop will be semi-permanent, I know there will be times when I take it with me. Thus it has to be as lightweight as possible. If there would be significant advantages in going with a heavier laptop, I might reconsider this, but as far as I can see the only advantage with a heavier machine is bigger/better screen, keyboard (all of which I find irrelevant) and maximum memory capacity (which I would find useful, but not enough of an argument for me). The sub-1.5kg laptops with the 5600U CPU on the market that I have found are:

Lenovo X250 1.42kg 12.5″ 1366×768
Lenovo X1 Carbon (3rd gen) 1.34kg 14″ 2560×1440
Dell Latitude E7250 1.25kg 12.5″ 1366×768
Dell XPS 13 1.26kg 13.3″ 3200×1800
HP EliteBook Folio 1040 G2 1.49kg 14″ 1920×1080
HP EliteBook Revolve 810 G3 1.4kg 11.6″ 1366×768

I find it interesting that Lenovo, Dell and HP each have two models that meets my 5600U/sub-1.5kg criteria. Regarding screen, possibly there exists models with other screen resolutions. The XPS 13, HP 810 and X1 models I looked had touch screens, the others did not. As screen is not important to me, I didn’t evaluate this further.

I think all of them would suffice, and there are only subtle differences. All except the XPS 13 can be connected to peripherals using one cable, which I find convenient to avoid a cable mess. All of them have DisplayPort, but HP uses DisplayPort Standard and the rest uses miniDP. The E7250 and X1 have HDMI output. The X250 boosts a 15-pin VGA connector, none of the others have it — I’m not sure if that is a advantage or disadvantage these days. All of them have 2 USB v3.0 ports except the E7250 which has 3 ports. The HP 1040, XPS 13 and X1 Carbon do not have RJ45 Ethernet connectors, which is a significant disadvantage to me. Ironically, only the smallest one of these, the HP 810, can be memory upgraded to 12GB with the others being stuck at 8GB. HP and the E7250 supports NFC, although Debian support is not certain. The E7250 and X250 have a smartcard reader, and again, Debian support is not certain. The X1, X250 and 810 have a 3G/4G card.

Right now, I’m leaning towards rejecting the XPS 13, X1 and HP 1040 because of lack of RJ45 ethernet port. That leaves me with the E7250, X250 and the 810. Of these, the E7250 seems like the winner: lightest, 1 extra USB port, HDMI, NFC, SmartCard-reader. However, it has no 3G/4G-card and no memory upgrade options. Looking for compatibility problems, it seems you have to be careful to not end up with the “Dell Wireless” card and the E7250 appears to come in a docking and non-docking variant but I’m not sure what that means.

Are there other models I should consider? Other thoughts?

Portable Symmetric Key Container (PSKC) Library

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.

Using OATH Toolkit with Dropbox

Today there was an announcement that Dropbox supports two-factor authentication. On their page with detailed instructions there is (at the bottom) a link to the man page of the OATH Toolkit command line utility oathtool. OATH Toolkit is available in Ubuntu 12.04 and Debian Wheezy. (Note that the 1.10.4 version in Ubuntu does not support the base32 features.) There is not a lot of details in the documentation on Dropbox’s site on how to use oathtool, but I have experimented a bit with the setup and I’d like to share my findings. I assume you are somewhat familiar with the OATH Toolkit; if not I suggest reading my earlier introduction to OATH Toolkit.

To use OATH Toolkit’s command line utility to generate OTPs that are accepted by Dropbox, here is how you proceed. When you enable two-factor authentication on Dropbox’s site, you must select “Use a mobile app” and on the next screen with the QR code image, click the “enter your secret key manually” link. You will then be presented with a code that looks like this: gr6d 5br7 25s6 vnck v4vl hlao re

Now this code is actually space-delimitted base32 encoded data, without any padding. Since version 1.12.0, oathtool can read base32 encoded keys. However, parsing the raw string fails, so to make it work, you need to remove the spaces and add padding characters. I have yet to see any documentation on the Dropbox implementation, but I assume they always generate 16 binary octets that are base32 encoded into 26 characters like the codes that I have seen. The code is the cryptographic key used for the HMAC-SHA1 computation described in the RFC 6238 that specify OATH TOTP. If you study the base32 encoding you discover that 26 characters needs six pad characters. So converted into proper base32, the string would be gr6d5br725s6vnckv4vlhlaore======. Now generating OTPs are easy, see below.

jas@latte:~$ oathtool --verbose --totp --base32 "gr6d5br725s6vnckv4vlhlaore======"
Hex secret: 347c3e863fd765eab44aaf2ab3ac0e89
Base32 secret: GR6D5BR725S6VNCKV4VLHLAORE======
Digits: 6
Window size: 0
Step size (seconds): 30
Start time: 1970-01-01 00:00:00 UTC (0)
Current time: 2012-08-27 21:22:54 UTC (1346102574)
Counter: 0x2ACA9C5 (44870085)

125860
jas@latte:~$

Dropbox’s implementation is robust in that it requests a valid OTP from me, generated using the secret they just displayed, before proceeding. This verifies that the user was able to import the key correctly, and that the users’ OATH TOTP implementation appears to work. If I type in the OTP generated from oathtool this way, it allowed me to enable two-factor authentication and I agreed. From that point, signing into the Dropbox service will require a OTP. I invoke the tool, using the same arguments as above, and the tool will use the current time to compute a fresh OTP.

Reflecting on how things could work smoother, I suppose oathtool could be more permissive when it performs the base32 decoding so that the user doesn’t have to fix the base32 spacing/padding manually. I’ll consider this for future releases.

Small syslog server

My home network has several devices that do not have large persistent storage to keep log files. For example, my wireless routers based on OpenWRT doesn’t log to the limited local storage it has, and a Flukso energy metering device log power readings to a ramdisk. These devices log a fair amount of information that I ideally would like to keep for later analysis. I have never before seen a need to setup a syslogd server, thinking that storing logs locally and keeping regular backups of the machine is good enough. However, it appears like this situation calls for a syslogd server. I found an old NSLU2 in my drawer and installed Debian Squeeze on it following Martin Michlmayr’s instructions. I’m using a 4GB USB memory stick for storage, which should hold plenty of log data. I keep backups of the machine in case the USB memory stick wears out.

After customizing the installation to my preferences (disable ssh passwords, disable portmap/rpc.statd/exim4, installing etckeeper, emacs23-nox, etc) I am ready to configure Rsyslog. I found what looked like the perfect configuration example, “Storing messages from a remote system into a specific file”, but it requires me to hard code a bit too much information in the configuration file for my taste. Instead, I found the DynFile concept. With a file /etc/rsyslogd.d/logger.conf as below I can point any new device to my log server and it will automatically create a new file for it. And since the dates are embedded into the filename, I get log rotation suitable for rsync-style backups for free.

$ModLoad imudp
$UDPServerRun 514

$ModLoad imtcp
$InputTCPServerRun 514

$template DynFile,”/var/log/network-%HOSTNAME%-%$year%-%$month%-%$day%.log”
:fromhost-ip, !isequal, “127.0.0.1” ?DynFile
:fromhost-ip, !isequal, “127.0.0.1” ~

After this, I get log files written to /var/log/network-IP-YEAR-MONTH-DAY.log. For example:

pepparkaka:~# tail /var/log/network-192.168.1.47-2012-03-20.log 
Mar 20 13:40:21 192.168.1.47 avahi-daemon[1508]: Registering new address record for 192.168.1.47 on br-lan.IPv4.
Mar 20 13:40:21 192.168.1.47 avahi-daemon[1508]: Registering HINFO record with values 'MIPS'/'LINUX'.
Mar 20 13:40:21 192.168.1.47 sysinit: setting up led WAN LED (green)
Mar 20 13:40:21 192.168.1.47 kernel: ar71xx-wdt: enabling watchdog timer

Use uci to configure the OpenWRT boxes to send log messages to this server:

uci set system.@system[0].log_ip=192.168.1.51
uci commit

Update! By default rsylog performs reverse lookups of incoming requests. This easily causes problems in case your DNS server is unreachable. Rsyslogd appears to have a long timeout for DNS queries, so if you expect incoming log messages to end up in the log when they are sent, think again. In my testing, it can take minutes until they end up in the log. For me, reverse DNS lookups does not add anything of value. To disable DNS lookups, make sure rsyslogd is invoked with the ‘-x’ parameter. On Debian, this is done by adding ‘-x’ to /etc/defaults/rsyslog like this:

RSYSLOGD_OPTIONS=”-c4 -x”

Unattended SSH with Smartcard

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