Certificates for XMPP/Jabber

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I am revamping my XMPP server and I’ve written down notes on how to set up certificates to enable TLS.

I will run Debian Jessie with JabberD 2.x, using the recent jabberd2 jessie-backport. The choice of server software is not significant for the rest of this post.

Running XMPP over TLS is a good idea. So I need a X.509 PKI for this purpose. I don’t want to use a third-party Certificate Authority, since that gives them the ability to man-in-the-middle my XMPP connection. Therefor I want to create my own CA. I prefer tightly scoped (per-purpose or per-application) CAs, so I will set up a CA purely to issue certificates for my XMPP server.

The current XMPP specification, RFC 6120, includes a long section 13.7 that discuss requirements on Certificates.

One complication is the requirement to include an AIA for OCSP/CRLs — fortunately, it is not a strict “MUST” requirement but a weaker “SHOULD”. I note that checking revocation using OCSP and CRL is a “MUST” requirement for certificate validation — some specification language impedence mismatch at work there.

The specification demand that the CA certificate MUST have a keyUsage extension with the digitalSignature bit set. This feels odd to me, and I’m wondering if keyCertSign was intended instead. Nothing in the XMPP document, nor in any PKIX document as far as I am aware of, will verify that the digitalSignature bit is asserted in a CA certificate. Below I will assert both bits, since a CA needs the keyCertSign bit and the digitalSignature bit seems unnecessary but mostly harmless.

My XMPP/Jabber server will be “chat.sjd.se” and my JID will be “simon@josefsson.org”. This means the server certificate need to include references to both these domains. The relevant DNS records for the “josefsson.org” zone is as follows, see section 3.2.1 of RFC 6120 for more background.

_xmpp-client._tcp.josefsson.org.	IN	SRV 5 0 5222 chat.sjd.se.
_xmpp-server._tcp.josefsson.org.	IN	SRV 5 0 5269 chat.sjd.se.

The DNS records or the “sjd.se” zone is as follows:

chat.sjd.se.	IN	A	...
chat.sjd.se.	IN	AAAA	...

The following commands will generate the private key and certificate for the CA. In a production environment, you would keep the CA private key in a protected offline environment. I’m asserting a expiration date ~30 years in the future. While I dislike arbitrary limits, I believe this will be many times longer than the anticipated lifelength of this setup.

openssl genrsa -out josefsson-org-xmpp-ca-key.pem 3744
cat > josefsson-org-xmpp-ca-crt.conf << EOF
[ req ]
x509_extensions = v3_ca
distinguished_name = req_distinguished_name
prompt = no
[ req_distinguished_name ]
CN=XMPP CA for josefsson.org
[ v3_ca ]
subjectKeyIdentifier=hash
basicConstraints = CA:true
keyUsage=critical, digitalSignature, keyCertSign
EOF
openssl req -x509 -set_serial 1 -new -days 11147 -sha256 -config josefsson-org-xmpp-ca-crt.conf -key josefsson-org-xmpp-ca-key.pem -out josefsson-org-xmpp-ca-crt.pem

Let’s generate the private key and server certificate for the XMPP server. The wiki page on XMPP certificates is outdated wrt PKIX extensions. I will embed a SRV-ID field, as discussed in RFC 6120 section 13.7.1.2.1 and RFC 4985. I chose to skip the XmppAddr identifier type, even though the specification is somewhat unclear about it: section 13.7.1.2.1 says that it “is no longer encouraged in certificates issued by certification authorities” while section 13.7.1.4 says “Use of the ‘id-on-xmppAddr’ format is RECOMMENDED in the generation of certificates”. The latter quote should probably have been qualified to say “client certificates” rather than “certificates”, since the latter can refer to both client and server certificates.

Note the use of a default expiration time of one month: I believe in frequent renewal of entity certificates, rather than use of revocation mechanisms.

openssl genrsa -out josefsson-org-xmpp-server-key.pem 3744
cat > josefsson-org-xmpp-server-csr.conf << EOF
[ req ]
distinguished_name = req_distinguished_name
prompt = no
[ req_distinguished_name ]
CN=XMPP server for josefsson.org
EOF
openssl req -sha256 -new -config josefsson-org-xmpp-server-csr.conf -key josefsson-org-xmpp-server-key.pem -nodes -out josefsson-org-xmpp-server-csr.pem
cat > josefsson-org-xmpp-server-crt.conf << EOF
subjectAltName=@san
[san]
DNS=chat.sjd.se
otherName.0=1.3.6.1.5.5.7.8.7;UTF8:_xmpp-server.josefsson.org
otherName.1=1.3.6.1.5.5.7.8.7;UTF8:_xmpp-client.josefsson.org
EOF
openssl x509 -sha256 -CA josefsson-org-xmpp-ca-crt.pem -CAkey josefsson-org-xmpp-ca-key.pem -set_serial 2 -req -in josefsson-org-xmpp-server-csr.pem -out josefsson-org-xmpp-server-crt.pem -extfile josefsson-org-xmpp-server-crt.conf

With this setup, my XMPP server can be tested by the XMPP IM Observatory. You can see the c2s test results and the s2s test results. Of course, there are warnings regarding the trust anchor issue. It complains about a self-signed certificate in the chain. This is permitted but not recommended — however when the trust anchor is not widely known, I find it useful to include it. This allows people to have a mechanism of fetching the trust anchor certificate should they want to. Some weaker cipher suites trigger warnings, which is more of a jabberd2 configuration issue and/or a concern with jabberd2 defaults.

My jabberd2 configuration is simple — in c2s.xml I add a <id> entity with the “require-starttls”, “cachain”, and “pemfile” fields. In s2s.xml, I have the <pemfile>, <resolve-ipv6>, and <require-tls> entities.

Some final words are in order. While this setup will result in use of TLS for XMPP connections (c2s and s2s), other servers are unlikely to find my CA trust anchor, let alone be able to trust it for verifying my server certificate. I’m happy to read about Peter Saint-Andre’s recent SSL/TLS work, and in particular I will follow the POSH effort.

Laptop decision fatigue

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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

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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.

EdDSA and Ed25519 goes to IETF

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After meeting Niels Möller at FOSDEM and learning about his Ed25519 implementation in GNU Nettle, I started working on a simple-to-implement description of Ed25519. The goal is to help implementers of various IETF (and non-IETF) protocols add support for Ed25519. As many are aware, OpenSSH and GnuPG has support for Ed25519 in recent versions, and OpenBSD since the v5.5 May 2014 release are signed with Ed25519. The paper describing EdDSA and Ed25519 is not aimed towards implementers, and does not include test vectors. I felt there were room for improvement to get wider and more accepted adoption.

Our work is published in the IETF as draft-josefsson-eddsa-ed25519 and we are soliciting feedback from implementers and others. Please help us iron out the mistakes in the document, and point out what is missing. For example, what could be done to help implementers avoid side-channel leakage? I don’t think the draft is the place for optimized and side-channel free implementations, and it is also not the place for a comprehensive tutorial on side-channel free programming. But maybe there is a middle ground where we can say something more than what we can do today. Ideas welcome!

Laptop Buying Advice?

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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?