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16 Mar 2012

Samsung Galaxy SII WiFi Calling: A Quick Look

After switching from my venerable Nexus One to a new Samsung Galaxy SII (SGS2) from T-Mobile, I was intrigued to discover that it has a fairly neat WiFi calling ability. This feature lets the phone use a wireless IP access point to place calls, in lieu of the normal cellular data network. On one hand it’s a bit of a ripoff – even though you’re using your own Internet rather than T-Mobile’s valuable spectrum, they still use up your minutes at the same rate; however, it’s nice if you travel to a place with crummy cell service but decent wireless Internet.

When the feature is enabled, the phone will switch preferentially to WiFi for all calls, once it pairs to an access point. (It can be disabled if you’d prefer it to not do this.) There are still some very rough edges: the biggest issue is that there’s no handoff, so if you place a call over WiFi and then walk out of range of the AP, the call drops. Whoops.

I was curious how the calls were actually handled on the wire, and in particular how secure things were. To this end, I decided to run a quick Wireshark analysis on the phone, while it was connected to my home WiFi AP.

The setup for this is pretty trivial, and out of scope of this entry; basically you just need to find a way to get the packets going to and coming from the phone to be copied to a machine where you can run Wireshark or tcpdump. You can do this with an Ethernet hub (the old-school method), via the router’s configuration, or even via ARP spoofing.

With Wireshark running and looking at the phone’s traffic, I performed a few routine tasks to see what leaked. The tl;dr version of all of this? In general, Android apps were very good about using TLS. There wasn’t a ton of leakage to a would-be interceptor.

Just for background: Gmail and Twitter both kept almost everything (except for a few generic logo images in Twitter’s case) wrapped in TLS.

Facebook kept pretty much everything encrypted, except for other users’ profile images, which it sent in the clear. This isn’t a huge issue, but it does represent minor leakage; the reason for this seems to be that Facebook keeps the images cached on a CDN, and the CDN servers don’t do SSL, apparently. I’m not sure what sort of nastiness or attacks this opens up, if any (perhaps social engineering stuff, if a motivated attacker could recover your friends list), but it’s worth noting and keeping in mind.

I next confirmed that text messages (SMSes) aren’t sent in the clear. They are not, although I’m not 100% sure they’re even sent over the data connection – it’s hard to tell, among the SIP keepalives, whether a SMS went out via the WiFi connection, or if the phone used the actual cell-data connection instead. Sometime when I’m in a location without any GSM coverage but with WiFi, I’ll have to test it and confirm.

Last, I made a quick call. This is what I was most interested in, since encrypted SIP is surprisingly uncommon – most corporate telephony systems don’t support it, at least not that I’ve seen or worked with. It wouldn’t have surprised me much at all if the SIP connection itself was all in the clear. However, that doesn’t seem to be the case. The call begins with a sip-tls handshake, and then there are lots of UDP packets, all presumably encrypted with a session key negotiated during the handshake. At any rate, Wireshark’s built-in analysis tools weren’t able to recover anything, so calls are not script-kiddie vulnerable. Still, I’m curious about what sort of certificate validation is done on the client side, and how the phone will react to forged SSL certs or attempts by a MITM to downgrade the connection.

Certainly lots of room for further experiments, but overall I’m relieved to see that the implementation isn’t obviously insecure or vulnerable to trivial packet sniffing.

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