German police have hired a company to create Trojans capable of capturing traffic from Skype and SSL, leaked documents appear to show.
The two scanned documents , which appear on the Wikileaks website in their German form, are difficult to verify, but one appears to describe how a security company, Digitask, was asked to create a “Skype Capture Unit” based around Trojans planted on targeted PCs covertly transferring data to a remote server.
“As requested by you, we hereby submit an offer for a surveillance method of the encrypted VoIP protocol Skype,” says the company hired to carry out the hacking.
The other document relates to an apparent exchange between the Bavarian police, the Ministry of Justice and the Prosecution office, on how to divide up the costs of this system. According to Wikileaks, this originally appeared on German website Piratenpartei, but was removed, speculating that this could be due to legal threats.
The description of the Skype Capture Unit is dated 4 September 2007 with a ‘delivery timescale’ for a targeted system in weeks, implying that the system could have been operational for some time.
The precise capabilities of the system are hard to gauge, but its creators appear confident of being able to capture Skype's voice, chat and video sessions, and possibly also hacking SSL. Since both are encrypted, it is likely that the data streams are first copied to remote servers first before being decrypted once the key used to scramble them has been recovered.
“Encryption of communication via Skype poses a problem for surveillance of telecommunications. All traffic generated by Skype can be captured when watching a Dial-in- or DSL link, but it cannot be decrypted. The encryption of Skype works via AES wih a 256-Bit key. The symmetric AES keys are negotiated via RSA keys. The public keys of the users are confirmed by the Skype-Login-Server when logging in. To watch Skype-communication it thus becomes necessary to realise other approaches than standard telecommunications surveillance,” the document explains.
How Digitask intended to get the Trojans on to targeted PCs without breaking German law is not explained.
This is no cheap hack for nosy policemen – each successfully compromised machine would net Digitask 3,500 euros per month for at least three months. SSL interception is said to cost 2,500 euros.
The interest German authorities have in hacking Skype has known about for some time. In November 2007 – after the time period covered by leaked documents – a source in the German police suggested that they had been unable to crack Skype’s encryption.
Unencrypted VoIP calls are a very different matter. Also in November, VoIP expert Peter Cox created a VoIP-hacking proof-of-concept program called SIPtap, which does exactly what the Skype Capture Unit claims to do, minus the ability to do this for encrypted Skype calls.