----- Original Message ----- From: "Axel H Horns" <axel.h.horns@gmx.net> To: <cryptography@metzdowd.com> Sent: Wednesday, April 28, 2004 4:49 AM Subject: Can Skype be wiretapped by the authorities?
Is something known about the details of the crypto protocol within Skype? How reliable is the encryption?
See e.g.
http://www.financialcryptography.com/mt/archives/000076.html
Can Skype be wiretapped by the authorities? With collaboration of the Skype operator? Without?
What do you mean with "operator"? AFAIK, the system is fully peer-to-peer (http://www.skype.com/skype_p2pexplained.html ). Regarding the crypto, at http://www.skype.com/help_faq.html#Technical they say: What type of encryption is used? Skype uses AES (Advanced Encryption Standard) - also known as Rijndel - which is also used by U.S. Government organizations to protect sensitive, information. Skype uses 256-bit encryption, which has a total of 1.1 x 1077 possible keys, in order to actively encrypt the data in each Skype call or instant message. Skype uses 1536 to 2048 bit RSA to negotiate symmetric AES keys. User public keys are certified by Skype server at login. OK, so "Rijndael" is misspelled and the RSA-based key exchange does not provide forward secrecy, but apart from that it doesn't smell like snake oil. Not too bad, at least. BUT, unfortunately, the implementation is closed source, so there are no guarantees that the software is not GAKked. And yes, definitely an opensource (and multiplatform) alternative would be a cool thing to have. A message I posted a while ago to the list p2p-hackers was reposted by Eugene Leitl to cypherpunks (http://www.mail-archive.com/cypherpunks@minder.net/msg81814.html ) but the couple of followups it elicited didn't seem to center the issues I raised. I didn't reply then because I'm not a subscriber of cypherpunks any longer, so I'd like to take this occasion for doing it here now: Major Variola (ret) commented (indented lines, followed by my comment): [...]
Skype claims to use RSA-based key exchange, which is good for multi-party conferencing but does not preserve forward secrecy. Maybe some variant of ephemeral D-H authenticated by RSA signatures, with transparent renegotiation every time someone joins the conference, could do the job better.
RSA (ie persistant keys) may be an option but MUST NOT be required, for secrecy reasons as mentioned. (At worst RSA keys can be used once, then discarded. Lots of primes out there :-) Well, I don't see why RSA signatures (only for authentication of the key exchange) could endanger forward secrecy. Also, this is *voice*, ie biometric auth, so public-key-web-o-trust verislime scam is unnecessary at best. It's not only voice, it's also IM-style text chat. And even with voice, biometric authentication becomes awkward to use with conference calls. [...]
One could always implement a brand new network, using Distributed Hash Table algorithms such as Chord or Kademlia,
We don't give a flying fuck as to which shiny new algorithm you use, although were we a graph theory wonk, we might care. The issue here is that DHT algorithms allow to implement a fully distributed directory, which means one much more resistant to attacks (especially from institutional attackers) than implementations based on centralized servers (see, in a related fild, the different destinies of Napster and its distributed successors such as Overnet or the less efficient Gnutella). Still, a full search takes O(log(n)) steps, making them practical for implementing directory/presence services. [...]
but it would be much easier to rely from the very beginning upon a large number of nodes (at least for directory and presence functionality, if not for the reflectors which require specific UDP code).
What the NAT world (yawn) needs is free registry services exploitable by any protocol. Those NAT-users with RSA-clue can sign their registry entry. Not only that: NATted agents cannot be "called" unless they first register with some reflector on the open Internet. And centralized reflectors are, again, easy to attack, and also expensive to operate, as the bandwidth requirements are substantial (all the traffic flows through them): see e.g. John Walker's analysis of the reasons that led him to abandon SpeakFreely at http://www.fourmilab.ch/speakfree/ . Thomas Shaddack suggested to leverage on Jabber, but: 1. Jabber uses TCP as transport, and therefore can't be efficiently used as transport for telephony, i.e. using encapsulation of the voice packets in the Jabber protocol in order to traverse NAT devices. 2. Jabber is based on a client-server paradigm similar to e-mail. Running a Jabber server requires an always-on machine with its own domain name; and, although dynamic DNS can help, the model again tend to be hierarchical, easy to attack etc. That pretty much rules it out also for session initiation, directory/presence etc. The beauty of Skype, encryption aside, is that it's based on an overlay network solely based on P2P servents, relies (if their FAQ tells the truth) upon NO central registry for presence and directory services, and each client that runs non-NATted can transparently act as reflector supporting NATted users. Plus, all this (including, besides voice, text-based instant messaging) works with zero configuration with an idiotproof UI. Enzo --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to majordomo@metzdowd.com ----- End forwarded message ----- -- Eugen* Leitl <a href="http://leitl.org">leitl</a> ______________________________________________________________ ICBM: 48.07078, 11.61144 http://www.leitl.org 8B29F6BE: 099D 78BA 2FD3 B014 B08A 7779 75B0 2443 8B29 F6BE http://moleculardevices.org http://nanomachines.net [demime 1.01d removed an attachment of type application/pgp-signature]