[smb at cs.columbia.edu: Skype security evaluation]

[Travis H.]

That's a fairly interesting review, and Skype should be commended for
hiring someone to do it.  I hope to see more evaluations from vendors
in the future.

However, I have a couple of suggestions.

My understanding of the peer-to-peer key agreement protocol (hereafter
p2pka) is based on section 3.3 and 3.4.2 and is something like this:

A -> B: N_ab
B -> A: N_ba
B -> A: Sign{f(N_ab)}_a
A -> B: Sign{f(N_ba)}_b
A -> B: Sign{A, K_a}_SKYPE
B -> A: Sign{B, K_b}_SKYPE
A -> B: Sign{R_a}_a
B -> A: Sign{R_b}_b

Session key SK_AB = g(R_a, R_b)

0) The p2pka allows us to use a peer as a signing oracle for nonces by
performing steps 1 through 4.  Only the one-wayness of f (specified
only as "modified in a standard way") stands in the way of arbitrary
forgery, which would allow us to bypass the security on steps 3, 4, 7,
and 8.  It would not stop us from knowing the session key, since there
is no restriction on the form of R_a or R_b.

1) It's not clear that the identity certificates are bound to a
[externally visible] network [source] address at registration time. 
IMHO, this would be a good idea.

2) He implicitly ignores the fact that the skype key is a trusted CA,
so skype can impersonate anyone (or delegate that impersonation by
signing a bogus ID).  This is obvious to a cryptographer but should be
mentioned for the layperson.  An evaluation should explicitly specify
who must be trusted by whom, and everyone must trust the Skype
registrar.

3) It looks like the peer-to-peer communication involves the same key,
SK_AB, in both directions, opening the door for keystream re-use, but
there's 64 bits of presumably random salt so it shouldn't be very
common.

Vagueness:

1) They use an unencrypted 2-byte CRC on each packet between peers. 
Undetected modification to a packet is possible, since the CRC is
computed over the encrypted data and stored en clair.  In this case,
arbitrary bits can be flipped, the CRC recomputed, and no future
packets depend on the current packet, so there's no tell-tale garbling
afterwards like there is in most other block modes.  He alludes to
this in section 3.4.4 but doesn't really specify the impact, merely
compares it to WEP.

2) The session established with the Skype server during registration
is protected with a 256-bit key, which is random, but he doesn't say
how the client and Skype agree on it.

3) It's not clear why they used rc4 instead of ICM to generate key
material, but at least it's not being used for confidentiality.

4) The details of the random number generation are vague ("makes a
number of win32 calls").

5) The details of the SK_AB key composition are vague ("combined in a
cryptographically-sound way"), shown by g in the p2pka above.

6) It doesn't say who sends the nonces first --- is it the recipient
of the connection, or the initiator?  Can we DoS people by repeated
connections triggering digital signatures?

7) It doesn't say whether it's a TCP or UDP protocol, what ports it
uses, etc.  I'm curious if it will work through NAT at both ends.

8) The skype server's timeout on login passwords can be used for a
denial-of-service against the registration protocol and doesn't affect
username guessing (fixed password variable username, a/k/a "reverse
hack").

9) It doesn't specify how the salts used in ICM mode are communicated.

10) It doesn't specify how streams are created and numbered.

It'd be nice to see the protocol clearly specified and analyzed via
automated means (finite state analysis via murphy, etc.).

Obsession with performance:

He makes no fewer than six comments about performance (of the AES
code, of the modular exponentiation, of the primality testing, of
modular inversion, of multi-precision arithmetic libraries, and SHA-1
implementation), which should normally be the least of anyone's
worries, especially cryptographers.  Is this is a security evaluation,
or a performance test?

However, since we're talking about real-time audio streams, perhaps
some discussion of the bandwidth and especially latency of the p2p
protocol would be in order.  Unfortunately, there's no quantification
("... performs favorably in terms of clock cycle per encryption").

Trust us:

Finally, the whole thing is closed source, so none of it is easily
verifiable.  We just have to take his word on it, and often he just
offers opinions (see the complaints of vagueness above).

Summary:

All that having been said, I still have more confidence in Skype than
I did before reading the paper.
--
http://www.lightconsulting.com/~travis/  -><-
"We already have enough fast, insecure systems." -- Schneier & Ferguson
GPG fingerprint: 50A1 15C5 A9DE 23B9 ED98 C93E 38E9 204A 94C2 641B

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