Comment by tptacek | Hacker News Reader

Comment by tptacek | Hacker News Reader

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Karsten Nohl: also the real deal.

Here, for us, are the nut grafs:

In early 2011, Nohl’s team started toying with the OTA protocol and noticed that when they used it to send commands to several SIM cards, some would refuse the command due to an incorrect cryptographic signature, while a few of those would also put a cryptographic signature on this error message.

With that signature and using a well known cryptographic method called rainbow tables, Nohl was able to crack the encryption key on the SIM card in about one minute. Carriers use this key to remotely program a SIM, and it is unique to each card.

This is a little vague and I don't understand the OTA protocol like, at all, but what it sounds like is that there is a case in some implementations of SIM OTA where (a) errors for improperly signed messages are noisy, (b) those errors include some of the plaintext of the improperly signed message, and (c) the error message itself has a signature that is intended to be valid only for the error.

Possible next steps: (i) you can table-solve for the signature (presumably this is a MAC, not a signature) for your intended message due to the way plaintext hits the error message, or (ii) you can table-solve for the plaintext of a previously unknown ciphertext by taking that ciphertext, flipping a bit to invalidate the signature, and collecting the error signature.

I'm guessing its more the latter (send an error an then use the result to deduce the key) That being the case, then someone duplicating the Raspberry Pi micro cell site could passively attack any phone that came within range.

Makes you wonder if the FBI snooping cell tower already does this :-)

Makes me wish I had the burner phone concession at DefCon.

Yep, that appears to be the case:

http://www.theregister.co.uk/2013/07/22/mobile_gsm_sim_card_...

An attack similar in spirit to this breaks RADIUS.

At one point he says that he thinks it will take the black hats six months or so to figure out the exploit, but then, in the passage you have quoted, he gives what sound (to my non expert ear) like fairly massive clues. Is it possible he has revealed too much?

Well, he's talking about it at Black Hat, right? Black Hat talks are usually full-disclosure.

Looks like it. https://www.blackhat.com/us-13/briefings.html#Nohl

He'll be presenting the same subject at OHM on 03/08/2013 too: https://program.ohm2013.org/event/169.html

Its all about full disclosure for ethical reasons. Can't say that about them white hats.

I don't know what this comment means. Could you, or someone else, explain it? Who are the "white hats"? Why do they have different perspectives on full disclosure than other people? Who are those other people?

I think hondje's comment was intended to be sarcastic, for the rest see

https://en.wikipedia.org/wiki/White_hat_(computer_security)

Maybe a riff on the burgeoning "green hat" practice of selling exploits instead of simply disclosing them, versus black hats releasing them publicly for free?

I wouldn't have revealed that the second bug is a buffer overflow. The knowledge of the existence of a buffer overflow vulnerability on SIM cards is much more dangerous than knowledge of a buffer overflow on, say, a desktop O/S. Six months is a long time for a well funded organization to fuzz such a small piece of software.

On a different note, I find it extremely frustrating that the community still has to deal with buffer overflow bugs in 2013. Hardware bounds checking architectures have existed for half a century, and SIM cards are a perfect example of special use devices that would benefit from this.

The buffer overflow is only accessible after you have broken the crypto stuff, no?

I strongly suspect that the attack uses the known plaintext of the error message to solve directly for the DES key (which is only an effective 56 bits). I wouldn't be surprised if it used the old FIPS DES-based MAC.

This appears to be confirmed - from https://srlabs.de/rooting-sim-cards/ :

  A rainbow table resolves this plaintext-signature tuple to
  a 56-bit DES key within two minutes on a standard computer.
  The cracked DES key enables an attacker to send properly
  signed binary SMS, which download Java applets onto the SIM.

It's particularly sad that the same key is used for the MAC in both directions (network-to-SIM and SIM-to-network).

Also sad that it's refereed to as a signature.

I still haven't been able to find the particular documentation around class 2 SMS verification. Most of the things I'm finding talk about IMSI using HMAC-SHA1 for OTA validation. Is this at a lower level?

Edit: Found this: http://www.3gpp.org/ftp/tsg_sa/wg3_security/TSGS3_33_Beijing...

In my company I started to issue smart cards instead of simple magnetic cards, because the tech is so hard to implement I started to question if this was a mild case of security through obscurity. I really hope this isn't the case, many banks store passwords in those cards, including us. This password cannot be easily transported back to our servers.

Anyway, there is something missing, as a security measure SIM cards should burn out if you keep calling it.

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