<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>pja</author><text>I had a chat to a friend who&amp;#x27;s worked in the mobile industry for decades. He said that 4G phones (possibly 3G too, I don’t recall) only use the shared secret key for the initial sign-on to the parent mobile network. Thereafter, new keys are generated and stored at both ends and it&amp;#x27;s these keys that are used to authenticate the end points and bootstrap encrypted connections. New keys are re-issued at intervals, although I don’t know what the interval time is.&lt;p&gt;So whilst it would be possible to decrypt phone connections if you had your hands on the original secret Ki stored in the SIM, you&amp;#x27;d have to record every connection between the phone and the network in order to obtain all the subsequent keys as well &amp;amp; if you miss out on the initial sign-on, or any individual re-keying then you’ll be shut out of that phone’s radio communications thereafter.&lt;p&gt;I imagine the NSA would be willing to try and do this for some target networks, but where they already have internal network access (US&amp;#x2F;UK&amp;#x2F;Five Eyes, any other network they&amp;#x27;ve hacked into) it would be a lot of pointless effort.&lt;p&gt;The fake base station attack presumably works by forcing a downgrade to 2G, which is another approach, but one that requires local assets on the ground within phone range (unless you can do something with high gain antennas pointed at a specific target phone from a distance? That sounds hard, but the NSA likes hard as we know - throwing resources at something isn’t a problem for them.)</text><parent_chain><item><author>scintill76</author><text>Can anyone elaborate on why it&amp;#x27;s supposedly only a problem for 2G?  &amp;quot;If someone intercepted the encryption keys used in 3G or 4G SIMs they would not be able to connect to the networks and consequently would be unable to spy on communications.&amp;quot;  Why not?  I feel like there is a &amp;quot;merely&amp;quot; missing from this sentence -- if so, what more than keys do they need to spy?&lt;p&gt;Are they basing this on the specific type of key discussed in the documents?  I don&amp;#x27;t know a lot about it, but I&amp;#x27;m inclined to believe there are valuable keys burned-in to 3G+ cards too.&lt;p&gt;I also wonder if there is a downgrade attack to force 2G, so that those keys are not completely worthless.</text></item></parent_chain></comment><story><title>Gemalto&apos;s findings of its investigations into the alleged hacking of SIM cards</title><url>http://www.gemalto.com/press/Pages/Gemalto-presents-the-findings-of-its-investigations-into-the-alleged-hacking-of-SIM-card-encryption-keys.aspx</url></story>
<instructions>Your goal is to analyze the following comment and estimate how highly it will be upvoted by the Hacker News community.</instructions><comment><author>thornjm</author><text>Hopefully someone more knowledgable can weigh in, but as I understand the key stored on a 3G sim is more useful for authentication&amp;#x2F;identification rather than encryption.&lt;p&gt;3G&amp;#x2F;4G somehow uses random, short lived keys for encrypted communication, which change frequently enough to be a pain.&lt;p&gt;EDIT: It has been a while since I studied this, but I believe the shared key is used for trust - that this isn&amp;#x27;t a fake base station and the client is who they say they are. Then they use the equivalent of public key cryptography to establish short lived encryption keys. Stealing keys would probably enable a MitM only?</text><parent_chain><item><author>scintill76</author><text>Can anyone elaborate on why it&amp;#x27;s supposedly only a problem for 2G?  &amp;quot;If someone intercepted the encryption keys used in 3G or 4G SIMs they would not be able to connect to the networks and consequently would be unable to spy on communications.&amp;quot;  Why not?  I feel like there is a &amp;quot;merely&amp;quot; missing from this sentence -- if so, what more than keys do they need to spy?&lt;p&gt;Are they basing this on the specific type of key discussed in the documents?  I don&amp;#x27;t know a lot about it, but I&amp;#x27;m inclined to believe there are valuable keys burned-in to 3G+ cards too.&lt;p&gt;I also wonder if there is a downgrade attack to force 2G, so that those keys are not completely worthless.</text></item></parent_chain></comment><story><title>Gemalto&apos;s findings of its investigations into the alleged hacking of SIM cards</title><url>http://www.gemalto.com/press/Pages/Gemalto-presents-the-findings-of-its-investigations-into-the-alleged-hacking-of-SIM-card-encryption-keys.aspx</url></story>