On Tue, Jul 30, 2019, 8:21 AM Scott Fluhrer (sfluhrer) <sfluh...@cisco.com> wrote:
> During the physical meeting in Montreal, we had a discussion about > postquantum security, and in particular, on how one might want to negotiate > several different ‘groups’ simultaneously (because there might not be one > group that is entirely trusted, and I put ‘groups’ in scarequotes because > postquantum key exchanges are typically not formed from a Diffie-Hellman > group). > > > > At the meeting, there were two options presented: > > > > Option 1: as the supported group, we insert a ‘hybrid marker’ (and include > an extension that map lists which combination the hybrid marker stands for) > > For example, the client might list in his supported groups > hybrid_marker_0 and hybrid_marker_1, and there would be a separate > extension that lists hybrid_marker_0 = X25519 + SIKEp434 and > hybrid_marker_1 = X25519 + NTRUPR653. The server would then look up the > meanings of hybrid_marker_0 and 1 in the extension, and then compare that > against his security policy. > > In this option, we would ask IANA to allocate code points for the various > individual postquantum key exchanges (in this example, SIKEp434 and > NTRUPR653), as well a range of code points for the various hybrid_markers.. > > > Option 2: we have code points for all the various combinations that we may > want to support; hence IANA might allocate a code point X25519_SIKEp434 and > another code point for X25519_NTRUPR653. With this option, the client > would list X25519_SIKEp434 and X25519_NTRUPR653 in their supported groups.. > > In this option, we would ask IANA to allocate code points > for all the various combinations that we want allow to be negotiated. > > > > I would like to make an argument in favor of option 1: > > > > - It is likely that not everyone will be satisified with “X25519 plus > one of a handful of specific postquantum algorithms”; some may prefer > another elliptic curve (for example, x448), or perhaps even a MODP group; I > have talked to people who do not trust ECC); in addition, other people > might not trust a single postquantum algorithm, and may want to rely on > both (for example) SIKE and NewHope (which are based on very different hard > problems). With option 2, we could try to anticipate all the common > combintations (such as P384_SIKEp434_NEWHOPE512CCA), however that could > very well end up as a lot of combinations. > - There are likely to be several NIST-approved postquantum key > exchanges, and each of those key exchanges are likely to have a number of > supported parameter sets (if we take the specific postquantum key exchange > as analogous to th ECDH protocool, the “parameter set” could be thought of > an analogous to the specific elliptuc curve, and it modifies the key share > size, the performance and sometimes the security properties). In fact, one > of the NIST submissoins currently has 30 parameter sets defined. Hence, > even if NIST doesn’t approve all the parameter sets (or some of them do not > make sense for TLS in any scenario), we might end up with 20 or more > different key exchange/parameter set combinations that do make sense for > some scenario that uses tLS (be it in a tranditional PC client/server, a > wireless client, two cloud devices communicating or an IOT device). > - In addition, we are likely to support additional primitives in the > future; possibly National curves (e.g. Brainpool), or additional > Postquantum algorithms (or additional parameter sets to existing ones).. > Of > course, once we add that code point, we’ll need to add the additional code > points for all the combinations that it’ll make sense in (very much like we > had to add a number of ciphersuites whenever we added a new encryption > algorithm into TLS 1.2). > > > Are people actually going to use hybrid encryption post NIST? The actual deployments today for experiment have all fit option 2 and hybrids are unlikely in the future. My objection to 1 is it gets very messy. Do we use only the hybrids we both support? What if I throw a bunch of expensive things together? No reason we need a hybrid scheme! It seemds reasonable to me that the combination of these two factors are > likely to cause us (should we select option 2) to define a very large > number of code points to cover all the various options that people need. > > > > Now, this is based on speculation (both of the NIST process, and > additional primitives that will be added to the protocol), and one > objection I’ve heard is “we don’t know what’s going to happen, and so why > would we make decisions based on this speculation?” I agree that we have > lack of knowledge; however it seems to me that a lack of knowledge is an > argument in favor of selecting the more flexible option (which, in my > opinion, is option 1, as it allows the negotiation of combinations of key > exchanges that the WG has not anticipated). > > > > My plea: lets not repeat the TLS 1.2 ciphersuite mess; lets add an > extension that keeps the number of code points we need to a reasonable > bound. > > > > The costs of option 1? > > - It does increase the complexity on the server a small amount (I’m > not a TLS implementor, however it would seem to me to be only a fairly > small amount) > - It may increase the size of the client hello a small amount (on the > other hand, because it allows us to avoid sending duplicate key shares, it > can also reduce the size of the client hello as well, depending on what’s > actually negotiated) > > IMHO, the small increase in complexity is worth the lack of complexity in > the code point table, and the additional flexibility it gives. > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls >
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