Re: [Cfrg] ISE seeks comments on: draft-cakulev-ibake-02.txt
"Jim Schaad" <ietf@augustcellars.com> Fri, 22 October 2010 23:04 UTC
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From: Jim Schaad <ietf@augustcellars.com>
To: 'David Wagner' <daw@cs.berkeley.edu>, cfrg@irtf.org
References: <201010191940.o9JJe6qT026149@taverner.cs.berkeley.edu>
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Date: Fri, 22 Oct 2010 16:15:49 -0700
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Subject: Re: [Cfrg] ISE seeks comments on: draft-cakulev-ibake-02.txt
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This document, if published, would be strictly as an informational draft. It is not on standards track in any way. Jim > -----Original Message----- > From: cfrg-bounces@irtf.org [mailto:cfrg-bounces@irtf.org] On Behalf Of > David Wagner > Sent: Tuesday, October 19, 2010 12:40 PM > To: cfrg@irtf.org > Subject: Re: [Cfrg] ISE seeks comments on: draft-cakulev-ibake-02.txt > > [I tried sending this two weeks ago, but it never seems to have hit the list. I > apologize for any duplicates anyone may receive. > This is regarding draft-cakulev-ibake-02.txt.] > > draft-cakulev-ibake-02.txt's abstract says: > > "Cryptographic protocols based on public key methods are based on > > certificates and large scale public key infrastructure (PKI) to > > support certificate management. The emerging field of Identity Based > > Encryption protocols allows to simplify the infrastructure > > requirements via a Key Generation Function (KGF) while providing the > > same flexibility. However one significant limitation of Identity > > Based Encryption methods is that the KGF can end up being a de-facto > > key escrow server with undesirable consequences. Another observed > > deficiency is a lack of mutual authentication of communicating > > parties. Here, Identity Based Authenticated Key Exchange (IBAKE) > > Protocol is specified which does not suffer from the key escrow > > problem and in addition provides mutual authentication and a perfect > > forward and backwards secrecy." > > Here are my notes, after a quick informal review: > > I don't immediately see any practical advantage over standard PKI. > IBAKE doesn't seem to address the hard problems or limitations of PKI. > The practical limitations of PKI mainly arise from the way it is used in a larger > system, not from the crypto-mathematics themselves. (See, e.g., writings from > Peter Gutmann, Carl Ellison, or Bruce Schneier.) I don't see how IBAKE changes > the game in any significant way. > > For instance, IBAKE "assumes that the Initiator and the Responder trust a third > party" (the key generator). If it's acceptable to trust a third party, then one > could equally well trust a Certificate Authority. I do not see how IBAKE > simplifies the management, revocation, authorization, or other issues > associated with standard PKI. I see no advantage here. > > The Internet-Draft claims that by making the keys date-dependent, IBAKE can > address key revocation. But as far as I can see, PKI can support a similar > functionality by issuing certificates with appropriately chosen expiration dates. > In either case, the requirement for tighter time synchronization may have some > practical consequences. > > Some technical comments (many are nitpicky or at least addressable): > > * I have questions about the security of this protocol. It does not > come with a proof of security, which is the accepted standard for new > proposals of this sort. The scheme has apparently not been previously > published in an appropriate peer-reviewed scientific conference. > > I will illustrate some examples of specific potential concerns, which a > proof of security would put to rest. The IBAKE protocol uses encryption > to authenticate the parties, and relies upon the encryption to bind > various plaintext entities securely to each other, which requires careful > analysis (for instance, if the encryption method is malleable, then > various active attacks may be possible, so any proof of security would > need to account for this, implicitly or explicitly). As another example, > Message_1 and Message_3 have the same format, raising the possibility of > cut-and-paste attacks that copy an intercepted Message_1 and introduce > it as Message_3, or vice versa (any proof of security would need to > demonstrate, either implicitly or explicitly, that this cannot cause > any problems). As a third example, the protocol specification does > not explicitly specify that, after decrypting, each party should check > that the identities in the decrypted plaintext match what is expected. > It's not obvious whether this might have any security consequences; > a proof of security would address that concern. These examples are not > intended to be exhaustive, and the main value of a proof of security is > that it addresses even concerns we may not have anticipated (as opposed > to solely those concerns that we were able to identify in advance). > > * The protocol specification does not require the parties to check > that the elliptic curve points they receive are indeed elements of > the curve and members of the group. I wonder if this has any > consequences. Can a malicious Initiator achieve anything interesting > by sending a value of xP that is not actually on the curve? > > * The security analysis doesn't make explicit the consequences > of active man-in-the-middle attacks, where the attacker colludes with > the KGF (or compromises the KGF). The security analysis says it is > assumed this will not happen. > > * The protocol specification could provide stronger guidance on the > choice of x and y. It says that they should be random and seems to > assume they will be chosen anew for each session, but it might help to > specify explicitly for implementors that they must be chosen anew for > each session. If the Responder re-uses y for multiple sessions with the > same Initiator, then it becomes possible for an active attacker to replay > past sessions, fooling the Initiator into thinking that he is talking > with the Responder when he is actually not. If the Responder's message > is a non-idempotent command to the Responder, these replay attacks could > be problematic. The Internet-Draft could clearly state that no value > of x,y may ever be reused across sessions, not even in a new session > with the same (Initiator,Responder)-pair. > > * The protocol does not describe how messages are associated with > or bound to a particular session. There is no session ID. The > Internet-Draft does not discuss issues such as concurrent session > establishment. > > > Overall, it seemed to me that the IBAKE Internet-Draft did not make a > compelling argument for IBAKE, and I felt that its comparison of PKI vs IBAKE > was unconvincing. Even if the technical issues listed above were addressed, I > feel that the failure to articulate a clear advantage over standard PKI would be > sufficient to prevent advancement of this Internet-Draft. > > Therefore, if I have understood the situation accurately, I do not recommend > that this Internet-Draft be advanced for standardization. > > Caveat: This is based upon a quick look at the I-D. There may well be > significant errors or oversight in my analysis, for all I know. > _______________________________________________ > Cfrg mailing list > Cfrg@irtf.org > http://www.irtf.org/mailman/listinfo/cfrg
- Re: [Cfrg] ISE seeks comments on: draft-cakulev-i… David Wagner
- [Cfrg] ISE seeks comments on: draft-cakulev-ibake… Nevil Brownlee
- Re: [Cfrg] ISE seeks comments on: draft-cakulev-i… Jim Schaad