From: Amir Herzberg <AMIR@newgenpay.com>

Date: Sun, 25 Mar 2001 10:57:44 +0100

Message-ID: <078EE8822DCFD411AAA1000629D56ADC05227A@IMP01>

To: "Xml Encrypt (E-mail)" <xml-encryption@w3.org>

Date: Sun, 25 Mar 2001 10:57:44 +0100

Message-ID: <078EE8822DCFD411AAA1000629D56ADC05227A@IMP01>

To: "Xml Encrypt (E-mail)" <xml-encryption@w3.org>

Corrections to my last note: > Joe was concerned about this suggestion: > > > > It depends on how post-image resistant the hash function in > > use is. If SHA-1 > > is used then it will add most of what you expect, MD-5 it will add > > significantly less than expected (there are known collisions in the > > compression function). > > Actually, the property we need here is unrelated to > collisions, Sorry, a stupid error, of course we DO care for collisions in a commitment application and therefore SHA-1 is indeed preferable. > But let's agree first on the need, and then we can define a > specific function which we can all feel is secure enough (we > could even use > the simple, efficient and yet proven-secure construction in [CMR]). But I don't recommend it - I think while [CMR] is relatively efficient and practical, we should allow a solution which is roughly as efficient as regular hash. In fact, Shai Halevi noticed that for the Notice that in the application we discuss, of committing to some x using a randomizer r, the ransomizer may be different for each commitment and kept secret until proving the commitment. In this case, Shai Halevi noted that the trivial c(x)=h(x||r) function I suggested the proof is very simple: the binding follows from the collision-resistance of h(), and the secrecy follows since c(x) is indistinguishable from random for all x. For this case, where rand is revealed, I don't know how to prove it (other than in the random-oracle model). > > So while it provides resistance right > > now, it does > > not necessarily provide resistance for the future, in order > > to make it as > > resistant as possible (with currently known hash algorithm > > types) you should > > actually pre and post pend some random data. Clearly this constructions, c(x)= h( r1 || x || r2), where both r1 and r2 are random and kept encrypted (secret) until proving commitment, is fine as well. Best regards, Amir Herzberg CTO, NewGenPay Inc. See our demo and overview/tutorials on secure e-commerce in http://www.NewGenPay.comReceived on Sunday, 25 March 2001 02:54:11 UTC

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