Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as a RG document
Watson Ladd <watsonbladd@gmail.com> Thu, 08 January 2015 21:51 UTC
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Date: Thu, 08 Jan 2015 16:51:18 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
To: Andrey Jivsov <crypto@brainhub.org>
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Subject: Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as a RG document
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On Thu, Jan 8, 2015 at 4:36 PM, Andrey Jivsov <crypto@brainhub.org> wrote: > On 01/05/2015 11:15 AM, Alexey Melnikov wrote: >> >> This message starts 2 weeks adoption call (ending on January 19th 2015) >> on: >> >> https://www.imperialviolet.org/cfrgcurve/cfrgcurve.xml >> >> as the starting point for the CFRG document which describes an algorithm >> for safe curve parameter generation for a particular security level and >> also recommends a specific curve (2^255-19) for the 128-bit security >> level. >> >> Please reply to this message or directly to CFRG chairs, stating whether >> you support (or not) adoption of this document. If you do not support >> adoption of this document, please state whether you support adoption of >> any alternative document or whether you want a particular change be made >> to the document before adoption. > > > > For http://tools.ietf.org/html/draft-agl-cfrgcurve-00: >> >> >> 9. Elliptic Curve Diffie-Hellman >> >> > ... >> >> z_2 = E * (AA + a24 * E) >> // Conditional swap; see text below. >> (x_2, x_3) = cswap (s_t, x_2, x_3) >> (z_2, z_3) = cswap (s_t, z_2, z_3) >> Return x_2 * (z_2^(p - 1)) > > > should be > > Return x_2 * (z_2^(p - 2)) > >> 8. Wire-format of field elements > > >> When transmitting field elements in the Diffie-Hellman protocol >> below, they MUST be encoded as an array of bytes, x, > > > I would like to see an uncompressed format allowed as (x,y) as an optional > format. This avoids ~10% performance penalty for some applications, v.s. the > case when only (x) is sent. > > the y above can be calculated for "free" at the end of Montgomery ladder > calculation due to the above z_2^-1 inverse calculation, using a method > described in section 5 (p.13-14) in > http://ed25519.cr.yp.to/ed25519-20110705.pdf. The code would be needed > anyway for decompression with signatures that use compressed point. Three points: 1: There are recurring security issues caused by not sending compressed points, as well as additional complexity 2: We're not talking about signatures in this draft. 3: Options are bad. > > Likewise, receiving (x,y) allows a conversion to a projective form of > twisted Edwards coordinates for "free" for a peer. At the same time a peer > is free to ignore y and proceed with the Montgomery ladder multiplication > exactly as described in sec 9. > > Therefore, (x,y) on the wire allows the most suitable choice of > implementation for ECDH, whereas (x) limits it to Montgomery ladder given > that one needs to overcome the 10% penalty of recovering y with a square > root. > > Many applications and protocols will require generation of a new ECDH key > pair (i.e. they cannot reuse it). For these applications fixed-base scalar > multiplication on Edwards curve is much faster. These applications may > prefer to use the same code for variable-base scalar multiplication as well. > Likewise, the code reuse argument applies to applications that support > signatures on Curve25519. 4: Fixed based scalar optimizations already require different code from variable base for maximal speed: combs vs. radix. The Montgomery ladder is so small that using it for variable base multiplication isn't a major complexity increase. > > I support the draft with the above tweak. > > > _______________________________________________ > Cfrg mailing list > Cfrg@irtf.org > http://www.irtf.org/mailman/listinfo/cfrg -- "Those who would give up Essential Liberty to purchase a little Temporary Safety deserve neither Liberty nor Safety." -- Benjamin Franklin
- [Cfrg] Adoption of draft-agl-cfrgcurve-00 as a RG… Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Adam Langley
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Stephen Farrell
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Watson Ladd
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Tony Arcieri
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Blumenthal, Uri - 0558 - MITLL
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Adam Langley
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Ilari Liusvaara
- [Cfrg] (please make draft an IETF document first)… Rene Struik
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Paul Lambert
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … David Leon Gil
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Michael Hamburg
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Alyssa Rowan
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Dan Brown
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … David Gil
- Re: [Cfrg] (please make draft an IETF document fi… Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Sean Turner
- Re: [Cfrg] (please make draft an IETF document fi… Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Watson Ladd
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Andrey Jivsov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Adam Langley
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Andrey Jivsov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Watson Ladd
- [Cfrg] options (was: Re: Adoption of draft-agl-cf… Stephen Farrell
- [Cfrg] No longer talking about Adoption of draft-… Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Joppe Bos
- Re: [Cfrg] options (was: Re: Adoption of draft-ag… Paul Hoffman
- Re: [Cfrg] options Andrey Jivsov
- Re: [Cfrg] draft-agl-cfrgcurve-00 point format (w… Alyssa Rowan
- Re: [Cfrg] draft-agl-cfrgcurve-00 point format Andrey Jivsov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Robert Ransom
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Ilari Liusvaara
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Robert Ransom
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Tony Arcieri
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Alexey Melnikov
- Re: [Cfrg] Adoption of draft-agl-cfrgcurve-00 as … Stephen Farrell
- [Cfrg] (technical flaws to be corrected in next v… Rene Struik
- Re: [Cfrg] (technical flaws to be corrected in ne… Adam Langley
- Re: [Cfrg] (technical flaws to be corrected in ne… Rene Struik
- Re: [Cfrg] (technical flaws to be corrected in ne… Adam Langley