Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS] Additional Elliptic Curves (Curve25519 etc) for TLS ECDH key agreement)
Vadym Fedyukovych <vf@unity.net> Sun, 12 January 2014 22:31 UTC
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Date: Mon, 13 Jan 2014 00:29:44 +0200
From: Vadym Fedyukovych <vf@unity.net>
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Subject: Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS] Additional Elliptic Curves (Curve25519 etc) for TLS ECDH key agreement)
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On Sun, Jan 12, 2014 at 07:13:16AM -0800, Robert Ransom wrote: > On 1/12/14, Adam Back <adam@cypherspace.org> wrote: > > > So actually Bernstein went in the opposite direction, not only using > > sub-group size, but double sub-group size hash, basically because he could > > without increasing the signature size, and thereby slightly even further > > reducing the dependency on hash security and hash properties. I do not > > consider its necessary, just its because he could, slightly more security > > almost for free. But I think an EdDSA variant that used a 512-bit curve > > could safely use a 512-bit hash, because even the double width hash is > > over-engineering. > > It can for the hash of the message. > > > EdDSA also uses the deterministic DSA k trick (computed from m and x the > > private key). > > Deterministic generation of message keys is the primary reason that > EdDSA requires a double-length hash function. > > EdDSA relies on the hash function having double-length output in two ways: > > * Message key generation relies on the output being noticeably longer > than the group order in order to generate *uniform* exponents. Choosing a message key (an initial random in interactive system) from a large interval is a well-known idea for a group of a hidden order. For a group of known order, reducing almost anything modulo group order would likely result in quite a non-uniform distribution. It is non-trivial to see how/whether hash of roughly twice-bitlength of group order would be better than hash smaller than group order. Hash bitlength is rather non-critical while choosing a challenge, that should be just a large enough set. Vadym Fedyukovych
- [Cfrg] Thoughts on a Next-Generation Elliptic Cur… Alyssa Rowan
- Re: [Cfrg] Thoughts on a Next-Generation Elliptic… Robert Ransom
- Re: [Cfrg] [TLS] Additional Elliptic Curves (Curv… Alyssa Rowan
- [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS] Ad… Adam Back
- Re: [Cfrg] [TLS] Additional Elliptic Curves (Curv… Daniel Kahn Gillmor
- Re: [Cfrg] [TLS] Additional Elliptic Curves (Curv… Robert Ransom
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Robert Ransom
- Re: [Cfrg] [TLS] Additional Elliptic Curves (Curv… Adam Back
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Adam Back
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Mike Hamburg
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Vadym Fedyukovych
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Watson Ladd
- Re: [Cfrg] EdDSA and > 512 curve & hash (Re: [TLS… Vadym Fedyukovych