Date: Wed, 29 Jun 2016 16:13:17 -0400
From: Peter Todd <pete@petertodd.org>
To: Jonas Schnelli <dev@jonasschnelli.ch>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20160629201317.GA4855@fedora-21-dvm>
References: <87h9cecad5.fsf@rustcorp.com.au>
	<577224E8.6070307@jonasschnelli.ch>
	<CAP+0UNKqDknS-w6QyCJ0_ra71YfsDDtSdSBYoguUicW2oNMLvQ@mail.gmail.com>
	<5774149E.1010105@jonasschnelli.ch>
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In-Reply-To: <5774149E.1010105@jonasschnelli.ch>
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Subject: Re: [bitcoin-dev] BIP 151 use of HMAC_SHA512
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On Wed, Jun 29, 2016 at 08:34:06PM +0200, Jonas Schnelli via bitcoin-dev wr=
ote:
> > Based on previous crypto analysis result, the actual security of SHA512
> > is not significantly higher than SHA256.
> > maybe we should consider SHA3?
>=20
> As far as I know the security of the symmetric cipher key mainly depends
> on the PRNG and the ECDH scheme.
>=20
> The HMAC_SHA512 will be used to "drive" keys from the ECDH shared secret.
> HMAC_SHA256 would be sufficient but I have specified SHA512 to allow to
> directly derive 512bits which allows to have two 256bit keys with one
> HMAC operation (same pattern is used in BIP for the key/chaincode
> derivation).

What's the rational for doing that "directly" rather than with two SHA256
operations? (specifcially SHA256(0 . thing), SHA256(1 + thing) for the two
parts we need to derive)

Reducing the # of basic cryptographic primitives you need to implement a
standard needs is a good thing.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

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