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To: Rusty Russell <rusty@rustcorp.com.au>
References: <87h9cecad5.fsf@rustcorp.com.au>
From: Jonas Schnelli <dev@jonasschnelli.ch>
Message-ID: <577224E8.6070307@jonasschnelli.ch>
Date: Tue, 28 Jun 2016 09:19:04 +0200
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Cc: bitcoin-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] BIP 151 use of HMAC_SHA512
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From: Jonas Schnelli <dev@jonasschnelli.ch>
To: Rusty Russell <rusty@rustcorp.com.au>
Cc: bitcoin-dev@lists.linuxfoundation.org
Message-ID: <577224E8.6070307@jonasschnelli.ch>
Subject: Re: BIP 151 use of HMAC_SHA512
References: <87h9cecad5.fsf@rustcorp.com.au>
In-Reply-To: <87h9cecad5.fsf@rustcorp.com.au>

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> To quote:
>=20
>> HMAC_SHA512(key=3Decdh_secret|cipher-type,msg=3D"encryption key").
>>
>>  K_1 must be the left 32bytes of the HMAC_SHA512 hash.
>>  K_2 must be the right 32bytes of the HMAC_SHA512 hash.
>=20
> This seems a weak reason to introduce SHA512 to the mix.  Can we just
> make:
>=20
> K_1 =3D HMAC_SHA256(key=3Decdh_secret|cipher-type,msg=3D"header encrypt=
ion key")
> K_2 =3D HMAC_SHA256(key=3Decdh_secret|cipher-type,msg=3D"body encryptio=
n key")

SHA512_HMAC is used by BIP32 [1] and I guess most clients will somehow
make use of bip32 features. I though a single SHA512_HMAC operation is
cheaper and simpler then two SHA256_HMAC.

AFAIK, sha256_hmac is also not used by the current p2p & consensus layer.=

Bitcoin-Core uses it for HTTP RPC auth and Tor control.

I don't see big pros/cons for SHA512_HMAC over SHA256_HMAC.

</jonas>

[1]
https://github.com/bitcoin/bips/blob/master/bip-0032.mediawiki#child-key-=
derivation-ckd-functions


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