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To: Arthur Chen <arthur.chen@btcc.com>,
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From: Jonas Schnelli <dev@jonasschnelli.ch>
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Date: Wed, 29 Jun 2016 20:34:06 +0200
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Subject: Re: [bitcoin-dev] BIP 151 use of HMAC_SHA512
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From: Jonas Schnelli <dev@jonasschnelli.ch>
To: Arthur Chen <arthur.chen@btcc.com>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <5774149E.1010105@jonasschnelli.ch>
Subject: Re: [bitcoin-dev] BIP 151 use of HMAC_SHA512
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> Based on previous crypto analysis result, the actual security of SHA512=

> is not significantly higher than SHA256.
> maybe we should consider SHA3?

As far as I know the security of the symmetric cipher key mainly depends
on the PRNG and the ECDH scheme.

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).

Keccak would be an alternative but we probably don't want to introduce
another new hash type just for the encryption.

</jonas>


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