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From: Rusty Russell <rusty@rustcorp.com.au>
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Subject: [bitcoin-dev] BIP 151 use of HMAC_SHA512
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To quote:

> HMAC_SHA512(key=ecdh_secret|cipher-type,msg="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.

This seems a weak reason to introduce SHA512 to the mix.  Can we just
make:

K_1 = HMAC_SHA256(key=ecdh_secret|cipher-type,msg="header encryption key")
K_2 = HMAC_SHA256(key=ecdh_secret|cipher-type,msg="body encryption key")

Thanks,
Rusty.