HMAC has proven security property. It is still secure even when underlying crypto hashing function has collision resistant weakness. For example, MD5 is considered completely insecure now, but HMAC-MD5 is still considered secure. When in doubt, we should always use HMAC for MAC(Message Authentication Code) rather than custom construction
On Wed, Jun 29, 2016 at 9:00 AM, Rusty Russell via bitcoin-dev < bitcoin-dev@lists.linuxfoundation.org> wrote: > Jonas Schnelli <d...@jonasschnelli.ch> writes: > >> 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") > > > > 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. > > Good point; I would argue that mistake has already been made. But I was > looking at appropriating your work for lightning inter-node comms, and > adding another hash algo seemed unnecessarily painful. > > > AFAIK, sha256_hmac is also not used by the current p2p & consensus layer. > > Bitcoin-Core uses it for HTTP RPC auth and Tor control. > > It's also not clear to me why the HMAC, vs just > SHA256(key|cipher-type|mesg). But that's probably just my crypto > ignorance... > > Thanks! > Rusty. > _______________________________________________ > bitcoin-dev mailing list > bitcoin-dev@lists.linuxfoundation.org > https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev > -- Xuesong (Arthur) Chen Senior Principle Engineer BlockChain Technologist BTCC
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