Date: Wed, 21 Aug 2019 17:04:35 +0000
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
From: Dr Maxim Orlovsky <orlovsky@protonmail.com>
Reply-To: Dr Maxim Orlovsky <orlovsky@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Storm: escrowed storage and messaging at L2/L3
Precedence: list

Hi ZmnSCPxj,

> The Deaf Bob Attack
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

> -   If Alice does not publish the HTLC settlement transaction, then Bob w=
ill eventually enter the N2 state and get the stake+reward.
> -   If Alice does publish the HTLC settlement transaction, without Bob gi=
ving the encrypted data, then Bob can just use the hashlock and reveal the =
decryption key.
>     -   The decryption key is useless without the encrypted data!

That is the main attack already addressed by couple of other guys in differ=
ent forms; and you have provided the most detailed explanation of it, thank=
 you. I was working for its solution over the last day and I'd like to prop=
ose the following update: https://github.com/storm-org/storm-spec/issues/6#=
issuecomment-523497555

Briefly, I introduce two new intermediary HTLC transactions (called confirm=
ation and fallbacks). Alice has now a choice to sign HTLC fallback tx if sh=
e didn't get the data, and in that case she will get her money (reward) bac=
k and Bob's stake as a compensation. Bob can "appeal" to this by confirming=
 that he had hold the data for Alice. Bob does this by providing a "preimag=
e" to the secret hashed by Alice.

This secret is composed at setup time by Alice, and she uses her newly-deri=
ved public key for both funding transaction output and deterministically de=
finition of some small portion of the source data. This portion is double-h=
ashed to 160-bit hash and included into HTLC fallback tx by Alice as a hash=
 lock. Later, when Bob wants to prove that he still has the data available,=
 he see the published HTLC transaction, extracts Alice public key and uses =
it to get the same deterministic piece of the source data as Alice. Bob com=
putes a single hash on the date, which gives him a preimage to unlock the h=
ash lock from HTLC transaction output before Alice will spend it (Alice's o=
utput is timelocked).

This solution requires Bob to select decription key at setup time, when he =
pre-signs the transaction, as you correctly have pointed out:
> If Bob already selected the decryption key at setup time, then Bob can ig=
nore Alice.

Now, we need to aviod situation where by selecting the encryption/decryptio=
n pair Bob knows which part of the data he needs to provide Alice in PCP pr=
oof, and can discard the rest of the data. This can be mitigated by requiri=
ng that the data have to be encrypted using EC multiplication with some fac=
tor provided by Alice at the request time.

The only case for possible cheating now is Alice not needing data anymore a=
nd avoiding paying the full amount for the storage. However, this can be a =
part of the Bob business risk and may be covered by some insurance + Alice'=
s reputation & taken into account by Bob at setup time.

Kind regards,
Maxim Orlovsky