Response to a journalist about including arbitrary data in the block chain

arbitrary-data.txt

On Mon, Mar 19, 2018 at 10:07:12AM -0700, [redacted] wrote:
> Hi David,
> I'm working on a piece about how the Bitcoin blockchain can accommodate
> arbitrary data, potentially making it illegal in certain countries and
> circumstances. The paper about this can be found here:
>
> https://fc18.ifca.ai/preproceedings/6.pdf
>
> I'm wondering whether you might be available to comment before 1pm PT today.
>
> If so, I'd be curious to hear what you think of the researcher's claims and
> whether you think there's a way for Bitcoin's blockchain to prevent this
> issue.

Hey,

I've only quickly skimmed the above linked paper, but the inclusion of
arbitrary data in the block chain has concerned members of the technical
community for a long time.  For example, in late 2010, Bitcoin creator Satoshi
Nakamoto wrote a response to an early attempt to add arbitrary Domain
Name System (DNS) data to the block chain called BitDNS, saying

    "Piling every proof-of-work quorum system in the world into one
    dataset doesn't scale.

    "Bitcoin and BitDNS can be used separately.  Users shouldn't have to
    download all of both to use one or the other.  BitDNS users may not
    want to download everything the next several unrelated networks
    decide to pile in either.

    "The networks need to have separate fates.  BitDNS users might be
    completely liberal about adding any large data features since
    relatively few domain registrars are needed, while Bitcoin users
    might get increasingly tyrannical about limiting the size of the
    chain so it's easy for lots of users and small devices."

    Source:
    https://bitcointalk.org/index.php?topic=1790.msg28917#msg28917

    Note: BitDNS took Nakamoto's advice and became Namecoin, one of the
    earliest altcoins.

The concerns about illegal, immoral, or simply questionable arbitrary
data in the block chain have also been raised many times in discussion
and have lead to actual technical problems before, such as the
DOS/Stoned incident:

    https://en.bitcoin.it/wiki/DOS/STONED_incident

Again based only on a quick skim of the paper, I'd say it doesn't
contain anything novel that hasn't been known to Bitcoin developers for
years.  However, it does look like a nice writeup.

As for mitigation, there have been several proposals but none has
gained widespread support in the technical community and (in my opinion)
the topic is not considered a high priority at the moment.  The best known
proposal to date is P2SH^2 by Bitcoin Core developer Gregory Maxwell; my
article about that is here:

    https://en.bitcoin.it/wiki/P2SH%C2%B2

The references section of that article also links to other resources.
The article by applied cryptographer and occassional Bitcoin Core
contributor Peter Todd is probably the closest read to your topic.

Ultimately, however, I don't believe it's entirely possible to prevent
users from including arbitrary data in a decentralized block chain.  The
best mitigations known are merely to make it very expensive to publish
arbitrary data on a per-byte basis.

Good luck with your article,

-Dave