Yesterday, the Guardian wrote that “researchers have discovered unknown persons are using bitcoin’s blockchain to store and link to child abuse imagery, potentially putting the cryptocurrency in jeopardy.”

Is that true? This is a serious allegation. Unfortunately, both the Guardian story and a article which rejects these claims make grossly inaccurate statements. Furthermore, the cited study makes false statements about Bitcoin in its abstract. Aside from the sensational claims in the abstract, and the even more sensational claims in mainstream media articles about it, the paper is fairly thorough and accurate – but contains no new insights or discoveries, and duplicates prior work without credit. This is a common pattern: researchers make modest claims about something, an editor exaggerates them in the abstract to get attention, and then allows ignorant journalists to make an even more dramatic exaggeration in the press.

So does the Bitcoin blockchain contain illegal content? Not really.

While the media and the public like simple and definitive answers, getting to the truth of this claim requires understanding something about how Bitcoin works.

Bitcoin is a payment network. For the most part, the network itself only records the destination addresses of payments and the amount sent. There is no need for the network to store any arbitrary information which is not specific to a transaction. For example, unlike bank wires, there is not “memo” field in Bitcoin for adding “for pizza, love mom.” Aside from an 80 character field available for miners who sign blocks, the primary way to store non-payment information in the blockchain to use fake destination addresses for transactions. It’s kind of like one of the crank calls in The Simpsons:

Moe: Hello, Moe’s Tavern. Birthplace of the Rob Roy.
Bart: Is Seymour there? Last name Butz.
Moe: Just a sec. Hey, is there a Butz here? Seymour Butz? Hey, everybody! I want a Seymour Butz! [the entire bar laughs; realizes] Wait a minute… Listen, you little scum-sucking pus-bucket! When I get my hands on you, I’m gonna pull out your eyeballs with a corkscrew!

As you might imagine, this is a very inefficient way to store information. Bitcoin transactions have size limitations, so one can either send very small files or split files among many transactions. Since the Bitcoin network charges senders based on transaction size, sending large files is expensive, and much more so with the increase in the price of Bitcoin. The more popular Bitcoin becomes, the more expensive it becomes to insert non-trivial amounts of information.

This is why most images stored in the Blockchain so far were placed there when Bitcoin was cheaper and are tiny, low-resolution images (sample embedded “image” follows):

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Len was our friend.
A brilliant mind,
a kind soul, and
a devious schemer;
husband to Meredith
brother to Calvin,
son to Jim and
Dana Hartshorn,
coauthor and
cofounder and
Shmoo and so much
more.  We dedicate
this silly hack to
Len, who would have
found it absolutely
--Dan Kaminsky,
Travis Goodspeed
P.S.  My apologies,
BitCoin people.  He
also would have
LOL'd at BitCoin's
new dependency upon
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Here’s the address for the above tribute. It cost 1 BTC to send or $8500 at the current price of Bitcoin. Not a very cost-effective way to share illicit files, is it?

Still, based on the above, it seems like it would be possible to store illegal information on the Blockchain if one were able to make it very compact and spend enough money on it. Even if there is nothing illegal in the Blockchain yet (and the study presents no evidence of such information, despite articles which state otherwise), it could always be added in the future.

However, here is an important point: arbitrary information in Bitcoin can only be included by steganography, and cannot be read without tools which have nothing to do with the primary function of Bitcoin. Steganography is “hiding data in plain sight” – in other words, using information flows in ways other than they were intended to and that are not visible to normal users without special tools. Steganography has been known since 440 BC when Herodotus mentioned two examples in his Histories.

There is no way to prevent information from being hidden in any communications channel. For example, two criminals could conduct a series of bank transfers where the monetary amount itself encodes a message with illegal content. There is no way to detect or prevent such a message. However —  the payment network itself has no capability to decode such a message and is not designed for such a use. It’s actively hostile to such a use since all transactions (whether we’re talking about Bitcoin or bank transfers) incur a cost and can store very limited data.

Here is a screenshot I took of a Bitcoin transaction which contains the entire whitepaper where Satoshi Nakamoto presented Bitcoin:

As you can see, the output (aka destination) field contains a hex-encoded alpha-numeric string, which no Bitcoin client can convert into a human-readable message – because that is not their purpose. Furthermore, using Bitcoin to share secrets is a terrible idea. Not only is the amount of information that can be stored very limited, but the information is public for the world to see. Worse, Bitcoin transactions require spending Bitcoin and have the potential to trace back the transaction to a real-world Bitcoin purchase.

A final note: a major inaccuracy is the paper’s claim that “clearly objectionable content such as links to child pornography, which is distributed to all Bitcoin participants.”  The paper provides no such evidence, and only mentions that it found unspecified “nudity of a young woman.”  More importantly,  99.9% of Bitcoin users use a “light” client, which does not contain the full blockchain. Light clients defer blockchain validation to online servers which store the full node.

Here is an earlier and more accurate paper which contains a more fair analysis of the possibilities for data insertion on the Bitcoin blockchain.