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Would it be possible to make using Bitcoin illegal by posting child porn or similar highly illegal content in the message of a transaction? There are methods for having 'thin clients' that don't download the entire transaction history - would these be immune to an attack like this? How difficult would it be to track the sender of such material?

  • I don't think there is space for images in the blockchain (I could be mistaken) but you could add an "illegal number" like 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 .. .. .. easily enough. – LateralFractal Oct 25 '13 at 4:40
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    Can you make it illegal to operate a package delivery service by shipping child porn or other illegal content? – David Schwartz Oct 25 '13 at 5:22
  • @DavidSchwartz Your comparison is lacking. A better comparison would be "Can you make it illegal to receive a delivery of 1000s photos, containing a small number of child porn or other illegal content I wasn't aware of and didn't notice?" Then yes, it's possible. – SK19 Mar 20 '18 at 20:24
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    So, 4 years later... This happened: Child abuse imagery found within bitcoin's blockchain – JPhi1618 Mar 20 '18 at 20:59
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Would it be possible to make using Bitcoin illegal by posting child porn or similar highly illegal content in the message of a transaction?

Bitcoin, as a decentralised provision network of sorts, may have the same protections as given to ISPs and content providers. So unless the government is gunning for Bitcoin anyway - I'd say no, Bitcoin can not be rendered illegal this way.

There are methods for having 'thin clients' that don't download the entire transaction history - would these be immune to an attack like this?

Obviously if you don't download the illegal content nor upload any intent to download specifically that type of content - you should be safe with a thin-client. But it limits the security of Bitcoin network considerably if only thin-clients were possible.

How difficult would it be to track the sender of such material?

Not very difficult. Bitcoin is not an inherently anonymous protocol. If the communication between nodes is not encrypted with ephemeral TLS sessions, a mix service wouldn't provide any meaningful anonymity either.

  • "if you don't download the illegal content" Easier said than done when parts of the block chain could be downloaded automatically by the software. And fyi, tracking senders over the internet can easily made very difficult, see proxies and dark net. – SK19 Mar 20 '18 at 20:51
  • Can someone answer with a citation how you would be able to track someone using bitcoin? The link provided is a bit hard to understand. Does time effect how easy it is to track? For example if it happened a few years ago? – LateralTerminal Mar 20 '18 at 20:55
  • @LateralTerminal I think this would be a good question for this site :) – SK19 Mar 20 '18 at 22:12
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This might be a bit tricky because people generally tend to map binary data sequences to representations of real-life objects automatically, often conveniently slipping to lax and negligent assumptions. What is often left out is the importance of encoding, which is the essence that gives data a meaning. Without encoding, it's just a meaningless bitstream.

Now, the thing with the encoding is that, given a "right" encoding, you can basically turn any binary data into arbitrary "image" (or text; or give it any other meaning). Even this answer might be considered to convey an illegal material, if a carefully-crafted encoding scheme is used to interpret it.

When questioning the legality of binary data, a highly motivated prosecutor could surely come up with an encoding scheme which, when used to interpret the data in the Blockchain, would reveal an illegal material. However, a reasonable judge would probably dismiss it because it isn't the natural encoding intended to be used in the protocol. I'm not a lawyer, but I believe the intent would be a key factor here, so people using Bitcoin (and sharing the Blockchain) should not be prosecuted if they're only using it for the intended purpose, interpreting the data as specified in the Bitcoin protocol. Otherwise, anybody using the Internet could be prosecuted for anything.

On the other hand, a person who inserted such a data on purpose (re-encoding them to a compatible format and sending as a transaction output), could probably face a criminal prosecution, if reliably found.

Disclaimer: I'd like to add that law is not an exact science (actually is not exact nor science) and thus might be hard to grasp for technical people who strive to always find definite answers. Also, all decisions might be politically or otherwise motivated.

  • The "turn any binary data into arbitrary image" idea is simply wrong. An encoding cannot provide additional bits of information, only extend the representation of these bits. – SK19 Mar 20 '18 at 20:13
  • Not true. Even though the example was far-fetched and there are no "common" encodings doing this; you could surely come up with one. Example: take an image, xor it with some selected blockchain data and then make the result part of the definition of encoding function. Now, if you take the same data from blockchain and throw it into the just defined encoding function...yes, you get the image you always wanted to get :-) You can obviusly come up with any variation of this. It's silly, but so it is silly to interpret blockchain data using any other non-intended encoding. – Jozef Mar 21 '18 at 10:27
  • A character encoding is different from a program that gives a certain output on a certain input. A character encoding needs to return the same values for the same chunks of bytes. xor wouldn't work that way. You make it sound like an easy feast for a dedicated criminal programmer, but it really is not. – SK19 Mar 21 '18 at 11:01
  • I said "encoding", not "character encoding". There's infinite number of encodings and what I proposed would be a technically valid encoding. It was showed to make a point about natural and non-natural data interpretation in particular context. – Jozef Mar 21 '18 at 11:20

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