In the UK (and, I believe, the USA) child pornography is a "strict liability" issue — intent is irrelevant, mere possession is itself illegal. This is how teens get nailed for receiving unasked-for sexts from their under-age-of-consent SO.
As with illegal drugs and, in the UK, unlicensed firearms, this means the prosecution doesn't need to prove intent. (Mandatory — usually harsh — sentence terms are usually part of the package with strict liability offenses.)
There IS a mental element to possession in UK law:
> Section 160(1) creates an offence for a person to have any indecent photograph or pseudo-photograph of a child in his possession. [...] "Possession" involves both a physical and mental element. [...] The mental element is knowledge. A defendant must knowingly have custody and control of the photographs found on the device in question.
Downloading the blockchain, should it contain such images, would constitute "making", but it subject to similar mental requirements:
> The act of making or taking the indecent photograph or pseudo-photograph must be a deliberate and intentional act, done with the knowledge that the image made is, or is likely to be, an indecent photograph or pseudo-photograph of a child.
I suspect that not knowing that the blockchain does contain such images, not having knowledge of which transactions do, and that it requires specialised decoding rather than just being embedded per se/a default/intended usage would be enough to raise a defence.
So if a person sees you are about to have your phone searched by police they can send you child pornography and you will go to jail? It somehow doesn't sound like it would hold up in court. At least I feel like it should not.
We should probably be mentioning which laws (and subsections) we're talking about because:
"The presumption of mens rea is rebutted by express provision in the statute excluding the requirement of mens rea. Where the statute is silent as to the requirement the general presumption remains, however, the courts may look at other offences created under the same Act. If the other offences expressly require mens rea, the courts may well take the view that the omission to refer to such a requirement was deliberate and that Parliament intended to create an offence of strict liability." [0]
I don't know enough about UK laws or laws in general, but it should be easy to discredit such law. It only takes one rebel tech savvy teen to send his nude sext to whatever celebrity/politician email/phone he can guess while alarming the police - and the press to stir as much shit as possible.
Actually, pardon me for a minute, I have sci-fi short to write :).
In the UK, the police and prosecutors tend to take a common-sense approach: the "rebel tech savvy teen" would be the one who ended up prosecuted for possession.
(Similarly: possession of an unlicensed gun carries a stiff prison sentence. But if someone chucks a pistol on your lawn and you without delay call the police and ask them to take it away, you're probably safe. Picking it up and taking it inside is another matter, however ...)
The USA is a bit different. District Attorneys being elected means they have an incentive to bring charges against "soft" targets who'll take a plea bargain, i.e. hapless teens and people too poor to afford a decent defense lawyer.
"Unsolicited Photographs" is a specific defence to possession:
> The defence is made out if the defendant proves that the photograph in question was sent to him without any prior request by him or on his behalf and that he did not keep it for an unreasonable time.
But we are talking here about not exactly "possession". Imagine I sent you a funny video, but somehow, I appended it with child pornography image, which your video player will easily skip, unless you are specifically looking via forensic measures.
As with illegal drugs and, in the UK, unlicensed firearms, this means the prosecution doesn't need to prove intent. (Mandatory — usually harsh — sentence terms are usually part of the package with strict liability offenses.)