Illegal Hashes

First sentence is true. Second sentence is false - that's called a preimage attack. MD5 is broken for collisions, not preimage or second preimage.

> It is somewhat cheap and easy to produce a file with a specific SHA-1 hash.

No, that's called a preimage attack. SHA-1 is broken for collision, not preimage or second preimage.

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Please see this section for the 3 core properties of a cryptographic hash function: https://en.wikipedia.org/wiki/Cryptographic_hash_function#Pr... . They appear similar but are very different.

In a collision attack, you are allowed to manipulate both message0 and message1 as much as you like until hash(message0) = hash(message1). Because the attacker has so much choice, this is the hardest attack to defend against, and this is how almost every hash function has failed.

In a second preimage attack, you are given a target message0 (which you cannot change), and you can manipulate message1 as much as you want until hash(message1) = hash(message0). This would be like making a new SSL certificate with the same hash as another publicly available certificate belonging to someone else that you cannot modify.

In a preimage attack, you are given a target hash value h (which was generated from some secret message0 that is not known to you), and you can manipulate message1 as much as you want until hash(message1) = h. This would be like reversing a password hash back to a password.

It’s unclear that illegal numbers exist outside of intent to communicate the illegal idea. Otherwise all irrationals including pi and e are illegal. And it’s probably possible to view/interpret any data you have in some way that happens to coincide with an illegal number. If anyone was going to have legal trouble for coincidental unintentional hashes, then everyone has a problem, which is exactly why it’s unlikely to go there, it’s clearly absurd.

> Decoded into ASCII, that spells I hate the queen

This is an example of what I mean - while this hash is theoretically possible, note that hashes are not typically ever displayed as ASCII, so why ASCII? That was a narrative choice in this article. Hashes are almost unanimously displayed as hex, or base64, which have no spaces for good reasons. Side note, it’s extremely rare for a hash to ever make sense when interpreted as ASCII due to needing zero entropy in the first bit of every byte for that to happen (observe they are all zero).

«All—the detailed history of the future, the autobiographies of the archangels, the faithful catalog of the Library, thousands and thousands of false catalogs, the proof of the falsity of those false catalogs, a proof of the falsity of the true catalog, the gnostic gospel of Basilides, the commentary upon that gospel, the commentary on the commentary on that gospel, the true story of your death, the translation of every book into every language, the interpolations of every book into all books...» (J.L. Borges, The Library of Babel.)

More interesting is to analyze the probability of finding a file with a hash form a particular set. MD5 gives 160 bit; if it's a good hash that does not have large gaps in its values, this is roughly 1.46e+48 values.

Let the set of the forbidden values have 1.46 billion elements, that is, 1.46e+9. This should be enough for all the hashes of copyrighted works, and all the forbidden blasphemous utterances. Then the chance of hitting one of them is going to be 1e-39.

This chance is so low that it's hard to imagine. Take a mega-lottery with 10M = 1e+7 tickets; it's the chance of winning it six times in a row, by buying a single random ticket every time.

This is, of course, assuming that MD5 has no preimage attack; AFAIK such an attack is not known by now.

It is not too difficult to make up data that the first few bits of the hash match some specific number (although it is a bit slow). For example, you can adjust values of pixels in a picture so that if you make the hash of each scanline independently and then put together the first few bits of the hash of each scanline then it will make up some specific text. Or, to collide the first few bits of a commit (or a file) of a git or fossil repository.

You can also do many things involving XOR of hashes with other data (whether you have two or possibly more than two sets of data), etc.

But, copyright is bad anyways, and laws that you cannot freedom of speech (e.g. to tell you if you hate or don't hate the queen), is also bad anyways.

For example, you can present two or more different bit strings that appear random, but when XOR'd together produces some "illegal" content.

Which of these is illegal to transmit?

When you imagine those people going through every available media and looking for insults against the royalty, and finding one way or another to interpret it that way so they could crack down on the writers - how insane they must have been, or worse malicious, and sometimes just sadistic.

If I were to offer some advice based on it, I would suggest that if you think a hash can be problematic, to reflect seriously on where the problem may lie.

https://news.ycombinator.com/item?id=24917679

The basic idea is that the bits can be illegal not based on the bits themselves, but on the intent with which they're created, the way they're presented, how they're transmitted, etc.

You see, nerves transmit impulses by allowing charged particles called ions to enter the nerve cell.

All actions a person does could probably be described as a series of ion movements.

If those ion movements result in you hitting your neighbor in the nose, then you will probably get in trouble.

Hence, illegal ion movements.

I've never heard this? Who can I pay to create a file with the specific hash "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef" and how much does it cost?

Also, try to not infringe where you live.

It's quite simple once you understand the basics of law.