If I live in the US can I be prosecuted for answers describing illegal activities? The internet allows for some level of anonymity and occasionally you will find posts that give detailed descriptions of how to perform an activity that is illegal. Examples may include how to covertly grow marijuana, or swindle money. If I live in the US and make posts or provide answers to questions that explain how to perform an activity that is illegal in the US, can I be prosecuted? This question is about sharing the "how to" information only, not actually performing any illegal activity.
In general this is protected by the first amendment. It is not in general a problem describing how one can one can do something illegal. But there are special cases to be careful with. You might want to do some research into the limits on free speech. It would be hard to provide an answer that fully covers all your different cases and you would need to be more specific about what illegal activity you want to describe. In describing how to do something illegal, you might accidentally share information that you are not allowed to share. When you post things online, this can be considered as publishing or exporting. Therefore certain export restrictions might apply. Also, It is illegal to publish bomb making manuals, with the knowledge or intent that this information be used to commit a federal crime of violence. See https://www.law.cornell.edu/uscode/text/18/842. There are restrictions on publishing material relating to cryptography without having an export license. Granted, this isn't necessarily related to publishing things that are illegal, but just to give an idea about how publishing/exporting knowledge can causes problems. See https://en.wikipedia.org/wiki/Export_of_cryptography_from_the_United_States ITAR (International Traffic in Arms Regulations) sets restrictions on what you can publish about arms. What you publish can't be “directed to inciting or producing imminent lawless action.” See for example https://en.wikipedia.org/wiki/Brandenburg_v._Ohio. One might imagine that you could get into trouble if someone interprets what you do as inciting or producing a lawless action. It might sound obvious, but you want to make sure that you have the right to share the information that you have. The information that you are providing might be copyrighted in some way.
All laws (federal, state and local) apply to everybody, unless you have diplomatic immunity. That is, unless e.g. the federal government decides as a matter of policy to ignore certain federal laws. California does not have a law generally prohibiting the use of marijuana, though public consumption is illegal, minor consumption is illegal, and possession over 28.5 grams is illegal. So that is one less law to be concerned with violating. The federal law still exists, and has not been repealed for anyone. However, the federal government by policy is not pursuing marijuana cases in legal contexts in those states that have legalized marijuana. The complication for foreign students is that there are also immigration laws whereby you may be deported for a drug offense (that link is full of technical details on immigration and drug laws, worth reading). The immigration laws basically make it easy to penalize a foreigner (for example you might be deemed "inadmissible" so you cannot be re-admitted to the US if you leave; it just depends on what their grounds are for action). For example, "a noncitizen is inadmissible as of the moment that immigration authorities gain substantial and probative 'reason to believe' the noncitizen has ever participated in drug trafficking," which does not mean a conviction. It is reported that in California, DHS officers have treated minor infractions as "convictions," which means to be safe you have to avoid even infractions. It appears (and hire an immigration attorney if you want to test this) that trouble only arises if there is reason to believe you are trafficking, if you are a drug addict or abuser, if you are "convicted" (not necessarily "tried and found guilty," it also includes certain legal maneuverings), or if you admit to drug use (even in the case of home use under doctor's orders, i.e. a California-legal context). This incidentally includes non-use but working for the marijuana industry. It is possible that you could get stopped on the street by a random immigration search, and if you are in possession, then... it is not guaranteed that possession of a small amount of marijuana, when caught by federal authorities, cannot lead to immigration problems.
You might get away with it under federal law. But there are potentially also state laws to worry about. Under Texas law 17.461: &quot;Pyramid promotional scheme&quot; means a plan or operation by which a person gives consideration for the opportunity to receive compensation that is derived primarily from a person's introduction of other persons to participate in the plan or operation rather than from the sale of a product by a person introduced into the plan or operation. A person commits an offense if the person contrives, prepares, establishes, operates, advertises, sells, or promotes a pyramid promotional scheme. An offense under this subsection is a state jail felony. As you can see, there's no requirement under this law that there be fraud involved.
Some people seem to believe that just because something happens 'in the internet' it is somehow outside normal jurisdictions. Wrong. In may be harder to investigate and prosecute crimes in the internet, but the laws apply all the same. There are some problems when it is unclear 'where in the world' something did happen -- in the jurisdiction of the perpetrator, the victim, or the service provider? But problems of jurisdiction apply e.g. to international fraud cases in the non-web-world as well. In many jurisdictions, the informed and voluntary consent makes some things legal which would otherwise be illegal. For instance, if two boxers get into the ring, it is understood that each of them did consent to be hit by the other. But usually two fighters could not legally agree to a fight to the death, because even if there are laws on assisted suicide, they do not apply to a fight. Insults, libel, and slander are not on the same level as homicide. There are jurisdictions where they are not prosecuted without the request of the victim. But an insult might also violate other laws, e.g. disturbing the peace. So don't bet on such an app unless you know for sure which jurisdictions are involved.
No, a later trial is not allowed A prosecutor can, and often will charge multiple related crimes, and all will be addressed at the same trial. But once a person has been acquitted on a given set of events, the same jurisdiction cannot re-try the same person on what is often called a lesser included offense. Nor on a greater offense implied by the same events. Not even if additional evidence comes to light. However, if an act (or set of acts) is a crime under both state law and Federal law, for example theft by deception (state crime) and wire fraud (federal crime) one jurisdiction may try the person even after ther has been an acquittal in the other. I think the same rule applies if an act is a crime within the jurisdiction of two different states, that both can trey the accused. In many cases prosecutors will choose not to bring the second trial, but they can if they see fit.
Since there are cases where it is legal to break into a car, is it legal to publish a guide on how to break into a car online? It is legal to publish this guide. Indeed, it is legal to do so even if there are no cases where it is legal to do so. Does it need to have a disclaimer saying to comply with all applicable laws? No. Could the author be held responsible if someone uses the instructions to illegally break into a car? Generally not. I could imagine that there might be some very specific and exceptional fact pattern where it might, but that would be the rare exception. But see man sentenced to twenty-years in prison after pledging support to ISIS and uploading a bomb making video related to that pledge.
Most of the question has nothing to do with the law, it's about technical how-to or how-does, which should be asked in Information Security SE. There are two possible legal questions: is it legal to break into a computer system and take a database of passwords, and it is legal to acquire such a database obtained by someone else. As should be known in the US, per 18 USC 1030, breaking into a computer is illegal in the US. Given that, it is extremely unlikely that Google illegally breaks into other computer systems to obtain passwords. The aforementioned law criminalizes accessing computers without authorization, not (just) &quot;taking&quot; stuff from computers without authorization. The law does not criminalize receipt of illegally obtained material. Passwords are not protected by copyright. If Google were to induce someone to break into a computer system to get passwords, that would be legally actionable, however there is no law penalizing innocent receipt of illegally-obtained passwords (insofar as they are not protected by copyright). It is not illegal to access the dark web, at least in the US (probably it is illegal in Saudi Arabia). Using stuff gotten from the dark web can easily be illegal (e.g. logging in to someone's bank account, or forging a passport). There are many services which monitor the dark web and report breaches, which is totally legal.
You can be prosecuted for the crime in the U.S., both at the federal level and at the U.S. state level (or both), completely without regard to what happened in the criminal justice process elsewhere. This is true in all of the scenarios you pose, for any offense, and with or without an extradition treaty (of course, unless the treaty had some anomalous provision to the contrary or deprived the U.S. of jurisdiction by statute rather than constitutionally of this crime). The U.S. Supreme Court determined in Heath v. Alabama, 474 U.S. 82 (1985) that the double jeopardy clause of the U.S. Constitution is applied separately with regard to each sovereign involved and that each state and the federal government count as separate sovereigns. In the pertinent part, it states: The dual sovereignty doctrine is founded on the common law conception of crime as an offense against the sovereignty of the government. When a defendant in a single act violates the &quot;peace and dignity&quot; of two sovereigns by breaking the laws of each, he has committed two distinct &quot;offences.&quot; United States v. Lanza, 260 U. S. 377, 260 U. S. 382 (1922). As the Court explained in Moore v. Illinois, 14 How. 13, 55 U. S. 19 (1852), &quot;[a]n offence, in its legal signification, means the transgression of a law.&quot; Consequently, when the same act transgresses the laws of two sovereigns, &quot;it cannot be truly averred that the offender has been twice punished for the same offence, but only that by one act he has committed two offences, for each of which he is justly punishable.&quot; Id. at 55 U. S. 20. In practice, the U.S. Justice Department and most state and local prosecutors are disinclined to prosecute a crime that has already been handled by another jurisdiction and often have official, but non-binding, policies to that effect. In part, this is because an acquittal in one jurisdiction makes it likely that it is a weak case, while a conviction in one jurisdiction often constitutes sufficient punishment. There could be an issue under the 8th Amendment to the U.S. Constitution (cruel and unusual punishment) over whether a punishment imposed in a U.S. conviction for a crime needs to consider the already severe punishment imposed in another jurisdiction for the same crime in order to prevent the cumulative punishment from being cruel and unusual. But, I am not aware of authoritative case law that resolves that constitutional question. Certainly, evidence of a prior punishment for the same offense could be presented at a sentencing hearing following a conviction in mitigation of the punishment that should be imposed. Indeed, in some states time served pursuant to a conviction for the same crime in another jurisdiction might statutorily count as &quot;time served&quot; for which the defendant is legally entitled to credit at sentencing.