user78: Suppose I had an ultra large ASIC from the future, and could perform a double spend on the core Bitcoin network. At this point in Bitcoin's development, it is no longer a game - there's real money involved.
Would performing a successful double spend be considered illegal (e.g. in the U.S / Europe)?

user153: Mining isn't necessarily any different between when done with 2% of mining power than with 52% -- you are simply performing hashes and solving blocks.
Now when you have more than 50%, the options available to you change.  You have the ability to do a transaction, to let that transaction get a confirmation maybe even, and then to release blocks that make it as if that original transaction never happened and to spend the coins a second time to secure the funds.
You really can't double spend accidentally.  So that would be an action performed with the intent to defraud.  Nearly everywhere fraud is illegal.
Now whether a Bitcoin double spend would be something that a jury would convict on, who knows.

user942: Double spending would be fraud.

user68849: One thing to consider is that the purpose of the 51% attack may not to be to double spend the same bitcoins.  The purpose could be to disrupt the network and profit from a short.  This kind of attack would also need to be considered.

user516: The only way I could see a double spend attack being illegal in any part of the world would be if there was some kind of law that automatically recognises international alternative currencies in the first place.
I'm no lawyer, but I've never heard any mention of such a law existing.
Proprietary systems that create currency, points or tokens often have some kind of contract or T&C's that promise to give you something in return for the units and they also have conditions that you must meet to use them. Bitcoin however has no such contract for users to agree to, and no central authority to form an agreement with. 
Further to that, even if a law in one particular country somehow made a double spend attack illegal, the international nature of Bitcoin means the attack could be performed from elsewhere.
Note that it may not be in your best interest to perform a double spend attack either. A successful, sizeable double spend would likely draw widespread attention and could shake the confidence in Bitcoin, maybe causing the price to plummet. Quietly mining 51% of the coins over a long period would likely earn you more profit.

user85: It's probably not criminal in many jurisdictions. It should be fraud, but it's not quite in most jurisdictions. For example, in the United States, the elements of fraud are:
a representation of an existing fact;
its materiality;
its falsity;
the speaker's knowledge of its falsity;
the speaker's intent that it shall be acted upon by the plaintiff;
plaintiff's ignorance of its falsity;
plaintiff's reliance on the truth of the representation;
plaintiff's right to rely upon it; and
consequent damages suffered by plaintiff.

All of these are clearly met by the typical double spend except the first one. There really is no "existing fact" that is misrepresented.
However, you can certainly be sued for unjust enrichment and breach of the implied covenant of good faith and fair dealing implicit in any agreement.

user1878: Let's ignore the crypto-currency part of Bitcoin, the 51% attack you're talking about is a form of computer hacking, and there are laws to protect against this.
My opinion is that any law that applies to "hacker 'penetration testing'" applies to hacking bitcoin in addition to whatever else the law can throw against such an attacker.

Don't do it! Don't do it! If you are in the US, the law is very broad.
  You don't want to even tiptoe up to the line.
The relevant law is the Computer Fraud and Abuse Act (18 U.S.C. 1030).
  In a nutshell (and simplifying slightly), under the CFAA, it is a
  federal crime to "intentionally access a computer without
  authorization or exceed authorized access". This language is very
  broad, and I imagine an ambitious prosecutor could try to use it to go
  after everything on your list except #1 (view source).
Orin Kerr, one of the leading legal scholars in this area, calls the
  statue "vague" and "extraordinarily broad", and has said that
  "no one actually knows what it prohibits".
And, as @Robert David Graham explains, there have been cases where
  folks were prosecuted, threatened with prosecution, or sued for doing
  as little as typing a single-quote into a textbox, adding a ../ to a
  URL, or signing up to Facebook under a pseudonym. It's pretty wild
  that this alone constitutes a federal offense, even if there is no
  malicious intent. But that's the legal environment we live in.
I'd say, don't take chances. Get written authorization from the
  company whose websites you want to test.

source
Although that above link applies to a specific company, I'm sure the distributed nature of Bitcoin could land a hacker in hot water in several countries, not to mention the attacker will also be wanted by the entire community of Bitcoin users (both good people and unsavory users).
If you want to test, be open and transparent about your actions (tell people on the forums) and only do your tests on the test network.  (yes there is a dedicated bitcoin network for testing that makes it pretty easy to apply a 51% attack on)

user1925: A double-spend would be blackletter law illegal, guys.  Fraud and theft include a wide variety of intentional deception wrongfully depriving someone of property - use of a government-minted currency is not, and never was, a required element.  Look at any larceny or fraud statute - it will refer to loss of or damage to person and property not "dollars."
Schwarz - when you double-spend, you are representing to the seller as an existing fact that you are passing a valid bitcoin, which is a misrepresentation.  Your representations as to fact are readily, and usually, implied from conduct in fraud and "larceny by trick." You don't have to literally state them in words.  this goes for more than bitcoin.  If you were considering stealing puppies to return them for a reward, it's a legal problem for the same reason.  But do I really have to tell you this?
Of course, these are just general legal notes, and not legal advice.  Before you consider undertaking any questionable business activity, consult an attorney.

78: <p>Suppose I had an ultra large ASIC from the future, and could perform a double spend on the core Bitcoin network. At this point in Bitcoin's development, it is no longer a game - there's real money involved.</p>

<p>Would performing a successful double spend be considered illegal (e.g. in the U.S / Europe)?</p>
 153: <p>Mining isn't necessarily any different between when done with 2% of mining power than with 52% -- you are simply performing hashes and solving blocks.</p>

<p>Now when you have more than 50%, the options available to you change.  You have the ability to do a transaction, to let that transaction get a confirmation maybe even, and then to release blocks that make it as if that original transaction never happened and to spend the coins a second time to secure the funds.</p>

<p>You really can't double spend accidentally.  So that would be an action performed with the intent to defraud.  Nearly everywhere fraud is illegal.</p>

<p>Now whether a Bitcoin double spend would be something that a jury would convict on, who knows.</p>
 942: <p>Double spending would be fraud.</p>
 68849: <p>One thing to consider is that the purpose of the 51% attack may not to be to double spend the same bitcoins.  The purpose could be to disrupt the network and profit from a short.  This kind of attack would also need to be considered.</p>
 516: <p>The only way I could see a double spend attack being illegal in any part of the world would be if there was some kind of law that automatically recognises international alternative currencies in the first place.</p>

<p>I'm no lawyer, but I've never heard any mention of such a law existing.</p>

<p>Proprietary systems that create currency, points or tokens often have some kind of contract or T&amp;C's that promise to give you something in return for the units and they also have conditions that you must meet to use them. Bitcoin however has no such contract for users to agree to, and no central authority to form an agreement with. </p>

<p>Further to that, even if a law in one particular country somehow made a double spend attack illegal, the international nature of Bitcoin means the attack could be performed from elsewhere.</p>

<p>Note that it may not be in your best interest to perform a double spend attack either. A successful, sizeable double spend would likely draw widespread attention and could shake the confidence in Bitcoin, maybe causing the price to plummet. Quietly mining 51% of the coins over a long period would likely earn you more profit.</p>
 85: <p>It's probably not criminal in many jurisdictions. It should be fraud, but it's not quite in most jurisdictions. For example, in the United States, the <a href="http://en.wikipedia.org/wiki/Fraud#United_States" rel="nofollow">elements of fraud</a> are:</p>

<pre><code>a representation of an existing fact;
its materiality;
its falsity;
the speaker's knowledge of its falsity;
the speaker's intent that it shall be acted upon by the plaintiff;
plaintiff's ignorance of its falsity;
plaintiff's reliance on the truth of the representation;
plaintiff's right to rely upon it; and
consequent damages suffered by plaintiff.
</code></pre>

<p>All of these are clearly met by the typical double spend except the first one. There really is no "existing fact" that is misrepresented.</p>

<p>However, you can certainly be sued for unjust enrichment and breach of the implied covenant of good faith and fair dealing implicit in any agreement.</p>
 1878: <p>Let's ignore the crypto-currency part of Bitcoin, the 51% attack you're talking about is a form of computer hacking, and there are laws to protect against this.</p>

<p>My opinion is that any law that applies to "<a href="https://security.stackexchange.com/search?q=%2b%5Bpenetration-test%5D%20%2blegal">hacker 'penetration testing'</a>" applies to hacking bitcoin in addition to whatever else the law can throw against such an attacker.</p>

<blockquote>
  <p>Don't do it! Don't do it! If you are in the US, the law is very broad.
  You don't want to even tiptoe up to the line.</p>
  
  <p>The relevant law is the Computer Fraud and Abuse Act (18 U.S.C. 1030).
  In a nutshell (and simplifying slightly), under the CFAA, it is a
  federal crime to "intentionally access a computer without
  authorization or exceed authorized access". This language is very
  broad, and I imagine an ambitious prosecutor could try to use it to go
  after everything on your list except #1 (view source).</p>
  
  <p>Orin Kerr, one of the leading legal scholars in this area, calls the
  statue "vague" and "<a href="http://www.volokh.com/2011/05/24/congress-considers-increasing-penalties-adding-mandatory-minimum-sentences-to-the-computer-fraud-and-abuse-act/" rel="nofollow noreferrer">extraordinarily broad</a>", and has said that
  "<a href="http://www.volokh.com/category/computer-fraud-and-abuse-act/" rel="nofollow noreferrer">no one actually knows what it prohibits</a>".</p>
  
  <p>And, as @Robert David Graham explains, there have been cases where
  folks were prosecuted, threatened with prosecution, or sued for doing
  as little as typing a single-quote into a textbox, adding a ../ to a
  URL, or signing up to Facebook under a pseudonym. It's pretty wild
  that this alone constitutes a federal offense, even if there is no
  malicious intent. But that's the legal environment we live in.</p>
  
  <p>I'd say, don't take chances. Get written authorization from the
  company whose websites you want to test.</p>
</blockquote>

<p><a href="https://security.stackexchange.com/a/6370/396">source</a></p>

<p>Although that above link applies to a specific company, I'm sure the distributed nature of Bitcoin could land a hacker in hot water in several countries, not to mention the attacker will also be wanted by the entire community of Bitcoin users (both good people and unsavory users).</p>

<p>If you want to test, be open and transparent about your actions (tell people on the forums) and <em>only</em> do your tests on the test network.  (yes there is a dedicated bitcoin network for testing that makes it pretty easy to apply a 51% attack on)</p>
 1925: <p>A double-spend would be blackletter law illegal, guys.  Fraud and theft include a wide variety of intentional deception wrongfully depriving someone of property - use of a government-minted currency is not, and never was, a required element.  Look at any larceny or fraud statute - it will refer to loss of or damage to person and property not "dollars."</p>

<p>Schwarz - when you double-spend, you are representing to the seller as an existing fact that you are passing a valid bitcoin, which is a misrepresentation.  Your representations as to fact are readily, and usually, implied from conduct in fraud and "larceny by trick." You don't have to literally state them in words.  this goes for more than bitcoin.  If you were considering stealing puppies to return them for a reward, it's a legal problem for the same reason.  But do I really have to tell you this?</p>

<p>Of course, these are just general legal notes, and not legal advice.  Before you consider undertaking any questionable business activity, consult an attorney.</p>
