Can governments spam or ddos the bitcoin network to death?

[Steve]

The end result of a successful attack on Bitcoin will be a newer and better Bitcoin2.  Bitcoin itself is merely the latest of many iterations on the concept of virtual currency, each of which being an improvement (in one form or another) on earlier iterations.  It would be pretty pointless (even detrimental) to attack Bitcoin on the belief that it will somehow hinder progress.  Competition would be more effective, but I suspect no competitor will be successful if it regresses in terms of features as compared with Bitcoin (including and especially Bitcoin's privacy features).

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[hardcore-fs]

This is how an intelligent adversary would destroy Bitcoin:

https://bitcointalk.org/index.php?topic=128900.msg1374756#msg1374756

The process works like this: First, use a deceptive narrative to create the illusion of a crisis then use the false crisis as an excuse to push a protocol change which reverses Bitcoin's fundamental nature.

In this case the narrative is that blockchain bloat is a problem that can't be solved without a protocol change, and that a reference client implementation detail like anti-spam rules are impossible to change.

The proposed solution is to change the protocol to allow outputs to be spent without a signed transaction. If that ever happens Bitcoin as we know it is dead. Miners would the means and the economic incentive to claim unspent output for their own use without the consent of their owners (otherwise known as stealing).

A smart adversary would combine this tactic with the appropriate carrot and stick incentives applied to key individuals to either neuter or destroy Bitcoin.

I had been giving the Bitcoin network some thought and how Bitcoin might be exploited, unfortunately  during my travels I answered this very question.
It would require way less work than above (in that it would not need to change the bitcoin protocol or system.)

I've already got the skeleton of the software written, it just needs to be 'scaled up'
I get bored easily, so  If these damned ASIC suppliers do not get their act together soon I'm thinking of writing a paper on it....

[Sergio_Demian_Lerner]

The best government attack is to legally forbid Bitcoin and force ISPs to block Bitcoin traffic. Then Bitcoin would start a cat and mouse race to obfuscate the protocol data/port and that would hurt Bitcoin and prevent a wider adoption of the system.
This attack probably costs a few hundred USD.

I suspect that you are grossly underestimating the costs associated with passing the law, enforcement of the law, and engaging in the "cat and mouse race".  Furthermore, it would have to be "forbid" in all countries.  Otherwise bitcoin would continue to exist and function outside the jurisdiction of the countries that forbid it.

Yes, I was.  I just wanted to point out that this is the most easy way to crack Bitcoin for goverments.

The best private company attack is to earn BTC by market manipulation. This attack probably costs 1M+ USD to control the coin.

I'm not sure that I'd consider this an attack.  This sounds like a legitimate business practice to me, although I'll admit I haven't taken a lot of time to consider all the implications.

In almost all countries market manipulation is forbidden. It is prohibited in the United States under Section 9(a)(2).

The best crybecriminal attack is to exploit vulnerabilities added on purpose to steal BTC from nodes. This attack probably costs 50K - 200K USD and may include bribery to one of the core devs, github or sourceforge.net programmers or ISP workers. Probably the easiest way is by doing a man-in-the-middle attack of Windows binary downloads, since nobody checks fingerprints.

You are mistaken when you say "nobody checks fingerprints". Many don't, but enough do.  Word would spread pretty fast once someone found that the software they downloaded didn't match the fingerprint.  

Then why there is no link to the fingerprint in the Bitcoin.org main page? Why is Bitcoin running on http and not on https anyway?
The fingerprint is only accessible through sourceforge.org (SHASUMS.asc) which is also not secured by http.
You see, it's very easy to forge the binary, the public key and the fingerprint.

The best gray hat hacker attack is to find ways to DoS the network in order to manipulate the coin price. This attack probably costs 10K - 20K USD.

Bitcoin is distributed/decentralized. There is no central server to mount a DoS attack against.  You might manage to mount a DoS attack against an individual (or a few individuals), but that wouldn't affect the rest of the network.

My own research says that it's quite easy and cheap to hire 5000 virtual IPs and, with some time, manage to be connected to every other Bitcoin node.