I think Stux could be modified to do something like this.
Not really. It simply doesn't make sense. If a third party wanted to attack the Bitcoin nodes with a virus, it would be
much easier for them to write a new virus for this purpose as opposed to changing an existing one like Stuxnet (which wasn't even very successful as a virus, to begin with). For the original creators of the virus, it would be much easier, too. They have a framework for this purpose, so it's much easier to use it to build a new malware from the modules they already have than to modify something that they have already built (and which is known to the anti-virus community).
The stux worm released on Iran was very sophisticated
It was nothing exceptional. Oh, sure, it has interesting properties, like being obviously written by a defense contractor (ever heard the saying that an elephant is a mouse built by a committee to government specifications? Well, Stuxnet is a virus built by a "committee" - several teams not communicating with each other and only producing code modules matching a specification), it was attacking a SCADA system, it was used as a weapon against a country, and it gained wide notoriety in the press. But, as a virus, it was nothing special.
If you want sophistication, how about Flame or Gauss? They were both written by the same outfit that came up with Stuxnet, using the same (or similar) famework.
Flame was
huge - about 20 Mb! Four years later, we still don't know everything it could do - because how do you analyze 20 Mb of compiled code and linked libraries?! It even had a virtual machine and a Lua interpreter for some of its parts. Command-and-control, replication on demand, SQL injection, audio and video interception, backdoors, zero-day exploits, keylogging, encryption, compression, Bluetooth sniffing... Flame had it all. It even used an
unknown till then collision attack to crack MD5 and fake Microsoft Update. (Microsoft stopped using MD5-based certificates because of Flame.)
Gauss, clearly produced by the same outfit, is my personal favorite, because it implemented an attack I predicted in the late 90s. Google "clueless agents" - Bruce Schneier has a
nice paper about them. Gauss has a practically textbook implementation of them. We don't know what it does. It looks for directory paths by doing H(H(path)) where H() is a cryptographically strong hash function and then H(path) is used as the decryption key. We do know H(H(path)) - it's in the virus - but we have no clue what the path is, so we can't compute H(path) and decrypt the encrypted payload of the virus. (I am over-simplifying here - the hash-of-hash is not done once but 1000 times and the key is not a simple H(path) but of a more complex data which is derived from the path.) Although the hash function is MD5 and the cypher is RC4, both of which are considered nowadays cryptographically insecure, in practice we haven't got a snowball's chance in hell of decrypting the payload of the virus and understanding what it does...
For a more technical description of the issue, see
this.