SANS ISC: Bitcoin – crypto currency of future or heaven for criminals?

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SANS ISC InfoSec Forums

There has been quite some coverage about Bitcoin in last couple of months. For those that did not pay attention, Bitcoin is a crypto currency that is decentralized and works in a peer-to-peer network. It is a pretty fascinating project by a Japanese researcher (maybe – his real identity has not been confirmed) and in case you are interested in it you can find some information at http://www.bitcoin.org/.

Some background

Couple of weeks ago I started doing some research on how Bitcoin works. I found it amazing that for a scheme so wide spread (there are probably tens of thousands, if not hundreds of thousands of active users) that not a lot of technical documentation is available, apart from Satoshi’s paper available on the main web site, which does not really go into implementation details.

One of the features of Bitcoin that gets mentioned quite often is its anonymity. Basically, Bitcoin has a digital wallet which allows you to process incoming and create new transactions. A user has one or more (preferably many) public/private key pairs which identify him. In the Bitcoin system, when you want to send Bitcoins to someone, you sign a transaction that is taking some of your Bitcoins (which you received through a transaction or mining – more about this later) to the destination address. All addresses are unique 40 digit hexadecimal numbers (RIPEMD160(SHA256(public key)) with some extra conversion to Base-58.

You can have as many as you want of these and this is one of the ways for Bitcoin to allow anonymity. Since you can use a different public/private key pair for every transaction (and you can transfer Bitcoins to your other addresses) it can be difficult (but not impossible) to track the owner. One thing to keep in mind is that all Bitcoin transactions are public – every node knows everything about every transaction.
There is some interesting research about tracking Bitcoin owners and Dan Kaminsky posted some good ideas at this year’s Black Hat.

How do you get new coins?

In order to confirm a transaction, it has to be included in a block. A block (https://en.bitcoin.it/wiki/Blocks) contains a hash pointing to a previous block (so the blocks are chained, this is what makes spoofing exponentially difficult with generation of more blocks), some other data and a Merkle root hash of all transactions validated by this block.

Now comes the best part – all this data is hashed together (SHA256(SHA256(block)) and the resulting hash has to satisfy some requirements. The requirements state that the resulting hash has to start with a certain number of zeros. So, for example if the resulting hash has 7 leading zeros it is valid. How do we find a valid block? Besides the payload a nonce is embedded to which gets constantly changed.

Simply speaking, the node that is generating the block brute forces all possible values until it finds a valid hash that satisfies the previously mentioned requirement. As you can see, this is an extremely complex task that, with fastest gear (and I’m talking about loads of GPU cards) can take days if not months.

So a logical question is: why would anyone do that? The node that finds a valid block (mines it, in Bitcoin’s terminology) gets awarded (currently) 50 Bitcoin. With 1 Bitcoin being around 7.3 USD currently, this means that for each solved block the node that found it gets ~350 USD. Sounds good?

Besides this, the solver also gets a certain fee for transactions that have been validated so in reality more than 50 Bitcoin will be awarded to the solver (this is the incentive to keep solving the tasks even after all Bitcoins have been awarded).

Finally, another important thing about blocks is that it should take approximately 10 minutes to solve a block. The network itself measures how long it took to solve 2016 blocks (it should be about two weeks) and modifies the difficultly accordingly (so if more people start solving this, the difficulty gets higher).

My CPU > your CPU

There are legitimate groups of users that join so called mining pools in order to find new valid hashes. The pool owner runs a special algorithm that sends partial tasks to all nodes participating in mining. Different pools have different rules, but today it is common that they share received Bitcoin between participating nodes, depending on how much each node has participated.

There are many open source, free Bitcoin mining programs that are specially optimized for GPU’s.
And imagine this – who has the most CPU power in the world (except government agencies)? Bot owners of course.
In other words, it was to be expected that bot owners will start playing this game – after they’ve stolen all valuable data off a machine, why wouldn’t they use its resources (CPU, GPU and power) to mine Bitcoins and make some extra cash (which even looks anonymous!).

Couple of months ago we started first seeing malware stealing Bitcoin wallets (basically doing transactions to their owners) and lately Bitcoin mining pools used by malware started being increasingly popular.
Modus operandi is typical here – malware drops legitimate bitcoin mining executables which join a pool operated by the botnet owner. In most cases I’ve seen so far they use standard protocols so be sure to check the 8333 TCP port. Bitcoin also uses IRC for initially finding other nodes so it might easily make your IDS/IPS shine like a Christmas tree (even if a legitimate user started it).

Perfect extortion weapon

Just about when I was to finish this diary (which will probably be only first in the series about Bitcoin), we received a very interesting e-mail from one of our readers who wanted to remain anonymous.

He received an e-mail from an attacker asking him to pay 100 Bitcoin to a certain address or his site will be a target of a DDoS attack. We’ve seen such extortion e-mails many times in the past (as always – do not pay) but using Bitcoin is a new twist.

As I previously wrote, while it is not 100% anonymous, it can be very close to this and, depending on how careful the attacker is, it can be very difficult to trace the transaction.

As Bitcoin is gaining more attention it will be interesting to see what future will bring. Rest assured that we will keep an eye on it.

--
Bojan
INFIGO IS