Before taking a step in Investing to Bitcoin, you might just want to read this first!
We have all seen the meteoric rise of the beloved cryptocurrency this year. It’s price as of yesterday shot over $US10,000. This has seen a flurry of new investors asking questions on how to enter the market. But before you do, take a moment to consider the risks like you would with any major investment. One of those risks is the use of online wallets and online exchanges that may not do the job of holding your money very well.
Today, a security research came to our knowledge and a writer found a very interesting read as well as a particularly nasty vulnerability open to major exploitation of unsuspecting investors. The original research was published by an anonymous researcher who wishes to remain anonymous due to the highly sensitive nature of the situation and the potential loss of large sums of money to many people.
He writes “Hi. Thanks for passing this along so it gets some attention. I was worried if I posted this somewhere it would mostly go unnoticed. Also, I'm trying to stay anonymous because I don't want to be accused of being the person who came up with this exploit or be blamed by any company for any damages. It's an interesting technical story so I thought I would share it.”
I have evidence that some bitcoin address generation code in the wild is using private keys that can easily be discovered on a regular basis. This is either intentional or by mistake. Some wallets have been compromised by what is probably an innocent looking piece of code. Furthermore, someone has been siphoning bitcoin on a regular basis since 2014 from them. Whether they discovered this by accident (like I did) or are the ones who installed the code themselves, I don't know. It looks like either a clever exploit or a coding error. It could also be yet another piece of malware, however as I explain below, I feel this is less likely the case. In order to fully understand how this works and how I discovered it, please read on.
I've been following bitcoin since I first heard of it in 2011. One of the things that fascinated me was the ability for someone to create private keys from just about anything using Sha256 (i.e. Sha256(password/phrase)). This, of course, is NOT a recommended way of obtaining a private key since if YOU can think of the word/phrase, someone else can too and the likelihood of your bitcoins being stolen is quite high. The most secure private keys are generated randomly. The probability of someone else being able to generate the same sequence of 32 random bytes is so close to 0, it is highly improbable anyone ever will (given the expected lifespan of the universe). If you peer into the blockchain, you will find that people have 'played' with the chain by sending small amounts of bitcoins to addresses corresponding to private keys generated using Sha256. For example, Sha256 of each word in the entire /usr/dict/words file found on most UNIX systems has had a small amount sent to it. There was a site called brainwallet.org that made it easy for you to convert a phrase into a private key + public address. (The code is still available on GitHub but has since been removed from the Internet). Try using phrases like "i find your lack of faith disturbing", "these aren't the droids you're looking for" or "satoshi nakamoto" as inputs to Sha256. You'll find the addresses corresponding to those private keys have had small amounts sent to them (and transferred out). It's quite obvious these were _meant_ to be found. It turns out there are a lot of these addresses. (Keep looking and you will easily find some.)
This is nothing new and has been known to the bitcoin community for a while. I always had the idea in the back of my mind to try and find other non-trivial examples of 'discoverable' private keys. That is something beyond Sha256(word/phrase). So I decided to try and hunt for buried bitcoin treasure. Perhaps I could find some bitcoin intentionally hidden by someone that hadn't yet been discovered? In the first couple weeks of June 2017, I finally devoted some time to the task. I honestly didn't expect to find much but I was amazed at what I ended up discovering. I began by writing a program to scan every block in the blockchain and record every public address that had ever been used. (Note: I didn't only store addresses for which the balance was greater than zero, I stored ALL of them which is why I believe I ended up accidentally discovering what I did.) There were only about 290 million at the time so this wasn't a big deal.
What follows is a description of my experiments and what led me to discover what I believe is either a scam or really bad coding error.
My first experiment was to see if anyone used a block hash as a private key. That would actually be a nifty way to 'compress' 32 bytes in your head. You would only have to remember the block height (which is only maybe 6 digits) and the corresponding larger 32 byte number would be saved for all time in the chain itself! Results: Success! I found 46 addresses that had some amount of bitcoin sent to them between 2009 and 2016.
As expected, these all had 0 balances either because the owner had taken them back or they were discovered by someone else. Here are two examples. You can use blockchain.info to see these hex values are actually blocked hashes from early in the chain.
This happened on/off up until mid-2016.
Nothing really alarming so far.
Similar to my first experiment, I then searched for addresses that were generated from the merkle root used as a private key. (BTW, I searched for both compressed/uncompressed keys, so each 32 bytes resulted in two address look-ups from my database).
I found 6 addresses again up until mid-2016. Even though every address I found had a 0 balance (again expected), I was having fun with my success!
Similar to Merkle root and block hashes, transaction ids (hashes) also seem to have been used as private keys. Still nothing alarming to me thus far.
I wondered at this point if anyone might have used repeated Sha256 on words. Why stop at just one iteration when you can easily do one million? Also, it becomes less likely to be discovered the more iterations you do. I found a bunch. Here are a few:
Sha256('sender') x 2
Sha256('receiver') x 2
Sha256('hello') x 4
Sha256('hello') x 65536
Sha256('password') x 1975 (This one's my favorite, someone's birth year?)
People were obviously burying bitcoin in the chain. Whether they expected the coins to be taken or not, we'll never know. But these methods were still highly 'discoverable' in my opinion.
My last experiment is the one that led me to believe someone was siphoning bitcoin from some service on a regular basis and has been since 2014.
Take a look at this private key:
The public address is: 13JNB8GtymAPaqAoxRZrN2EgmzZLCkbPsh
The raw bytes for the private key look like this:
Looks random enough. However, these bytes are actually sha256 of this public address!
I discovered this by performing Sha256 on all the public addresses I had collected from the setup of my experiments and then seeing if those addresses (from the generated private keys) were ever used. Bingo! Lots were coming up. I searched a fraction of the chain and found dozens. I also found these addresses had bitcoin sent to them very recently (within weeks/days of when I discovered them.)
I asked myself, "Why would someone do this?"
At first, I thought this was someone who thought they could get away with having to remember only one piece of information rather than two. Maybe they have one favorite address/private key combo and derived another from that one? I thought it was possible. You could keep doing this in a chain and derive as many as you wanted and only ever have to remember the first one. But I ruled this out for one simple reason; bitcoins transferred into those addresses were being transferred out within minutes or SECONDS. If someone generated these private keys for themselves, then why would the coins be almost immediately transferred out in every case I looked at?
Here are some more (complete list at end of this doc):
16FKGvEtu5KPMZqiTK4yjmsSZsJLyxz9fr from Sha256(1CRWfJdgVrfKLRS4G3vTMRhEQrCZZyHNMo)
1HwxL1vutUc42ikh3RBnM4v2dVRHPTrTve from Sha256(1FfmbHfnpaZjKFvyi1okTjJJusN455paPH) 1FNF3xfTE53LVLQMvH6qteVqrNzwn2g2H8 from Sha256(1H21ndKEuMqZbeMMCqrYArCdV8WeicGehB)
In every case I looked at, the coins were moved away within minutes or seconds. It was much more likely that a bot was waiting for those coins to show up. Also, transactions are STILL happening to this day on those addresses! But how can that bot know in advance that address was about to receive bitcoins?
A Scam or A Mistake?
It is at this point I formed a theory on what was really happening. It is likely that someone installed malicious code into the backend system of a mining pool, an exchange, or possibly wallet generation code. They are using public information so that they can discover the private keys easily and steal the coins on the side.
But why would they use Sha256(public_address)? Why not do Sha256(public_address + some super hard to guess random sequence) or just use a hard-coded address?
Well, I have a theory on that too. It can't be hard-coded or it would look suspicious in a source code repository. It's likely the code was introduced by someone who works (or worked) for some company connected to bitcoin (exchange/mining pool/gambling site/wallet). Code submitted by developers into source control systems usually goes through a code review process. It would be much easier to hide an innocent looking Sha256 operation inside the millions of lines of code that make up the backend. Sha256 is used all over the place in bitcoin and it wouldn't look suspicious. The function would be readily available. However, if code were to be submitted that performed Sha256(address + "secret_password1234xyz"), that would look VERY suspicious. My guess is someone has slipped in a routine that LOOKS harmless but is actually diverting bitcoin to their awaiting bot ready to gobble them up.
It's actually quite clever. No one can know the destination address in advance. You would have to keep performing Sha256 on all public addresses ever used to catch that one in a million transaction. Someone would be able to capture those coins by simply watching for a transaction into an address that corresponds to a private key generated from Sha256 of one of the existing public addresses. Keeping such a database is trivial and lookups are quick. To be fair, I suppose this could be a coding error. Anything is possible with a buffer overflow. I would love to see the code if this is ever found. Transactions were STILL happening right up until a couple weeks before I made this discovery! So I wrote a bot to try and 'catch' a transaction.
Within the FIRST 48 HOURS of my bot going live, on Jun 19, a whopping 9.5 BTC was transferred into an address for which I had the private key. This was approximately worth $23,000 USD at the time. I was shocked.
This is the address: 12fcWddtXyxrnxUn6UdmqCbSaVsaYKvHQp
The private key is: KzfWTS3FvYWnSnWhncr6CwwfPmuHr1UFqgq6sFkGHf1zc49NirkC whose raw bytes are derived from Sha256 of: 16SH69WgJCXYXWV58sxjTxonhgBh5HCZTt (which appears to be some random address previously used in the chain) BUT... I had failed to test my program sufficiently and it failed to submit the transaction!
The 9.5 BTC was sitting there for almost 15 minutes before being swept away by someone else. I honestly didn't think the first amount to cross my radar would be so high. The other samples I found from past transactions were for tiny amounts. It is quite possible that whoever moved them later out of the poisoned address actually owned them. Maybe someone else's sweeper bot only takes small amounts most of the time to avoid attention? At this point, I was pretty confident I was on to something not yet discovered by anyone else.
I _could_ have taken those 9.5 BTC and if this was known to others. Also, if you look into the history of that account, 12 BTC was transferred into it (and out right away) only one month earlier. No one has claimed any theft (to my knowledge) involving that address. I fixed my program (actually tested it properly this time) and let it run again. My program detected more transactions (2 within the next 48 hours). I coded my bot to ignore anything less than .1 BTC so I didn't move them. I didn't want to tip off the anyone that I knew what they were doing (if that was indeed the case). Another 3-4 days passed and the next hit my bot detected was for roughly .03 BTC (~$95USD). For some reason, this was not transferred out immediately like the rest. By this time it was July 4th weekend. I let this one sit too and it took a full 7 days before it was moved (not by me). It may have been the legitimate owner or a bot. We'll never know.
The destination address was: 1LUqqMzaigWJTzaP79oxsD6zKGifokrh7p The private key raw bytes were: c193edeeb4e7fb5c3e01c3aebd2ec5ac13f349a5a78ca4112ab6a4cbf8e35404
The plot thickens...
I didn't realize it at the time but that last transfer was into an address for a private key not generated from another public address like the first one. Instead, this address was generated from a transaction id! I had forgotten that I seeded my database with private keys generated with transaction ids as part of one of my earlier experiments. I didn't label them so I didn't know which were from Sha256(pub address) and which were from transaction ids. I found some hits at the time but when I checked the balances for those accounts, they were all zero and I didn't think anything of it. But now my database was detecting ongoing transfers into THOSE addresses (transacton id based) too! Okay, someone was possibly using information from the blockchain itself to ensure private keys were discoverable for the addresses they were funelling bitcoin into.
The interesting thing is I found a link between the 12fcWddtXyxrnxUn6UdmqCbSaVsaYKvHQp address (via sha of a public address) AND the 1LUqqMzaigWJTzaP79oxsD6zKGifokrh7p transfer (via the tx id as a key).
In the history of both of these addresses, you can see the BTC eventually ended up into this address: 1JCuJXsP6PaVrGBk3uv7DecRC27GGkwFwE Also, the transaction id was for the previous transaction to the one that put the BTC in the toxic (discoverable) address in the first place. Now it became even more clear. The malicious code sometimes used a recent transaction id as the private key for the doomed destination address. Follow the .03 BTC back and you will see what I mean, you eventually get to the txid = private key for that discoverable address.
The 1JCuJXsP6PaVrGBk3uv7DecRC27GGkwFwE address is ONE of the collection addresses. I have reason to believe there have been many over the years. This one only goes back to approximately March 2017. You can see in the history of this one address when they consolidated their ill-gotten gains into one transaction back to themselves.
I let my bot run longer. The next hit I got was for block hashes that were used as private keys (see Experiment #1). Sure enough, this address also had links to the 1JCuJXsP6PaVrGBk3uv7DecRC27GGkwFwE collection address! And remember my merkle root experiment? I believe those were also part of this. However, I have not linked those to this one particular collection address yet. In the end, I found a total of four different 'discoverable' private key methods being used. I made sure my database was filled with every block hash, merkle root, transaction id and Sha256(public address) for private keys and let my bot run. Transactions for all four types were showing up, again for tiny amounts which I ignored. By this time, I was watching BTC getting taken in small amounts regularly. Sometimes, I saw as many as 6 transactions fly by in one day.
How fitwear lost (and got back) 9 BTC
On Nov 12, my program saw 9 BTC transferred into an address that my database had the private key for. I had searched for that address too to see if anyone was claiming ownership but I didn't see anything. I decided to send a small amount to a well known puzzle address to give the transaction some public scrutiny in an anonymous way (1FLAMEN6, I'm still trying to solve this BTW). Shortly after, I became aware of fitwear's reddit post claiming theft after someone noticed the prize amount had been topped off and linked the two events together.
I contacted fitwear privately and returned their coins minus the small amount I sent to the puzzle address. Blockchain.info's original response to his support ticket, was that his system must have been compromised. However, if you read his post, he took every precaution including typing in the key for his paper wallet instead of copy/paste and using 2FA.
In his case, in Aug 2017, he imported the private key for his 1Ca15MELG5DzYpUgeXkkJ2Lt7iMa17SwAo paper wallet address into blockchain.info and submitted a test transaction. At some point between then and Nov 12, the compromised 15ZwrzrRj9x4XpnocEGbLuPakzsY2S4Mit got into his online wallet as an 'imported' address.
Together, we contacted blockchain.info and I relayed the information I just outlined above to them. Their security team investigated but found no evidence it was their system that was at fault. I suppose it's possible his system was somehow compromised back in August and managed to import a key into blockchain.info without him knowing it. Or someone else logged into his account, imported the key, then waited. I feel the malware/login explanations are much less likely because it looks like code attempting to 'hide in plain sight' to me. You wouldn't need to use Sha256(address) or block hash or txid or merkleroot if you were malware or an unauthorized login. You would at least salt or obscure the key with some bit of knowledge only you know so that only you could derive the private key (as mentioned earlier). The fact that information from the blockchain itself is being used indicates it may be some transaction processing logic. Also, fitwear took extreme precautions (you can read his reddit post for details). The origin of these poison destination addresses remains a mystery.
If it's the case that some wallet generation code is doing this, then it may be the case that we're seeing 'change' transactions. When you create a wallet, there maybe 20 addresses generated. They are all supposed to be random keys. If this rogue code creates one of them in this manner (based on the public address string of an earlier one), then at some point, your 'change' will get put back into it as the wallet 'round-robins' through the list.
fitwear's 15Z address sat unused until Nov 12 when fitwear transferred his 9 BTC into it using blockchain.info.
To see the connection, take a look at this:
echo -n "1Ca15MELG5DzYpUgeXkkJ2Lt7iMa17SwAo" | sha256sum
That hex number is the private key for 15ZwrzrRj9x4XpnocEGbLuPakzsY2S4Mit !!! fitwear insists he did not import the key for that address. Did Blockchain.info generate it or was it added by mallicious browser code? We may never know.
See below for the complete list of other Sha256 based addresses that suffer from the same issue. I believe this is happening for others. It's likely, that the small amounts usually taken are going unnoticed by the owners. What does this mean for bitcoin? Nothing probably. I believe the bitcoin network itself to be secure. However, as long as humans are involved in the services that surround it (mining pools, exchanges, online/mobile wallets) there is always a chance for fraud or error. The bitcoin network itself may be 'trustless', but anything humans touch around its peripheries is certainly not. And you need to use those services to get in/out of the network. So even with bitcoin, it still boils down to trust.
To be fair to blockchain.info, only Sha256(public address) (one in particular) was found to be present in one of their wallets. The other 3 methods I described above could be completely unrelated. And they could all possibly be a (really weird) software bug.
Here are 100+ addresses that received bitcoins whose private keys are the bytes resulting from Sha256 of another public address. Most of these came from a scan I did of old transactions, not while my bot was running. Blockchain.info told me they do not appear to have been generated by their system.
Also, the list of addresses I"m providing are only the subset that have already had some BTC transacted through them. There are likely hundreds more lying dormant inside people's wallets that have not been used yet.
:::At this stage we removed the full list of 100 addresses, more for the length of this blog post than for any other reason.
But the researcher goes onto the say
"I think this information should be made public so that other backend systems plugged into crypto networks can guard against this sort of 'hide in plain sight' attack. As stated earlier, I honestly set out to look for buried treasure and stumbled upon someone else's exploit.
Thanks to yt_coinartist's assistance in making this public."
Keeping your Bitcoin safe
A key step to protecting your cryptocurrency is to store anything of significant value in a hardware wallet—a physical device, like a USB drive, that stores your private keys and currency locally and isn’t connected to the Internet. Experts caution against storing large amounts of coins through cryptocurrency exchanges, or in digital wallet apps on your smartphone or computer.
The public-facing Internet offers an attacker too many inroads to attempt to infiltrate your wallet or trick you into giving them access. Secure hardware wallets like Trezor or the Ledger Nano S cost about $100 or less and have a straightforward setup. You just choose a PIN number and a recovery "seed" (usually a set of words and numbers) in case you forget your PIN or your wallet malfunctions. It's pretty robust security, so make sure you keep copies of your PIN and seed somewhere accessible to you, but not to home intruders. Recovering currency stored in a hardware wallet after losing both the PIN and the seed is a whole thing.