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1  Bitcoin / Development & Technical Discussion / Question about extra data in blockchain. Malicious tempering vulnerability? on: July 20, 2018, 02:43:29 AM
Hello

This is a bit technical.

I tried looking and did not found anything really specific about this.

I was looking at analyzing the blockchain, mainly using the blkxxxxx.dat files generated by the bitcoin core client.

If you look at the structure of a block, in this file, you have:

- magic number (4 bytes) - 0xD9B4BEF9
- Block length (4 bytes) - length of the block that follow
- Block header (80 bytes)
- Transaction count (var int)
- Transactions (multiple bytes) - A list of all the transactions for this block

Now, the block length above could be larger than the size of 'useful' data that follow (header/transactions).  So, there may be extra data at the end of the block, and indeed, this has surfaced in the news that there was some dodgy data that have been found there (but the subject of this topic is not on what's been found).

Now, my question is about the immutability of this 'extra' data.  The immutability of the blockchain is primarily based on the hash of the block header alone (only the block header is used when computing cryptographic hash).  However, the header itself contain a cryptographic merkel hash of all the transactions, so effectively this also make the transactions immutable.  The magic number also is fixed by consensus, so that is also immutable.

But apart from that, the immutability pretty much ends there, meaning that the following data could be modified:
- The block length
- The extra data.

So, effectively, on the blockchain, a specific block could be represented in a 'practically' infinite many possible way.  One could for example grow any block he receive from the network, store any arbitrary data there, and re-broadcast this new block.  Then, any other nodes would accept it (as long as it is under 1MB of size).

Am I right in my assumption?  What are the implications?  Could someone 'pollute' the network and flood it with 1MB blocks (which would effectively give a 500GB blockchain)?  Possibly not to that extent, because all but the newer peers (who still don't have the blockchain yet) already have their own copy of the blocks, but from an academic point of view, if you were to query this rogue node, you would get a 500GB blockchain.

On a more darker side, I could see for example that some parties could hide data there in the hope that enough peers on the network eventually propagate them.  For example, some political parties or spy agencies could take some blocks and append state secrets.  With the current Bitcoin clients all relaying the block 'as is', this would effectively make this extra appended data as semi-immutable (by majority consensus).  This would be more effective for new blocks, where a party controlling many nodes could broadcast this 'new' tempered block very quickly.  In that case, the network would be mixed with possible a large percentage of nodes having one version of a block, and the other half having a different version.  Technically both version of that block would be valid, and that alone would not create any fork.

However, this could very easily and effectively be fixable with a soft fork, by ensuring that any node receiving a block does strip any extra data (make the 'block length' the real size of the block/transactions set).  That would be backward compatible with any existing clients, and really be a 'free' fix.

I have not tried, but this could be easily be verifiable by querying many nodes for a specific block, and check if that all return the exact same block (taking a block which was found to have dodgy data in it).

Any thought, comments?

Regards,
Eric Hoffman
2  Alternate cryptocurrencies / Altcoin Discussion / Re: [ANNOUNCE] Digital Currency Widget: bitcoin / altcoin price widget for android on: February 19, 2016, 08:01:29 PM
Is there something that need to be done for caVirtex (now Kraken) API?  Does not update anymore.

Regards,
Eric
3  Economy / Speculation / Gox gone, China gone, we need a new beacon exchange... on: March 28, 2014, 12:24:02 PM
So, with China now going down, I think it's time Bitcoin exchanges ecosystem settle for a new reference.  Who could it be?

We need a Bitcoin-friendly country.  Why not Canada? :-)  And now with Jim Flaherty gone, we may at least have another break on Bitcoin pressure...  Smiley

4  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][BETA][EXCHANGE][REALTIME] CoinEX realtime exchange on: March 17, 2014, 01:52:09 PM
Thanks for the "unofficial" follow-up Smiley

Thankfully I scraped everything back from CoinEx last week (dumped alts->BTC and transferred to my wallet, including tons of ContinuumCoins that was laying there) Roll Eyes

Moral to the story, don't keep anything hot on an exchange, pool, etc.
5  Economy / Speculation / Re: Finally, GOX dead... Best thing that could happen to Bitcoin... on: February 26, 2014, 01:44:23 AM
Why do people assume they have ran off with everyone's bitcoins? Can they really get away with something like this?

I didn't imply that they left with the coins...  I do believe rather that they did get eaten slowly and surely with transaction malleability.  It's a shame that they didn't had two level of balance verification.  They just assumed that their algorithm worked.  Some transactions happened to not make it through and being "rejected".  Gox assumed that it was just because the transactions got lost, due for example to too little transaction fees, and just re-issued it.

You take all gox transactions, and discard the "failed" ones, and what you were left did balance.  But no check was done in the other way, checking if the sum of users balances did match Gox sum of BTC balances (hot + cold wallets).  So, those "failed" transactions really did pass...  just with different transaction ID, but still eating from Gox wallet.

What will happen now is that even if Gox come back, they will probably have lost the public trust.
6  Economy / Speculation / Finally, GOX dead... Best thing that could happen to Bitcoin... on: February 25, 2014, 08:26:32 PM
... Well, not so to those who have lost their coins on the exchange, but if you look at Bitcoin as a whole, 50% of the time that BTC went down was due to Gox, and the other exchanges followed afterward.  We have come to rely on Gox for so long now.  Even though it's been shadowed when Bitpay started using an average of several exchanges instead of taking Gox price as the golden exchange rate, and with the Chinese taking over the (BTC) world, I'm sure everyone who followed BTC price had a Gox window open on their desktop.

Now it's over (let face it, even if it does re-open, it's as dead as 50btc.com, which suffered (presumably) the same problem).  So, people will get out and finally look past Gox.  We are free! Smiley

Whatever the price does from now on, be it if it goes up, or crash down, won't be because Gox ripple up or ankle the price down.

Just trying to see a bright side in those dark days...  Undecided
7  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][BETA][EXCHANGE][REALTIME] CoinEX realtime exchange on: February 22, 2014, 12:11:06 PM
What's happening with the pools, they keep going up and down...
8  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][MOON] Mooncoin: You know where it's headed! NEW SOURCE RELEASED 2/16/2014! on: February 21, 2014, 01:30:11 PM
4...  Well, actually 3.  We have found 26846.  KGW kick in at 26850.  So, we just have to find 26847, 26848 and 26849.  Then, KGW will be ON  Wink

Just 3!!!
9  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN] Velocitycoin- Client Update -Pools, Exchange, Trade, Mutli-language on: February 18, 2014, 11:07:45 AM
One of the reason why it's so low is that no exchanges seems to care about this coin (apart from some no-name Chinese sites).  If it would appear on Cryptsy or Coinex, it would for sure get a boost  Smiley

But the number 1 reason why it's tanking right now is IMHO because this is mainly a quick-money coin.  By that I mean that by the way it's designed, 90% of people jumped mining the cr@p of it when it came out, to get the maximum number of coins while it was worth it (while reward was high, and number of miners was low).  Now, most of the coins are mined, reward is very low (compared with initial reward) and so many people mine it that it's not worth mining it anymore in the eye of most people.  So, what you have is a coin where it still have to do it's proof (that takes time, months...), and 75%+ of the coins already mined, and especially most of the holders of those coins just DYING for this coin to go up and cash in.  So, don't be surprised that people get tired, seeing this cion tanking, decide to sell, dump, stop waiting, and killing any price raise with sudden selling, telling themselves "well, it's up 10%, dump now before the price may never reach that price again for a while...".  Remember, probably the majority of people who have the initial coins are there for quick money, and not true believer/holders.

I sincerely do hope that this coin do get up.  I didn't mine it and even if I was there at the beginning, I wouldn't have mined it either (don't have GPU rig, I'm more equipped on SHA256 side with ASIC).  But I did get some coins though on the... hummm... only non-Chinese exchange out there Roll Eyes
10  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [OSC]OpenSourcecoin-- Traded @ OpenEx, Coinex, Cryptsy on: February 18, 2014, 10:52:18 AM
No more news on this coin?  It seem to be holding up at around 2K Satoshi on Cryptsy lately.

I'm telling you, this coins is worth 0.001 (or 50x more than it is now)  Smiley  It just need some push, and some big exchanges taking it.  But I'm not worried, this should come sooner or later...
11  Economy / Speculation / Re: MTGOX hits $570 "A bug in the bitcoin software makes it possible for someone to" on: February 10, 2014, 01:08:32 PM
I would have said that Gox is dead, especially in light of spreading this FUD...  But then again, I'm sure we'll still find morons going back to it and maintaining it...  As there's still morons who went back to 50BTC after doing this same exact sh!t...
12  Economy / Speculation / Re: Did MtGox play people's money? Would explain hypothetical lack of liquidity on: February 10, 2014, 01:04:46 PM
And now...  They try to put the blame on the blockchain...  Lame excuses...  Same strategy as 50BTC...
13  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN] Velocitycoin- Client Update -Pools, Exchange, Trade, Mutli-language on: February 08, 2014, 08:32:55 PM
It's back yes...  But sadly, it hasn't changed the price of VEL...  Cry
14  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN] Velocitycoin- Client Update -Pools, Exchange, Trade, Mutli-language on: February 08, 2014, 11:10:58 AM
Site seem in maintenance mode or something...
15  Economy / Speculation / Re: Did MtGox play people's money? Would explain hypothetical lack of liquidity on: February 08, 2014, 05:27:22 AM
Oh, to add, the warnings are not new...

1 - Remember when the price skyrocketed from 30~40 to 200$+.  Then, bubble popped and what then happened?  April 11 2013, Gox halted the tradings for 12 hours (in hope to stop market crash).  https://www.mtgox.com/press_release_20130412.html
2 - June 20th 2013, cash withdrawal becoming slower and slower, Gox halt fiat withdrawal for 2 weeks.  https://www.mtgox.com/press_release_20130620.html
3 - Well, this is the events taking place right now.  This has been ongoing for at least one to two week (or more).  https://support.mtgox.com/entries/26501000-Statement-Regarding-BTC-Withdrawal-Delays

More and more sources are worried about Gox now.  Some news talk about the "Fall of Gox".  Good link above traderCJ Smiley  And http://www.youtube.com/watch?v=cbxD-0EVV00 among other...

Sad days for Bitcoin...  Apple withdraw Bitcoin apps, Gox, Russia Just Outlawed Bitcoin and Other Cryptocurrencies, ...
16  Economy / Speculation / Re: Did MtGox play people's money? Would explain hypothetical lack of liquidity on: February 08, 2014, 04:44:45 AM
Also, don't say comments like "It's just too big and famous to be illegitimate"...  If someone is so naive to think so, think about Madoff, or closer to my home town (in Quebec, Canada), Vincent Lacroix (Norbourg scandal), where even the highest and most serious banks have been duped.

As caratheodory say, don't underestimate people's greed.

Just think about all the corruption, from the small to the most elaborate plot (just look at the news surrounding the construction industry here in Quebec, with the "Commission Charboneau"...  Very elaborate system put in place by the Italian mafia...  Freemasonry secrecy at it's best!).
17  Economy / Speculation / Re: Did MtGox play people's money? Would explain hypothetical lack of liquidity on: February 08, 2014, 02:10:05 AM
Reviving my original post...  Time to re-consider my original taught?

Last time was with withholding fiat.  Now they are stopping any currency withdrawal...
18  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][MOON] Mooncoin: You know where it's headed! WE HAVE LIFTOFF! on: February 03, 2014, 01:53:26 AM
http://www.btc-8.com/?MOON_CNY

MOON for fiat in China, not sure if that is new, but it looks it.

There's trade there since at least a week, and it's been around that same value.

Right now, buy is 0.0019CNY (which is equivalent to BTC/MOON price of 38 Satoshi).
Sell is 0.00046 (which is equivalent to BTC/MOON price of 9~10 Satoshi).

So, nah, sorry, no chances for arbitrage  Roll Eyes heh!
19  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][MOON] Mooncoin: You know where it's headed! WE HAVE LIFTOFF! on: February 02, 2014, 07:21:34 PM
Some people will only learn of their mistakes when they delist this sh!t coin...  And some will never learn...

Going above 2 Satoshi was the mistake in the first place...  Call me troll if you want, but I prefer being a troll with BTC than being MOON-broke waiting for the messiah...
20  Alternate cryptocurrencies / Announcements (Altcoins) / Re: [ANN][MOON] Mooncoin: You know where it's headed! WE HAVE LIFTOFF! on: February 02, 2014, 11:27:17 AM
Hey, look at it, there's 50~100 BILLION MOON created in the first few days.  At 1 Satoshi, that makes 500~1000 BTC worth of MOON in the first few days/weeks.  Now, you see why those supposedly "whales" at 10 Satoshi are really just the fruit of few days worth of mining.

So, stop calling whales, and instead look at the light of why when we say that there just too many fu..ing coins and why it's really worth about half a Sato
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