Why nobody can withdraw bitcoins from one of the currency – s largest exchanges


The morning of Friday, Feb. 7, the vooraanstaand Japanese bitcoin exchange Mt. Gox announced that, due to technical problems, it would be putting bitcoin withdrawals on hold. Customers would still be able to contant their bitcoins ter for other currencies, or trade on the market. But getting bitcoins out of Mt. Gox would be unlikely.

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The price of a bitcoin tumbled overheen the weekend at the news, from harshly $850 to closer to $675.

This morning Mt. Gox published an update on its webpagina, stating the hold on bitcoin withdrawals wasgoed prompted by a bug te the software on which bitcoin operates:


A bug ter the bitcoin software makes it possible for someone to use the Bitcoin network to alter transaction details to make it seem like a sending of bitcoins to a bitcoin wallet did not occur when ter fact it did occur&hellip,This defect, known spil &ldquo,transaction malleability&rdquo, makes it possible for a third party to alter the hash of any freshly issued transaction without invalidating the signature, hence resulting ter a similar transaction under a different hash.

So what does that mean?

Transaction malleability

To understand transaction malleability, wij need to quickly voorkant some bitcoin basics. (You can read a more in-depth bitcoin explainer here).

Very first, bitcoin transactions: When one person, call hier Alice, transfers bitcoins to someone else, Bob, it does not involve withdrawing money from a bitcoin account. Instead, what Alice is doing is taking a transfer of bitcoins that she received from somebody else at some point ter the past and signing that transfer overheen to Bob.

When she does, a transaction record is created, containing the following: a reference to the previous transaction (the one ter which Alice received the money she is now conveying to Bob), a private digital signature that Alice uses to prove that the bitcoins ter the previous transaction were ter fact given to hier, the amount she is transferring to Bob, and a digital address (sort of like an email address) where Bob will receive the money.

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That transaction record is sent out to the network of bitcoin miners, who then check it. If the transaction is legit, it is added to the growing pulic ledger of every bitcoin transaction everzwijn, known spil the block chain. Alice has successfully transferred those bitcoins to Bob.

Now, for Bob to use thesis bitcoins &mdash, the ones he just received from Alice &mdash, he has to have a way of referencing that transaction, of pointing to it te the block chain. The bitcoin protocol makes this effortless by creating a unique ID for every single bitcoin transaction. That unique ID is generated by running the contents of the transaction record through what&rsquo,s called a hash function.

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The hash function takes the transaction record, does some sophisticated math, and spitsuur out a string of 64 characters, something like this:

That output string varies insanely with petite switches to the input transaction record, which is one of the reasons why it can serve so effectively spil a unique identifier. If you look at a transacation on any webstek that permits you to explore the block chain, you will always find the hash of the transaction sitting there spil a handy reference:

Now, wij can response the question of what transaction malleability is.

When a transaction record is sent out to the network of bitcoin miners, it&rsquo,s possible for a hacker to tweak it enough to switch the hash, but not enough to make the transfer invalid. One of the ways the hacker can do this is by slightly modifying the digital signature te a way that it is still recognized spil the right signature, but, again, results ter a fully different hash:

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So why is this a problem? Wij are, afterall, talking about legitimate transactions that are still coming from the right place and going to the right place. The only thing that is switching midstream is the unique ID used to reference that transaction. Doesn&rsquo,t seem like that big of a overeenkomst.

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But it complicates things for Mt. Gox, at least te theory. When somebody who trades on the exchange wants to take bitcoins out of the system, Mt. Gox has to transfer those bitcoins to hier from its own bitcoin wallet. When it does, it makes a note of the transaction by recording the hash. That way, if something goes awry, it has a handy list of references it can use to track down its transactions.

The potential hitch is that, if the hash is switched before it gets entered into the block chain, then the hash that Mt. Gox recorded for a transaction and its presente unique ID ter the public ledger won&rsquo,t match up.

This makes Mt. Gox indefenso, at least ter theory. A Mt. Gox customer could potentially withdraw bitcoins, have the hash switched, and then voorwaarde that she never actually received the bitcoins withdrawn. When Mt. Gox checks its records, it won&rsquo,t find the hash that it recorded for the transaction anywhere te the block chain. The result could be that Mt. Gox finishes up paying the customer more than merienda.

Transaction malleability is nothing fresh

Mt. Gox did not detect the kwestie of transaction malleability. It wasgoed very first identified at least three years ago, and has bot discussed by bitcoin software developers ter online forums. According to the bitcoin wiki, developers are working on switching the bitcoin software, so that only one version of a digital signature is valid, thus making transactions much less malleable.

Mt. Gox mentions te its press release that it is &ldquo,working with the bitcoin core development team and others to mitigate this punt.&rdquo,

One of the core developers, Greg Maxwell, has stated that it&rsquo,s possible for Mt. Gox to resolve the problem with transaction malleability by switching its own internal systems of accounting&mdash,that a fundamental switch to the bitcoin software isn&rsquo,t absolutely necessary.

&ldquo,This isn&rsquo,t news to mij&mdash,for years&mdash,and it&rsquo,s never bot a particularly large concern. This wouldn&rsquo,t make the top ten list of dangers ter the bitcoin technology,&rdquo, said Maxwell.


  1. aj319

    May 16, 2018 at 7:23 pm

    automatic cloud backup for user’s private keys (encrypted)

  2. Gill5

    May 17, 2018 at 1:02 am

    Top Ten accounts own 24.03% compared to 24.17% on May 8 and 25.54% on April 28.

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