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13 August 2013updated 26 Sep 2015 12:17pm

Android vulnerability hits Bitcoin apps and more

When a random number is not so random, security pays the price

By Alex Hern

Android users of Bitcoin are being advised to upgrade their apps and re-secure their wallets after the discovering of a weakness in a component of the operating system responsible for generating secure random numbers. The weakness also affects some secure communication networks, and renders users vulnerable to theft of their digital currency.

The weakness lies with the Android implementation of a piece of code which is supposed to spit out purely random numbers. Instead of working as it should, the numbers it produces aren’t as random as they seem. These numbers are used by Bitcoin users as the public and private keys in the series of mathematical problems which makes up the “blockchain”, the record of transactions. If they are slightly predictable, then as a result, it is theoretically possible to work out someone’s private key from the public signatures they post, and steal money contained in the wallet.

The vulnerability was highlighted by developer Mike Hearn, who created the Bitcoin Wallet app. That app has since been updated, as have Mycelium Wallet and blockchain.info, two other popular wallet apps for Android. Bitcoin.org, a key website for the decentralised development community, advises users to “rotate” their keys. “This involves generating a new address with a repaired random number generator and then sending all the money in your wallet back to yourself”, they write. “Once your wallet is rotated, you will need to contact anyone who has stored addresses generated by your phone and give them a new one.”

However, the weakness in the random number generator has the potential to affect more than just bitcoin apps. Any app which relies on the generator for security is at risk, particularly if the programme requires a public and a private key. The nature of the flaw makes it overly easy to determine a private key if given a public key generated around the same time; as a result, any app which uses a form of public key cryptography, where the security of the encrypted content relies on the public and private keys being unrelated, is at risk if those keys were generated using the faulty generator.

In practice, though, the Bitcoin community is at the most risk here. It’s one of the few situations where a public key is very public indeed, and the rewards for cracking it are so immediate that if people can try, they will. But it’s hardly a mortal wound; the apps can be updated, and wallets resecured. If Bitcoin is really in danger, it comes from a source which many advocates of the digital money are celebrating. Earlier this month, a Texas court officially declared Bitcoin a “currency” in order to take action against a man accused of running a Bitcoin Ponzi scheme. What sounds like much-needed mainstream recognition is actually a double-edged sword, though. As a currency, it is now fair game for regulators. And sure enough, the New York Department of Financial Services is looking into the “Wild West for narcotraffickers and other criminals”. Bitcoin will shortly need to grow up or shut up, it seems.

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