A new practical technique has been published that can recover secp256k1
private keys after observing OpenSSL calculate as little as 200 signatures:
http://eprint.iacr.org/2014/161.pdf
This attack is based on the FLUSH+RELOAD technique published last year. It
works by observing L3 CPU cache timings and forcing cache line flushes
using the clflush opcode. As a result, it is applicable to any x86
environment where an attacker may be able to run on the same hardware i.e.
virtualised hosting environments where keys are being reused.
I am not currently aware of any efforts to make OpenSSL's secp256k1
implementation completely side channel free in all aspects. Also,
unfortunately many people have reimplemented ECDSA themselves and even if
OpenSSL gets fixed, the custom implementations probably won't.
So, IMHO this is a sign for hot wallet users to start walking (but not
running) towards the exits of these shared cloud services: it doesn't feel
safe to sign transactions on these platforms, so hot wallets should be
managed by dedicated hardware. Of course other parts of the service, like
the website, are less sensitive and can still run in the cloud. I doubt the
researchers will release their code to do the side channel attack and it's
rather complex to reimplement, so this gives some time for mitigation.
Unfortunately the huge sums being held in some "bitbank" style hot wallets
mean that attackers are well motivated to pull off even quite complex
attacks.
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