On 10 Apr 2018, at 00:39, m...@musalbas.com wrote:

> The original disclosure didn't contain any information about the library
> in question, so I did some digging.
> I think that the vulnerability disclosure is referring to a pre-2013
> version of jsbn, a JavaScript crypto library. Before it used the CSRNG
> in the Web Crypto API, it tried to use nsIDOMCrypto, but incorrectly did
> a string comparison when checking the browser version.
> In practice though, this doesn't really matter, because
> navigator.appVersion < "5" returns true anyway for old browsers. The
> real issue is that modern browsers don't have window.crypto.random
> defined, so Bitcoin wallets using a pre-2013 version of jsbn may not be
> using a CSPRNG, when run on a modern browser.

Yes, it looks like high-quality entropy via crypto.getRandomValues was only
added in Tom Wu's latest version (v1.4) in July 2013.

Note that even with v1.4, it still does not use high-quality entropy for
Internet Explorer, because getRandomValues is provided under window.msCrypto
for that browser.


> As is noted though, even if a CSPRNG is used, the library passes the
> output of the CSPRNG through RC4, which generates some biased bits,
> leading to possible private key recovery.

I think this is the real issue: even if high-quality entropy is utilised, the
RNG is RC4-based, which is known to generate biased output.

Finally, note that even Chrome used RC4 for crypto.getRandomValues at one
point (as recently as 2015)!


Jason Davies, https://www.jasondavies.com/

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