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Last Notes

2023-06-07 18:11:34 UTC

- reply

📅 Original date posted:2018-04-10
📝 Original message:On 10 Apr 2018, at 00:39, mus at 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.
http://www-cs-students.stanford.edu/~tjw/jsbn/rng.js
> 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)!
https://bugs.chromium.org/p/chromium/issues/detail?id=552749
--
Jason Davies, https://www.jasondavies.com/

2023-06-07 18:11:33 UTC

- reply

📅 Original date posted:2018-04-09
📝 Original message:These issues all stem from the RC4-based RNG implementation (with insecure
fallback entropy) in Tom Wu's jsbn library, published here:
http://www-cs-students.stanford.edu/~tjw/jsbn/
Please refer to Tom Wu's URL, or this more up-to-date fork of Tom Wu's code
(published to NPM): https://github.com/andyperlitch/jsbn -- my repository on
GitHub was only ever intended to be a straight mirror of Tom Wu's code (created
over 7 years ago!). I'll probably delete my mirror repository given that there
are now better JavaScript bignum alternatives, and in light of this report.
Jason
> On 9 Apr 2018, at 22:11, mus at musalbas.com wrote:
>
> Here's the code in question: https://github.com/jasondavies/jsbn/pull/7
>
> Best,
>
> Mustafa
--
Jason Davies, http://www.jasondavies.com/