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Token generation may be vulnerable to Modulo Bias #780
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Reference: github-starred/vaultwarden#780
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Originally created by @zyuiop on GitHub (Aug 22, 2020).
Modulo Bias is an issue with random numbers generators where some numbers have a higher chance to be found than others. See an article about this bias and how to avoid it here: https://research.kudelskisecurity.com/2020/07/28/the-definitive-guide-to-modulo-bias-and-how-to-avoid-it/
Here is the line where the bias arises:
https://github.com/dani-garcia/bitwarden_rs/blob/eba22c2d9429b9f9bad5baff9b1d571b469645c9/src/crypto.rs#L67
From what I saw in the code, this seems to be used (only?) for two factor authentication, so this may not be very problematic.
@dani-garcia commented on GitHub (Aug 22, 2020):
This shouldn't be a problem in practice as long as users don't configure the token generation to be over 16 or so:
For example using a 16 digit length, the numbers at the start would have 1844 chances to appear (2^64 / 10^16), while the ones at the end would have 1855 which is basically close enough to random for this case I think.
But yeah using the max of 19 would give the first numbers in the range 50% more chance to appear, so I think we should at the very list make the limit lower.