Solved \XF\Util\Random::getRandomString() seems suspectible to timing attacks

[Kirby]

\XF\Util\Random::getRandomString() generates a cryptographically secure random value (by calling \XF\Util\Random::getRandomBytes()) but then calls base64_encode to generate a string from those bytes.

AFAIK, base64_encode is not constant time so could be vulnerable to timing attacks.

It might therefore be better to use sodium_bin2base64 instead (if available) or fallback to bundled ParagonIE_Sodium_Core_Base64_UrlSafe if not.

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