[FS#3613] Overestimated entropy in urgnd

[OpenWrt Bugs]

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User who did this - Kurt Roeckx (kroeckx) 

Attached to Project - OpenWrt/LEDE Project
Summary - Overestimated entropy in urgnd
Task Type - Bug Report
Category - Packages
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - High
Priority - Very Low
Reported Version - openwrt-19.07
Due in Version - Undecided
Due Date - Undecided
Details - I was looking at the entropy collected by the jitterentropy library that urngd is using. I've filed a bug against the library at https://github.com/smuellerDD/jitterentropy-library/issues/21

What I'm seeing it that is overestimates the amount of entropy collected. On my desktop machine this is at least a factor of 100. I've not looked at any of my devices using OpenWRT yet, but I expect it to be worse there.

Urngd has an oversampling factor, which is currently set to 2, meaning the factor is 2 time smaller. The oversampling in urngd currently collects twice as many bytes (64), and then tells the kernel that it contains 32 byte / 256 bit of entropy. The library also supports oversampling itself, it's a parameter to the jent_entropy_collector_alloc function.

I suggest that you set the oversampling factor to at least 1000.

More information can be found at the following URL:
https://bugs.openwrt.org/index.php?do=details&task_id=3613

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