why does RAND_add() take "randomness" as a "double"?

[Paul Dale]

Double makes sense.  Entropy is often estimated as a real value.

E.g. we have the aforementioned coin flipper feeding data serially.
Adding each bit sequentially means 0.125 bytes of entropy per call.

Not the best example....

Pauli
-- 
Oracle
Dr Paul Dale | Cryptographer | Network Security & Encryption 
Phone +61 7 3031 7217
Oracle Australia


-----Original Message-----
From: Laszlo Ersek [mailto:lersek at redhat.com] 
Sent: Wednesday, 22 May 2019 12:15 AM
To: openssl-users at openssl.org
Cc: Jian J Wang <jian.j.wang at intel.com>; edk2-devel-groups-io <devel at edk2.groups.io>; Lu, XiaoyuX <xiaoyux.lu at intel.com>; Ard Biesheuvel <ard.biesheuvel at linaro.org>
Subject: why does RAND_add() take "randomness" as a "double"?

(resending, with my subscription to <openssl-users at openssl.org> completed)

Hi OpenSSL Developers,

(cross-posting <openssl-users at openssl.org> and <devel at edk2.groups.io>,)

OpenSSL commit [1] changed the representation of the "entropy amount" -- later renamed to "randomess" in [2] -- from "int" to "double". I've read the commit message:

commit 853f757ecea74a271a7c5cdee3f3b5fe0d3ae863
Author: Bodo Möller <bodo at openssl.org>
Date:   Sat Feb 19 15:22:53 2000 +0000

    Allow for higher granularity of entropy estimates by using 'double'
    instead of 'unsigned' counters.
    Seed PRNG in MacOS/GetHTTPS.src/GetHTTPS.cpp.

    Partially submitted by Yoram Meroz <yoram at mail.idrive.com>.

and also checked "MacOS/GetHTTPS.src/GetHTTPS.cpp" at the same commit.
But, I'm none the wiser.

Can someone please explain what is gained by using a floating point type here?

Is it really a relevant use case that entropy is fed from an external source to OpenSSL such that truncating the amount to a whole number of bits would cause significant lossage? (Admittedly, it could be relevant if the individual randomness bit counts were in the (0, 1) interval, both boundaries exclusive.)

Using floating point for randomness representation is a problem for environments that prefer to avoid floating point altogether, such as
edk2 ("UEFI") firmware

Thanks,
Laszlo

[1] https://github.com/openssl/openssl/commit/853f757ecea7
[2] https://github.com/openssl/openssl/commit/f367ac2b2664