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#!/usr/bin/env bash
# https://github.com/kubernetes/kubernetes/blob/84beab6f26609527752ff441c155813b783fe145/cluster/gce/gci/configure-helper.sh#L44
# secure_random generates a secure random string of bytes. This function accepts
# a number of secure bytes desired and returns a base64 encoded string with at
# least the requested entropy. Rather than directly reading from /dev/urandom,
# we use uuidgen which calls getrandom(2). getrandom(2) verifies that the
# entropy pool has been initialized sufficiently for the desired operation
# before reading from /dev/urandom.
#
# ARGS:
# #1: number of secure bytes to generate. We round up to the nearest factor of 32.
function secure_random {
local infobytes="${1}"
if ((infobytes <= 0)); then
echo "Invalid argument to secure_random: infobytes='${infobytes}'" 1>&2
return 1
fi
local out=""
for (( i = 0; i < "${infobytes}"; i += 32 )); do
# uuids have 122 random bits, sha256 sums have 256 bits, so concatenate
# three uuids and take their sum. The sum is encoded in ASCII hex, hence the
# 64 character cut.
out+="$(
(
uuidgen --random;
uuidgen --random;
uuidgen --random;
) | sha256sum \
| head -c 64
)";
done
# Finally, convert the ASCII hex to base64 to increase the density.
echo -n "${out}" | xxd -r -p | base64 -w 0
}
secure_random $1
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