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Re: [bug-bash] $RANDOM not Cryptographically secure pseudorandom number

From: Martijn Dekker
Subject: Re: [bug-bash] $RANDOM not Cryptographically secure pseudorandom number generator
Date: Sun, 20 Jan 2019 15:39:45 +0100
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.11; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

Op 19-01-19 om 23:10 schreef Chet Ramey:
> On 1/19/19 2:45 PM, Martijn Dekker wrote:
>> Op 16-01-19 om 02:21 schreef Quentin:
>>> If you really need some quality CSPRNG values, I'd suggest adding a
>>> $SECURE_RANDOM variable that just reads from /dev/urandom.
>> IMHO, this would clearly be the correct approach. I don't know of any
>> 21st century Unix or Unix-like system that doesn't have /dev/urandom. I
>> would really like to see shells adopt this idea -- hopefully all with
>> the same variable name.
> OK, this is a reasonable approach. Since /dev/urandom just generates
> random bytes, there's a lot of flexibility and we're not subject to
> any kind of backwards compatibility constraints, especially not the
> 16-bit limit. What do you think would be the best way to present that
> to a user? As a 32-bit random number? A character string you can use to
> create filenames? Some other form?

I'd say numbers would be the most useful, as these are the easiest to
convert into anything else using shell arithmetic and parameter
expansions. E.g. to create a random character string for a temporary
file name, you could do

filename_suffix() {
    for ((i=0; i<10; i++)) do
        printf '%s' "${chars:$(( SECURE_RANDOM % length + 1 )):1}"

(which would of course already work with RANDOM but that would be
totally insecure, as in not effectively eliminating the risk of collisions).

- Martijn

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