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Re: How to convert a md5sum back to a timestamp?

From: Stephane Chazelas
Subject: Re: How to convert a md5sum back to a timestamp?
Date: Thu, 1 Aug 2019 07:50:12 +0100
User-agent: NeoMutt/20171215

2019-07-31 22:36:18 -0500, Peng Yu:
> Hi,
> Suppose that I know a md5sum that is derived one of the timestamps
> computed below. Is there a way to quickly derive what the original
> timestamp is? I could make a database of all the timestamps and their
> md5sums. But as the total number of entries increases, this solution
> will not be scalable as the database can be big. Is it there any
> better solution to this problem?
> for i in {1..2563200}; do date -d "-$i minutes" +%Y%m%d_%I%M%p; done

seq -f '-%g minutes' 2563200 | date -f - +%Y%m%d_%I%M%p

would be an improvement as it would only run one date
invocation, but you'd still need to run one md5sum for each of
those lines. coreutils md5sum in itself is not slow, but forking
a process and loading a command and linking its libraries is,
that's not a bug in coreutils itself.

You'd be better off using perl/python which can also compute MD5
sums by themselves without having to invoke a separate utility.

If you want to do it in a shell, you can use ksh93 which if
built as part of ast-open will have a builtin md5sum command
(which you enable with "builtin md5sum" or can invoke with
"command /opt/ast/bin/md5sum")

Something like:

#! /bin/ksh93
builtin md5sum mktemp || exit
tmp=${ mktemp; }
trap 'rm -f -- "$tmp"' EXIT

now=${ printf '%(%s)T' now; }
for ((i = 1; i <= 2563200; i++)) {
  t=${ printf '%(%Y%m%d_%I%M%p)T' "#$((now - 60*i))"; }
  printf %s "$t" > "$tmp"
  sum=${ md5sum < "$tmp"; }
  if [ "$sum" = "$sum_to_find" ]; then
    printf '%s\n' "Timestamp: $t ($i minutes ago)"
    exit 0
exit 1

(here using a tempfile as using a pipe would mean  forking
extra processes)

That would be orders of magnitude faster than running one
coreutils md5sum for each timestamp, but still orders of
magnitude slower than doing it in python/perl because of all the
shell interpretation and I/O overhead.

(in any case, note that the builtin versions of mktemp, printf,
md5sum, [  in ksh93 have nothing to do with GNU coreutils').


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