blog/data/2014-03-25-trusted-timestamping.txt

   1 Title: Public Trusted Timestamping services for everyone
   2 Tags: english, sikkerhet
   3 Date: 2014-03-25 12:50
   4
   5 <p>Did you ever need to store logs or other files in a way that would
   6 allow it to be used as evidence in court, and needed a way to
   7 demonstrate without reasonable doubt that the file had not been
   8 changed since it was created?  Or, did you ever need to document that
   9 a given document was received at some point in time, like some
  10 archived document or the answer to an exam, and not changed after it
  11 was received?  The problem in these settings is to remove the need to
  12 trust yourself and your computers, while still being able to prove
  13 that a file is the same as it was at some given time in the past.</p>
  14
  15 <p>A solution to these problems is to have a trusted third party
  16 "stamp" the document and verify that at some given time the document
  17 looked a given way.  Such
  18 <a href="https://en.wikipedia.org/wiki/Notarius">notarius</a> service
  19 have been around for thousands of years, and its digital equivalent is
  20 called a
  21 <a href="http://en.wikipedia.org/wiki/Trusted_timestamping">trusted
  22 timestamping service</a>.  <a href="http://www.ietf.org/">The Internet
  23 Engineering Task Force</a> standardised how such service could work a
  24 few years ago as <a href="http://tools.ietf.org/html/rfc3161">RFC
  25 3161</a>.  The mechanism is simple.  Create a hash of the file in
  26 question, send it to a trusted third party which add a time stamp to
  27 the hash and sign the result with its private key, and send back the
  28 signed hash + timestamp.  Anyone with the document and the signature
  29 can then verify that the document matches the signature by creating
  30 their own hash and checking the signature using the trusted third
  31 party public key.  There are several commercial services around
  32 providing such timestamping.  A quick search for
  33 "<a href="https://duckduckgo.com/?q=rfc+3161+service">rfc 3161
  34 service</a>" pointed me to at least
  35 <a href="https://www.digistamp.com/technical/how-a-digital-time-stamp-works/">DigiStamp</a>,
  36 <a href="http://www.quovadisglobal.co.uk/CertificateServices/SigningServices/TimeStamp.aspx">Quo
  37 Vadis</a>,
  38 <a href="https://www.globalsign.com/timestamp-service/">Global Sign</a>
  39 and <a href="http://www.globaltrustfinder.com/TSADefault.aspx">Global
  40 Trust Finder</a>.  The system work as long as the private key of the
  41 trusted third party is not compromised.</p>
  42
  43 <p>But as far as I can tell, there are very few public trusted
  44 timestamp services available for everyone.  I've been looking for one
  45 for a while now. But yesterday I found one over at
  46 <a href="https://www.pki.dfn.de/zeitstempeldienst/">Deutches
  47 Forschungsnetz</a> mentioned in
  48 <a href="http://www.d-mueller.de/blog/dealing-with-trusted-timestamps-in-php-rfc-3161/">a
  49 blog by David Müller</a>.  I then found a good recipe on how to use
  50 over at the
  51 <a href="http://www.rz.uni-greifswald.de/support/dfn-pki-zertifikate/zeitstempeldienst.html">University
  52 of Greifswald</a>.</p>
  53
  54 <p><a href="http://www.openssl.org/">The OpenSSL library</a> contain
  55 both server and tools to use and set up your own signing service.  See
  56 the ts(1SSL), tsget(1SSL) manual pages for more details.  The
  57 following shell script demonstrate how to extract a signed timestamp
  58 for any file on the disk in a Debian environment:</p>
  59
  60 <p><blockquote><pre>
  61 #!/bin/sh
  62 set -e
  63 url="http://zeitstempel.dfn.de"
  64 caurl="https://pki.pca.dfn.de/global-services-ca/pub/cacert/chain.txt"
  65 reqfile=$(mktemp -t tmp.XXXXXXXXXX.tsq)
  66 resfile=$(mktemp -t tmp.XXXXXXXXXX.tsr)
  67 cafile=chain.txt
  68 if [ ! -f $cafile ] ; then
  69     wget -O $cafile "$caurl"
  70 fi
  71 openssl ts -query -data "$1" -cert | tee "$reqfile" \
  72     | /usr/lib/ssl/misc/tsget -h "$url" -o "$resfile"
  73 openssl ts -reply -in "$resfile" -text 1>&2
  74 openssl ts -verify -data "$1" -in "$resfile" -CAfile "$cafile" 1>&2
  75 base64 < "$resfile"
  76 rm "$reqfile" "$resfile"
  77 </pre></blockquote></p>
  78
  79 <p>The argument to the script is the file to timestamp, and the output
  80 is a base64 encoded version of the signature to STDOUT and details
  81 about the signature to STDERR.  Note that due to
  82 <a href="http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=742553">a bug
  83 in the tsget script</a>, you might need to modify the included script
  84 and remove the last line.  Or just write your own HTTP uploader using
  85 curl. :) Now you too can prove and verify that files have not been
  86 changed.</p>
  87
  88 <p>But the Internet need more public trusted timestamp services.
  89 Perhaps something for <a href="http://www.uninett.no/">Uninett</a> or
  90 my work place the <a href="http://www.uio.no/">University of Oslo</a>
  91 to set up?</p>