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.  Both email, FTP and HTTP can be used to
  29 request such signature, depending on what is provided by the service
  30 used. Anyone with the document and the signature can then verify that
  31 the document matches the signature by creating their own hash and
  32 checking the signature using the trusted third party public key.
  33 There are several commercial services around providing such
  34 timestamping.  A quick search for
  35 "<a href="https://duckduckgo.com/?q=rfc+3161+service">rfc 3161
  36 service</a>" pointed me to at least
  37 <a href="https://www.digistamp.com/technical/how-a-digital-time-stamp-works/">DigiStamp</a>,
  38 <a href="http://www.quovadisglobal.co.uk/CertificateServices/SigningServices/TimeStamp.aspx">Quo
  39 Vadis</a>,
  40 <a href="https://www.globalsign.com/timestamp-service/">Global Sign</a>
  41 and <a href="http://www.globaltrustfinder.com/TSADefault.aspx">Global
  42 Trust Finder</a>.  The system work as long as the private key of the
  43 trusted third party is not compromised.</p>
  44
  45 <p>But as far as I can tell, there are very few public trusted
  46 timestamp services available for everyone.  I've been looking for one
  47 for a while now. But yesterday I found one over at
  48 <a href="https://www.pki.dfn.de/zeitstempeldienst/">Deutches
  49 Forschungsnetz</a> mentioned in
  50 <a href="http://www.d-mueller.de/blog/dealing-with-trusted-timestamps-in-php-rfc-3161/">a
  51 blog by David Müller</a>.  I then found
  52 <a href="http://www.rz.uni-greifswald.de/support/dfn-pki-zertifikate/zeitstempeldienst.html">a
  53 good recipe on how to use the service</a> over at the University of
  54 Greifswald.</p>
  55
  56 <p><a href="http://www.openssl.org/">The OpenSSL library</a> contain
  57 both server and tools to use and set up your own signing service.  See
  58 the ts(1SSL), tsget(1SSL) manual pages for more details.  The
  59 following shell script demonstrate how to extract a signed timestamp
  60 for any file on the disk in a Debian environment:</p>
  61
  62 <p><blockquote><pre>
  63 #!/bin/sh
  64 set -e
  65 url="http://zeitstempel.dfn.de"
  66 caurl="https://pki.pca.dfn.de/global-services-ca/pub/cacert/chain.txt"
  67 reqfile=$(mktemp -t tmp.XXXXXXXXXX.tsq)
  68 resfile=$(mktemp -t tmp.XXXXXXXXXX.tsr)
  69 cafile=chain.txt
  70 if [ ! -f $cafile ] ; then
  71     wget -O $cafile "$caurl"
  72 fi
  73 openssl ts -query -data "$1" -cert | tee "$reqfile" \
  74     | /usr/lib/ssl/misc/tsget -h "$url" -o "$resfile"
  75 openssl ts -reply -in "$resfile" -text 1>&2
  76 openssl ts -verify -data "$1" -in "$resfile" -CAfile "$cafile" 1>&2
  77 base64 < "$resfile"
  78 rm "$reqfile" "$resfile"
  79 </pre></blockquote></p>
  80
  81 <p>The argument to the script is the file to timestamp, and the output
  82 is a base64 encoded version of the signature to STDOUT and details
  83 about the signature to STDERR.  Note that due to
  84 <a href="http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=742553">a bug
  85 in the tsget script</a>, you might need to modify the included script
  86 and remove the last line.  Or just write your own HTTP uploader using
  87 curl. :) Now you too can prove and verify that files have not been
  88 changed.</p>
  89
  90 <p>But the Internet need more public trusted timestamp services.
  91 Perhaps something for <a href="http://www.uninett.no/">Uninett</a> or
  92 my work place the <a href="http://www.uio.no/">University of Oslo</a>
  93 to set up?</p>