Did you ever need to store logs or other files in a way that would allow it to be used as evidence in court, and needed a way to demonstrate without reasonable doubt that the file had not been changed since it was created? Or, did you ever need to document that a given document was received at some point in time, like some archived document or the answer to an exam, and not changed after it was received? The problem in these settings is to remove the need to trust yourself and your computers, while still being able to prove that a file is the same as it was at some given time in the past.
A solution to these problems is to have a trusted third party "stamp" the document and verify that at some given time the document looked a given way. Such notarius service have been around for thousands of years, and its digital equivalent is called a trusted timestamping service. The Internet Engineering Task Force standardised how such service could work a few years ago as RFC 3161. The mechanism is simple. Create a hash of the file in question, send it to a trusted third party which add a time stamp to the hash and sign the result with its private key, and send back the signed hash + timestamp. Both email, FTP and HTTP can be used to request such signature, depending on what is provided by the service used. Anyone with the document and the signature can then verify that the document matches the signature by creating their own hash and checking the signature using the trusted third party public key. There are several commercial services around providing such timestamping. A quick search for "rfc 3161 service" pointed me to at least DigiStamp, Quo Vadis, Global Sign and Global Trust Finder. The system work as long as the private key of the trusted third party is not compromised.
But as far as I can tell, there are very few public trusted timestamp services available for everyone. I've been looking for one for a while now. But yesterday I found one over at Deutches Forschungsnetz mentioned in a blog by David Müller. I then found a good recipe on how to use the service over at the University of Greifswald.
The OpenSSL library contain both server and tools to use and set up your own signing service. See the ts(1SSL), tsget(1SSL) manual pages for more details. The following shell script demonstrate how to extract a signed timestamp for any file on the disk in a Debian environment:
#!/bin/sh
set -e
url="http://zeitstempel.dfn.de"
caurl="https://pki.pca.dfn.de/global-services-ca/pub/cacert/chain.txt"
reqfile=$(mktemp -t tmp.XXXXXXXXXX.tsq)
resfile=$(mktemp -t tmp.XXXXXXXXXX.tsr)
cafile=chain.txt
if [ ! -f $cafile ] ; then
wget -O $cafile "$caurl"
fi
openssl ts -query -data "$1" -cert | tee "$reqfile" \
| /usr/lib/ssl/misc/tsget -h "$url" -o "$resfile"
openssl ts -reply -in "$resfile" -text 1>&2
openssl ts -verify -data "$1" -in "$resfile" -CAfile "$cafile" 1>&2
base64 < "$resfile"
rm "$reqfile" "$resfile"
The argument to the script is the file to timestamp, and the output is a base64 encoded version of the signature to STDOUT and details about the signature to STDERR. Note that due to a bug in the tsget script, you might need to modify the included script and remove the last line. Or just write your own HTTP uploader using curl. :) Now you too can prove and verify that files have not been changed.
But the Internet need more public trusted timestamp services. Perhaps something for Uninett or my work place the University of Oslo to set up?