What good, standard digital signature verification clients are widely or easily deployed?

Question

I'm looking for a good way to publish data along with a digital signature (and ideally a timestamp) which is as easy as possible to verify.

For example, imagine an election administrator who wants to publish election data which is to be audited (totals by candidate by precinct). We ideally don't want to require the public to have to acquire much if any special or proprietary software. Signatures which adhere to an open standard, can be validated on PC, Mac, Linux, and can be generated by open source software are preferred.

The data has to remain machine-readable, so a pdf isn't suitable, so if pdf is used, the data would be in attachments to the pdf.

E.g. if we generate XML Signatures (e.g. for IEEE 1622-2011 Election Markup Language (EML) files for elections), is there standard functionality in any common browsers or operating systems to validate the signatures? Is the user interface clear enough to make it clear to the user when the data looks good, when it is suspicious, and how to deal with common issues?

Other alternatives (and concerns):

• Publish SHA-256 hash in the newspaper (How to describe verification steps to the user?)
• Use a jar (signed zip) and jarsigner (is there a nice GUI for verifying jar signatures? X.509 PATH validation?)
• Use OpenOffice (A hassle to install)
• CMS or S-MIME (How can we publish an S-MIME message with attachments - public IMAP?)
• Use gpg (but admins need to get in the PGP web of trust, and users need to learn to understand it)
• Use s-http (a pity http://tools.ietf.org/html/rfc2660 never took off....)
• Use Excel (are there free clients that can check the signature? Open source software to generate it via a script?)
• "Just serve from a secure web site" (doesn't deal with a variety of attacks)

Answer 1

One option would be to digitally sign a message with the XML attachment with S/MIME. Use an online page on which users can enter their email address to which a copy will be sent.

Another option would be to use a signed PDF. The PDF standard allows PDF documents to be digitally signed and supports attached documents. The XML can be attached to the PDF as an attachment. The complete PDF including the attachments will be signed.

If you want to make it easier for end-users to check the signature, you should make sure that you use a universally trusted x.509 certificate or, use your own certificate and publish the thumbprint of the certificate somewhere.

Martijn Brinkers

Answer 2

Clients? That rather misses the point. It's really about file formats and protocols.

x509 is a cornerstone of encryption implementations. Although its just a starting point for implementing software on, it should probably be your starting point for looking for software.

About 10 years ago, I needed to implement a PKI for my then employers. The route I took was to set up a root Certification Authority using openSSL. At that time x509 verification was available for email required plugins for MSOutlook Express and Eudora (but not for the Bat!). IIRC, these are now built-in as standard. Wincrypt (file encryption) was available as a free download and Stunnel had just been ported to MSWindows.

Certainly 10 years is a long time in encryption circles - but the nice thing about x509 is it accomodates different algorithms / key sizes. It also builds on the same basis as LDAP.

More recently there are have been many more tools implemented using SSL type encryption - e.g. Gnu Anubis (OK, it doesn't actually add the sigs - but provides hooks for calling openssl), smart cards

Answer 3

Signatures which adhere to an open standard, can be validated on [...] Mac [...]

On the Mac, there's a Certificate, Key and Trust service which is built in all over the place. That means that you could send them S/MIME data and they'd be able to inspect the certificate and its trust path in the mail client. On the other hand, the UI just looks like a little tick or cross in a bar above the message. That's also the problem with using TLS - it works, but it works silently, and in some situations it fails silently too. Everything in between needs special knowledge to comprehend.

There is a third-party plugin for GPG in Apple Mail, but as you observed in your question registration and subsequent correct use of OpenPGP is hard.

Answer 4

If the example is a common use case (election official posting) then you might want to consider an official web page that provides server side cryptographic hash validation and the option to let citizens validate offline for themselves. The election office would post the results on a page that provides the result message, a hash of said message, and a function that confirms the hash is valid for the message on the server side (button action or automatic check with on page result). This scheme would address the trust chain by resolution of the site to a legitimate government agency URL. The attack surface would be the server side artifacts of message and hash and DNS. Considering the use case in the example is about publication and not securing original documentation this might suffice as the main point would be to avoid typos being reproduced in further dissemination (newspapers etc).