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Digital Signatures

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Introduction

The information on this page was compiled from a series of posts on my blog in December, 2007 about the use of digital signatures in the CAD industry. Note that some information and links that were current as of the original writing may now be out of date.

NOTICE: The contents of this page are Copyright 2007 ManuSoft, All Rights Reserved. The contents of this page may not be reproduced under any circumstances without permission from the author! Permission to print one hardcopy of this page for individual use is hereby granted.

DISCLAIMER: ManuSoft is in no way responsible for either the accuracy or completeness of information on this page! Neither ManuSoft, Owen Wengerd, nor any credited or uncredited contributors may be held liable for any damages which may result either directly or indirectly from the use of information on this page! In other words, use at your own risk!

Owen Wengerd, President
ManuSoft

Prelude

For many, the word "encryption" has a mysterious quality that invokes images of math virtuosos in secret bunkers working feverishly during wartime to break the enemy's coded communications. My first exposure to encryption came in 1996 when I began working with Paul Kohut on the first version of CADLock software for locking AutoCAD drawing files. After overcoming my initial struggle to understand the terminology and get a handle on the mathematics behind encryption, I realized that it wasn't nearly as mysterious and complicated as it first appeared.

I knew it would take a long time for encryption terminology to become standardized and commonly understood by laypersons. From the first days of CADLock, we recognized that the key to success for our software was going to be our ability to educate consumers about our technology, it's possibilities and its limitations, its strengths and its weaknesses, what it could do and what it could not do. I felt that we needed to be realistic and patient while we waited for the market to catch up with our technology at its own pace. In the meantime, we needed to resist any temptation to needlessly bandy about sexy buzzwords like "encryption" lest we delay our mission by further muddying the waters in an already crowded ocean of technical jargon.

This recognition of the need for patience and perseverance has led me on a personal crusade to prevent encryption terminology from being perverted or hijacked by overeager marketing departments and uninformed experts. I've also tried to nudge the learning process along by adding my two cents whenever the opportunity arises. With this last goal in mind, I have prepared the following three part essay about digital signatures, tailored for the CAD industry. This is not written to academic standards, nor do I claim to be the final authority on the subject. Let me be clear about my agenda: I hope that furthering the common understanding of encryption related technology such as digital signatures will indirectly help sell more CADLock software!

Philosophically Speaking

There is nothing magical about digital signatures. They are simply an electronic mark used to signify a promise and to cement faith in the document or transaction to which that promise is attached. Some day digital signatures will be as commonplace among CAD professionals as the human readable "wet stamp" is today. However it must also be said that, just as CAD did not make architects more creative, digital signature technology will not make engineers more trustworthy.

The technology to use digital signatures has been around for well over 20 years, and a wide variety of software tools exist today that make digital signatures easy to use on almost any platform. So why is the use of digital signatures not more widespread? Primarily because the utopian dream of a paperless world has yet to materialize.

So long as hardcopy on paper is regarded as the authentic "record" version of a document, digital signatures are virtually useless. Even if new buildings are designed completely in CAD, an architect's digital signature simply won't mean anything if downstream consumers of the building design require a paper blueprint. The true benefits of a paperless world simply can't be realized until every link in the chain has joined the digital club. So long as even a single node on the distribution tree requires a human readable method of verifying authenticity, the architect is forced to use a handwritten wet signature on paper blueprints from the very start.

Handwritten signatures can be scanned into an electronic format, but then they lose their putative value because a digital facsimile is so easily reproduced. Therefore only original and unique handwritten wet signatures are trustworthy in a human readable form. Digital signatures do not translate to paper because they are not human readable. Therefore digital signatures only have value for a document in electronic form. This maxim is fundamental to a proper understanding of the emerging digital signature technology: digital data can only be securely signed by computers; and human readable documents can only be securely signed by humans.

I see many people in the CAD industry looking for a hybrid solution to this paper vs. digital dilemma by using a "digitized signature" (i.e. handwritten on paper, then scanned into electronic format) so that printed output contains this digitized signature. This cannot even remotely be considered a digital signature! There is simply no such thing as a secure printed digitized signature. Yes, you can digitally sign a document that has an embedded digitized signature, but once it is printed, that digitized signature is not worth the paper it's printed on. Not only could that signature be stolen from the document and used maliciously, there is no way to tell from the signature alone whether or not it has already been stolen and used maliciously.

The only hybrid approach that is practical and legally sound is to use digital and wet signatures in parallel. Using an architect as an example, this would mean creating and disseminating two sets of signed documents: the CAD model of a building (perhaps converted into a common 2D format like PDF, DWFx, or XPS) with the architect's digital signature and paper blueprints with the architect's individually handwritten wet signature.

A number of companies claim to sell some sort of hybrid solution involving the creation of a secure digitized signature. This is simply capitalism -- selling something useless just because there is a market for it. A digitized signature is only secure if it is never used. I've heard the rationalization that the digitized signature printed on paper meets a psychological need for people accustomed to seeing one, even if it has no legal value. In my opinion, such pandering borders on deception and serves only to further alienate digital signature technology from those who would benefit from it. This sort of abuse is typified by an emailed PDF file consisting of a scanned document with a handwritten signature. In such cases, checking the sender's email address likely becomes the most reliable way to verify the document's trustworthiness. Spam filters bear witness to the fact that the sender's email address is the most important -- and often only -- criteria we use in determining the level of trust to place in emailed documents. This is an important point to keep in mind.

The act of applying a digital signature requires the use of a secure private encryption key, but the digital signature alone does not provide security in any way. Digitally signing a document does not prevent it from being modified or stolen. In that respect it is no different than a handwritten wet signature. However, unlike a wet signature that cannot be easily duplicated, a digital signature does not prevent a signed document from being copied. This does not make digital signatures inferior. On the contrary, making copies is necessary and commonplace in the digital world, so the fact that exact copies of digitally signed documents are still trustworthy is a huge benefit over handwritten signatures.

There are other notable differences between a digital signature and a handwritten signature. Digital signatures can be time-stamped, thus providing reliable evidence that a document was signed on or before a certain time. A disadvantage of digital signatures is that they are only as reliable as the digital ID used to create them. To combat this problem, a public clearinghouse of compromised digital IDs must be maintained, and auditing systems must be in place to immediately detect and report suspicious activity. Time stamped digital signatures created before a digital ID is compromised can be easily and certainly distinguished from compromised signatures if properly designed and competently managed infrastructure is in place. This is necessary to ensure that previously existing digital signatures survive a compromising event.

In conclusion, adequate technical solutions and time tested safeguards already exist for successfully using digital signatures in a professional environment, but digital consumers still need to learn how to work within the limitations of the technology. Building an acceptable comfort level with a technology that must by its nature completely replace something as significant and culturally ingrained as the handwritten signature will not be easy or quick, but it is inevitable.

Under the Hood

The basic requirements of a digital signature are that it must uniquely identify the signatory, it must be independently verifiable, and it must be invalidated if the signed data has changed. To understand how these objectives are achieved, let's start with the foundation of modern digital signature technology: public key encryption and the public key infrastructure, or PKI.

In public key encryption, a "key pair" consists of two parts: a public key and a private key. In simplistic terms, the public key is mathematically derived from the randomly generated private key using an algorithm known as a "one way function". A one way function makes it easy to calculate a public key if the private key is known, but extremely difficult to deduce the private key if the public key is known. The end result is a secret private key and an openly shared public key that are mathematically related in such a way that the public key can be used to decrypt data that was encrypted with the private key, and the private key can be used to decrypt data that was encrypted with the public key.

This interesting property of such a key pair gives rise to a number of useful capabilities. In the case of digital signatures, the act of signing data is essentially nothing more complicated than encrypting the data with a private key. If the data can be decrypted successfully with the signer's public key, then only the signer's private key could have been used to do the encrypting. In practice, this process is simplified so that the signer encrypts only a secure hash, or checksum, of the data to be signed. The recipient then calculates the hash from the raw data and compares the result with the "signed" hash after it is decrypted. If the values match, the digital signature and data are validated.

For this process to work properly, there need to be standard ways to package information about the algorithms used, and to provide important information about the keys themselves. This need is fulfilled by digital certificates. A digital certificate is a file or block of memory containing a public key along with ancillary data about the key and its owner. The certificate is itself digitally signed by the entity, usually a mutually trusted third party, that issued the certificate. This enables users to verify that the public key is valid and trustworthy.

A digital ID is the private key component of a key pair. Normally the private key is not stored together with the public key, but instead is stored in a separate physical location for security, usually requiring a password to access it. A key manager maintains links between the digital certificate and its associated private key. In many cases, it is convenient to use the term "digital ID" to mean both the public and private keys, even though they are physically separated.

It is almost always a good idea to time stamp digital signatures. Time stamping involves sending the digital signature to a time stamp authority, who then creates and returns a digitally signed time stamp that is uniquely and securely associated with the original digital signature. The time stamp can then be verified by third parties in the future by using exactly the same technique used to verify a digital signature.

I think these important terms deserve a review. A "digital certificate" is a public key, which is itself digitally signed by a mutually trusted third party. Your digital certificate represents your public digital identity, and it should be made freely available to anyone who wants or needs it. A "digital ID" is a digital certificate and the private key associated with the digital certificate. It isn't difficult to create your own self-signed digital ID, but a digital ID is only as good as the issuing authority that signs it. When you purchase a digital ID from a third party like VeriSign or Thawte, their reputation makes your digital ID more trustworthy.

Practical Guidelines

We use digital signatures every time we visit a secure web site. Visiting a secure web site involves an authentication process that includes verifying the identity of the server by ensuring that its digital certificate, or "server certificate", is signed by a trusted certificate authority. This verification process might involve verifying an entire chain of certificates from the actual server certificate up through one or more intermediate certificate authorities and ending with a trusted root certificate authority. This all takes place quickly and automatically before the web page is displayed in your web browser because the web browser includes built in logic to do this work without any user interaction. More importantly, the web browser warns us when the server certificate is expired or invalid.

The biggest obstacle when using digital certificates in a CAD environment today is not creating them, but easily and automatically verifying them at the receiving end. Even in a completely digital distribution system where everybody works from the CAD model, the various software tools we use to view and work with the model do not handle digital signature verification automatically in a standardized way. As long as downstream consumers of CAD data cannot easily and automatically ensure the trustworthiness of digital data, they will continue to rely on handwritten signatures on paper.

A second obstacle to the use of digital signatures is the difficulty in accepting that digitally signed data is only trustworthy while it remains in digital format, and therefore the digital file is the "record" document. There is substantial social inertia that must be overcome before a digital document can gain the same amount of trust as a paper document. Engineers and architects must deal with the specter of previously hidden meta data in their CAD models becoming part of their signed document, thereby exposing them to new liabilities that don't exist with paper drawings. Construction supervisors must learn to refer to the CAD model instead of relying on hardcopy blueprints when resolving disputes or establishing responsibility for errors. Here I think it should be noted that the use of a digitally signed model does not preclude the creation of hardcopy blueprints. Those can be created and "wet stamped" separately at the same time the CAD model is signed digitally; or they can be created in the field for reference without any signature at all.

AutoCAD has supported digital signatures for several years, but using the built in functionality is limited to only individual DWG files, lacks support for co-signing (more than one person signing), and forces the signed document to remain in the proprietary DWG format or lose its signature. These problems can be worked around by using third party tools, but doing so requires recipients to use the same tools.

Over the past few years, many government plan review bodies have amended laws and administrative rules to accommodate digital signatures as part of the plan review process. Without standardization, however, organizations still struggle to effect the necessary changes in their workflow. A lack of uniformity in terminology from one set of regulations to another adds to the confusion. If you are involved in amending or creating rules or regulations that enable the use of digital signatures, you should use generic and well defined terms of art in the regulations, but supplement these with practical guidelines that mention specific technologies, software tools, and file formats that will meet the legal requirements and that you are capable of working with.

If you are an architect, engineer, or CAD manager working to implement digital signatures into your firm's workflow, there are some concrete steps you can take to make the task easier. Start by segregating your distribution network into "digital-only" and "hardcopy" classes of downstream users. Begin the transition with the digital-only part of the network (perhaps only the plan reviewing authority, for example). Next, decide which file format to use for your digital "documents". Rather than signing CAD files, many companies start by signing 2D output files such as PDF, DWFx, or XPS. These files are essentially digital versions of the hardcopy documents, so they are more familiar to a wider audience and avoid some of the liability issues of exposing formerly hidden metadata that lives within the CAD model files.

You'll need to obtain a digital ID and establish internal policies for storing and accessing the digital ID so that only the owner of the digital ID ever has access to the private key. Windows includes a built in certificate manager that you can use to view and manage your digital IDs. To start the certificate manager, run the certmgr.msc management console by entering its name in the Start -> Run command window. Your digital certificate will be installed in your personal certificates folder along with a link to the private key stored in the Windows secure key repository. Make a backup of the digital ID by exporting it to a password protected PFX file. Once a backup is made, the private key should be marked as not exportable to further secure it.

If you want to create digitally signed AutoCAD DWG files, you can use the digital signature feature of AutoCAD to sign a drawing file either while saving it or after it is saved. You should also consider subscribing to a commercial time service (see What time is it?) to ensure that your signatures are accompanied by a reliable time stamp in case your digital ID becomes compromised at some point in the future. Third party tools like CADVault for AutoCAD even make it possible for different people to sign different parts of the CAD model, but such advanced functionality is not needed in most cases.

If you use different CAD software that does not support digital signatures natively, or if you choose to sign only the secondary files produced by exporting your CAD model to a different format, then you will need to use either tools specific to that format or third party tools that work with files of any format. Adobe Acrobat (PDF) and Microsoft's free XPS Viewer both provide integrated digital signature tools that use the same digital IDs that you would use in AutoCAD, Internet Explorer, or Outlook/Windows Mail, and both applications are easy for recipients to obtain and use.

Another popular tool for managing digital IDs and signing files is an open source tool called GnuPG. GnuPG utilizes encryption and key storage standards called OpenPGP. OpenPGP is not compatible with the X.509 standard used by Windows and many other encryption tools, however it is an attractive alternative when cost or closed source tools are a prohibitive barrier. There are many other digital signature resources available on the internet for those wanting more information, or needing specialized tools.

Unfortunately, no matter what software tools or file formats you use, today's CAD software and document viewers still do not provide the user experience that web browsers do when it comes to digital signatures. These problems can be overcome by end users, but ultimately they need to be addressed by the makers of the software tools we use. Software for handling digital data will need better user interfaces that allow users to easily specify which digital signatures should be trusted for which purposes, and provide requisite warnings when a document should not be trusted. I am confident that these improvements will come in the future, especially as more companies begin to use digital signatures in their workflow and demand for better digital signature support rises.

If you already use digital signatures with your CAD related documents, I would like to hear about it. Please leave a comment about your experiences, whether good or bad!

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