Biometrics and PKI

PKI stands for Public Key Infrastructure and refers to the infrastructure and procedures required to facilitate the management, distribution, storage, and revocation of certificates based upon public key cryptography. This in turn seeks to provide secure data exchange over third-party networks such as the Internet.

A PKI effectively provides a tool set with which organizations or private individuals can implement a level of data transmission security appropriate to their needs. In some cases this may be signing a message or document with a digital signature in order to verify its source, whilst in other cases it may mean total encryption of the message as well as digital signing. In addition to the desired privacy of information, a PKI seeks to provide:

• Integrity—to verify that a message or document is genuine and has not been manipulated or changed since its original creation and signing.
• Authentication—to verify the identity of the individual or organization sending the message.
• Non-repudiation—to ensure that the originator of the message or transaction can not subsequently disown it.

In addition, for many transactions undertaken over the Net, the users involved neither see, hear, or even know each other, leaving little scope for reaching intelligent conclusions as to the integrity of the received message, or the authenticity of the identity of the originator.

We therefore need a methodology to ensure authenticity and integrity of messages and transactions transported via the Internet, or indeed, any such untrusted network outside the immediate control of the user. PKI offers such a methodology, which can be used in a variety of scenarios, but is especially pertinent to the Internet.

In order to create a certificate for yourself or your organization, you need to make a certificate request, usually via a Registration Authority which acts as an intermediary between yourself and the Certificate Authority. There are two primary types of certificate request, known as PKCS#10 and RFC2511, with PKCS#10 perhaps being the most popular. The PKCS#10 certificate request typically consists of a version number, the certificate owners name (as in 'Distinguished Name' or Dname), the certificate owners public key and other attributes that the owner may wish to publish such as e-mail address, telephone number and so on. The Dname is an ordered set of owner attributes which includes the applicable two letter country code, the state or province, the locality or street, the organization name, the organization department, and the individual owners name.

When the Certificate Authority receives the request, it will check the authenticity and if satisfied, will sign and publish the certificate accordingly. The owner may wish to generate their own key pair and submit the public key to the Certificate Authority for inclusion in the certificate, thus keeping the private key totally in house. Alternatively, the Certificate Authority may generate the key pair and send the private key back to the certificate owner when the certificate is created and published.

Of course, it is possible for an individual or organization to generate keys and send the public key directly to trusted parties without formally going through the certification process. However, managing the ongoing situation could become extremely complicated, especially if the private key was compromised in any way. A trusted Certificate Authority thus provides a useful management function for those wishing to utilize a PKI. Part of this management includes maintaining the validity of public keys via regular updates and also maintaining a 'revocation' database for keys which have been revoked for one reason or another.

Figure 1:  Signing a message

In Figure 1, the originator of the message creates a hash from the document, encrypts the hash with his private key in order to create the digital signature and then sends both the message and the signature to the recipient. The recipient creates a hash from the message, decrypts the signature to recreate the original hash and then compares the two hash values. In practice, good quality available software streamlines this process for the user.

The benefits are twofold. Firstly, the recipient can have confidence that the received message has not been tampered with or altered in any way, because the two hash values match. Secondly, the recipient can have confidence as to the true identity of the sender, because he used the sender's public key to decrypt the digital signature. If we utilize message encryption and digital signatures within a PKI environment, our confidence in data exchange over untrusted networks is increased considerably.

One of the often repeated concerns lies in the area of key management, and in particular, the likelihood of your private key being misused or perhaps stolen. For example, if the operation of your private key is protected by a PIN, then this may easily be compromised at your workstation by someone who wishes to pretend to be you and makes it his or her business to discover that PIN. Similarly, if the private key is stored on your computer's hard disk, then how easy is it for someone to hack into your computer and copy this file? If someone acquires and is able to use your private key, then your PKI environment is powerless to protect you as this person could intercept messages meant for you and easily decrypt them. Furthermore they could pretend to be you within the context of important transactions, with all the implications that this entails. Key management and key security therefore become paramount within a PKI environment.

Let's bring in another old friend, the chip card or smart card as it is sometimes known. If we undertake key management functionality right on the card itself and maintain the private key in the secure area of the chip, then we can use the private key straight from here, removing the problems associated with storing the private key on the hard disk. The user now has absolute control over the key and can carry it around with him or keep it secure, just as he would with a physical key. If we now protect access to this private key via a biometric, we have created a considerably higher confidence level as to the true identity of the originator and digital signatory of a specific PKI message. In addition, we have dramatically reduced the possibility that the private key could be fraudulently acquired by a third party as we are physically securing it away from a hard disk or network drive.

We should also consider the user position in this context. Some people may be wary of having the biometric template on the chip card for fear of identity theft should the chip card be lost or stolen. Others are wary of using biometrics with a PKI because of the enhanced non repudiation that this offers, feeling that their anonymity is compromised and that third parties such as law enforcement agencies might use this against them. For every distinct view on the subject, it is likely that you will find an equally distinct opposing one. Clearly a solution which seems ideal for one group will not necessarily be acceptable to another and we should be cognizant of this reality. Perhaps the answer lies in developing the technological infrastructure that allows for all levels to be accommodated, and then letting the user choose to what degree they wish to use the functionality. For example, a particular solution may integrate biometrics, chip cards and PKI, but allow the user to choose whether they use a biometric or a PIN, how and where the biometric template is stored and other variables. This would then place the choice, and the responsibility for that choice with the user or user organization. If adopting a higher level of security unlocks enhanced functionality, or otherwise, as the case may be, then why not allow the user to choose accordingly? This is perhaps a thorny question, especially when we move into the territory of public applications, but these are the sort of questions we need to ask as the relevant technology continues to move forward. From a technical solution perspective, the integration of biometric and PKI models offers the potential for substantially enhanced confidence in data exchange over untrusted networks, especially in the areas of digital signing and non repudiation. Historically, the two camps have not always seen eye to eye on the subject, but maybe it is time to move closer together and understand the potential advantages and how these might be offered to user communities.