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D3.2: A study on PKI and biometrics

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Executive Summary

This document forms a comprehensive study on current implementations of digital signatures, public key infrastructure (PKI), biometrics and approaches for combined technical solutions. This is carried out from the legal and technological point of view, with focus put on privacy-enhancing implementations. 

This study explains the technical infrastructure and the functional principals of cryptographic algorithms used for electronic signing. Basic standards, definitions and terms including those introduced by the European Directive 1999/93/EC have been described in light of their importance in the on-going development of public-key infrastructures (PKI). Cryptographic signature schemes for various areas of application are described; they are solutions to meet security requirements and can be used to implement additional functionality such as non-deniability (undeniable signature scheme), resistance against forgery (fail-stop signature scheme) or included encryption (signcryption scheme). 

Security problems of PKI arise mainly from the use of general purpose machines for signing (secrecy of the private key may be compromised), the concept of trust in hierarchical systems (who trusts whom for what?), interoperability and compatibility aspects (such as certificate revocation, adoption of the X.509v3 standard for the internet and name schemes in cyberspace). In addition, the limited validity of key pairs raises a number of problems with for example the resigning of longer valid contracts or digital archives. The solution to those problems requires some technical effort and raises the costs of the infrastructure required for electronic signatures. From the privacy perspective the linkability of a certificate with the holder making her or him highly traceable when signing documents or transactions is a major problem of current PKI implementations. The concept of pseudonymous certificates is not very well adopted by the EU member states.  

This study describes legal requirements, legal effects and the probative values for the four types of electronic signatures defined in the Directive 1999/93/EC. Legal provisions on a European level on the use of pseudonyms for electronic signatures are also discussed. The intent of the European legislator towards pseudonymous electronic signatures is concluded. The European legislator tries to ensure that the member states do not prevent certification service providers from indicating pseudonyms in certificates whilst at the same time leaving the choice to give legal effect to pseudonymous signatures up to them. Certificate providers should be obliged to communicate their conditions to the signatory; they could indicate limitations in a qualified certificate which have to be recognisable to third parties. The requirements of a service provider which offers pseudonymous certificates are concluded; a main topic is that they are liable for the damage resulting from any inaccuracy and incompleteness of the information contained in the certificates.  

Using the established economic model of diffusion of technologies into a market, the currently available electronic signatures are compared with five key factors of success defined for technological products and solutions. While a good performance towards compatibility and complexity with today’s PKI is observed, the relative advantage against investigated substituting solutions, triability and observability need substantial improvements, at least in some European member states. This matches with the observed diffusion of electronic signatures in the investigated European markets.  

From the economic perspective, the diffusion of PKI in the European market has been notably less successful than expected. This study suggests six concrete measures to improve the diffusion into the market: 


  • To shift costs in order to achieve a fair distribution 

  • Measures to reach the critical mass of users 

  • Increasing awareness and knowledge about this technology 

  • To especially target the user group called ‘early adopters’ 

  • To increase triability e.g. by trial versions of electronic signatures 

  • To further reduce complexity of the needed private infrastructure 


In a case study on mobile signatures two approaches are investigated: (1) a server based approach that is independent from the client and (2) a client based approach using an improved SIM-card. Technical aspects, basic designs and typical processes needed for mobile signing are presented and discussed. A number of possible applications including multilateral secure financial transactions and integration in a user controlled digital identity management system are discussed. 

From the technological perspective, biometrics is a promising enrichment for the factors (or channels) of authentication. In this study terms and established categories of biometric methods are introduced. Five passive (physiological) and two active (behavioural) methods which demonstrate current state-of-the-art techniques are described and investigated with respect to technical strengths and weaknesses as well as privacy aspects. In contrast to the situation two years ago biometrics are now tested with higher enrolment numbers under operative conditions. Notably there is a gap between high quality of biometrics under laboratory conditions and limitations observed under practical implementation.  

In addition to active and passive biometrics, soft biometrics are discussed with respect to their current area of application. Currently they are not used for authentication and verification, but have found applications for statistical purposes e.g., the assignment of an individual to a specific group such as an ethnic group (see FIDIS Deliverable 7.2 for further information on group profiling). 

From a technological and economic perspective all biometric methods used today for authentication and verification depend on the following factors: 

  • Quality (low False Acceptance Rates (FAR), secure systems with e.g. high tamper resistance and compliance to the privacy criteria of the European Art. 29 Data Protection Working Party (WP 29)) 

  • Convenience (easy and quick enrolment, use and maintenance, low False Rejection Rate (FRR)) 

  • Costs for the needed infrastructure 

In this study technological weaknesses with respect to quality and convenience of the biometric methods were analysed. Many of these methods cannot be used by all people, for example iris recognition (e.g. people with aniridia) or genetic fingerprinting (e.g. monozygotic twins). In addition some methods cannot be used in certain environments. This applies to for example fingerprinting (people doing hard labour may have severely worn papillary structures) or face recognition (light conditions are vital for the quality of this method). For these cases alternative methods are needed when used for authentication or verification. Notably, current active and behavioural biometrics are not as reliable as passive biometrics. Perhaps the most notable aspect of biometrics is the connection between quality (in this case FAR) and convenience (in this case FRR) – it is not possible to optimise both factors at the same time due to the inherent connection between them. 

Based on an international discussion on biometrics raised in the late 1990s and the work of the European WP 29, privacy criteria for the use of biometrics have been developed and, at least on the European level, are well agreed. Implementation of these criteria seems to be possible, in principle, with all of the investigated methods. In this study a number of current initiatives, projects and technical available systems looking into implementation of these criteria are described.  

Figure 1‑: Magic triangle of biometrics


Many questions with respect to implementation of privacy criteria are still open from the perspective of currently available solutions. In some cases it is not known if privacy-critical information e.g., concerning health, can be extracted from templates. This is notably not thoroughly investigated in areas where numerous different algorithms are used to generate templates, e.g. for face recognition. Further research is necessary in this area, however it is expected that this will not be driven by the economic sector due to the lack of economic interest. 

Summing up those factors we can characterise biometrics by a “magic triangle” () deriving from a model originally established for project management. This magic triangle is defined through (1) quality, (2) convenience and (3) costs. At least currently it is impossible to optimise all of these factors at the same time – implementations of biometrics seem to always be a compromise with focus on one, at best two of these factors while the remaining factor or factors show significant weaknesses. Most notably, even when optimised with respect to quality, today’s available biometric systems are simply unsuitable for access control solutions with high security requirements.

In general we observe a far reaching international standardisation of methods and data formats for biometrics which are used for forensic purposes. This applies especially to fingerprinting and genetic fingerprinting. Other biometric methods suffer from the lack of standardisation; e.g., face recognition and hand geometry. In accordance to the privacy criteria described by the WP 29, biometrics which offer no common templates for the area of application, e.g., international authentication of individuals, should not be used. In addition, some algorithms and resulting template formats are subject to patents or copyright which will have an influence on the standardisation process. A case study on secure storage of biometric data on smart cards shows current research and development with respect to standardisation in the area of authentication technologies using biometrics. 

The case study is followed by a broader view on social, economic and political implications of the planned introduction of biometrics, e.g. in ID documents such as passports. The complexity, the total costs of the introduction of biometrics in large scale projects and the draw backs on society seem to be underestimated. More research is therefore required to establish the interaction of such systems pertaining to the implementation of biometric technologies and to consider the potential implications of their deployment to other fabrics of the socio-political sphere which in their own turn will affect the economy. Especially in countries where the implementation of biometrics in ID documents is an opportunity for e-government services or for cooperation between public and private sector, there needs to be more research on how such an implementation will reframe the economic and commercial national and international relations.  

Looking into current development of biometrics themselves we expect further improvements in the near future. Technological trends are: 

  • Improvement of sensors e.g. for taking images or aliveness detection 

  • Improvements in algorithms to generate templates 

  • Developments in methods used to discriminate acceptance from rejection 


This will result in improved accuracy, and hence security of biometric solutions. In addition further integration of systems e.g., bio-chips for genetic fingerprinting, will lead to improvements of functionality and further areas of application.  

However, it seems likely that implementations of biometric systems will remain a compromise between several factors, and limitations of enrolment and application of certain biometrics to specific groups will remain. 

It is possible for biometrics to offer significant enhancements to the PKI model. The co-operation of these technologies potentially brings a lot to the area of secure data exchange and general encryption. In the discussion on whether biometrics could enhance PKIs, the distinction has been drawn between (1) the use of biometrics in order to secure the access to the private key by the user, (2) the use of the biometric as an electronic signature and (3) the use of biometric characteristics as a private key in a PKI environment. Currently, most of those approaches are either still at the concept level, or in a pilot phase. Certainly within this structure, it appears that there is great promise for the continued integration of these two technologies. 

Within the FIDIS Network of Excellence the results of this document are to be used for further research leading to several future deliverables. One area of research is the application of biometrics and PKI in ID documents such as passports, ID cards or e-health cards.  

Certain aspects discussed in this document are also relevant for future work in the area of ID theft / ID fraud and ambient intelligence where authentication plays a central role. Certainly social and legal aspects of today’s technically weak authentications are an on-going matter of discussion there. From an ethical and legal perspective the question of how consent for transfer and processing of personal data defined by the European Directive 95/46/EC can be applied to passive authentication, e.g. using active biometrics, is still open. The solution to this issue is vital for the development of socially acceptable and commercially implementable concepts for ambient intelligent environments and thus also a key area of research within FIDIS. 




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