Issues regarding digital signature infrastructure and digital certificate management.

Robu, Raul ; Filip, Ioan

1. INTRODUCTION

The high performances reached by the information technology during the last years and its decreasing costs made it possible to present, manipulate and archive documents more and more in an electronic format. In this context, an essential role is played by the digital signature, the instrument through which the content of an electronic document and its issuer are authenticated in an almost infallible manner.

The digital signature is implemented using cryptographic methods based on two keys: a private key and a public key (Diffie & Hellman, 1976). The base principle of these methods is: a hash function--such as MD5, SHA1 (NIST, 1995), SHA2 (NIST, 2002)--is applied on document, in order to extract from the document a binary code, known as hash value (we note it H1), representing essential data. The hash value is encrypted (Rivest et al., 1978) with the private key of the signer, obtaining a cryptogram which represents it self the signature and which is attached to the original document. Signature validation procedure is achieved by extracting the hash value from the signed document (we note it H2), decrypting the cryptogram which is attached to it by using the public key, in order to obtain the initial hash value (H1). If the two hash values are identical (H1 and H2), the signature is valid.

2. THE DIGITAL SIGNATURE INFRASTRUCTURE

The required infrastructure to implement a digital signature system mainly consists from: digital certificates, server certificates, certification authorities (CA), facilities for the certificate management

2.1 Digital certificates

The digital certificate represents a collection of data in electronic format which confirm the connection between the validation data of the digital signature and a person, authenticating the identity of that person (Romanian Parliament, 2001). The most important data contained by a digital certificate are: public key, the name and the signature of the certification authority that issued the certificate, the holder's name, the cryptographic algorithm and the hash function used.

[FIGURE 1 OMITTED]

The most widely used digital certificate is the one with the number X.509, standardized ISO/IEC/ITU--version 3 (figure 1).

A system based on public key certificates also involves the existence of a Certification Authority, which releases certificates for a certain group of a key pair's owners (public and private). Each certificate contains the value of the public key and information that identifies in a unique mode the subject of the certificate (which can be a person, an application, a device or another entity which has the private key corresponding to the public key integrated in the certificate). The certificate represents a connection, impossible to fake, between a public key and a certain attribute of its possessor. The certificate is digitally signed by a Certification Authority, which confirms the identity of the subject. Once the certification infrastructure is established, a user of that public keys' infrastructure can obtain a public key for any of the users certified by that Certification Authority, simply by obtaining the certificate for that user and extracting the required public key (Patriciu et al., 2001).

2.2 Certificate servers

A certificate server, also known as a Cert-server or a Key-server, is in fact a database which allows the users to search or generate digital certificates. A Cert-server usually provides some administrative components which allow certification companies to maintain the security policies--for example, to permit storage only for the keys that fulfill certain conditions.

The most well known certificate servers are (Patriciu et al., 2001):

* Netscape Certificate Server: interoperates only between Netscape clients.

* Microsoft Certificate Server (IIS): interoperates between Netscape clients, as well as Entrust clients.

* Entrust Web/AC Server: permits SSL, S/MIME certificates to be released and published as well as the signing of objects for Netscape Enterprise Server, for MS IIS and Apache Stronghold servers.

2.3 Certification authorities

Because it is not practical for all the users in the world to trust a single organization or company concerning their secret communications, there are more Certification Authorities. They mutually detain certificates--not necessarily each one to the other one-, with the help of which an user having a certificate released by a certain authority--such as CA1--may obtain the public key of an user possessing a certificate released by another authority--such as CA3, for example -. Therefore, a so called certification chain or path is constituted -see figure 2-, in which, at the present time, all the great systems that distribute public keys are registered in.

A credible certification authority which does not have a digital certificate issued by another certification authority stands at the highest level of the previously mentioned hierarchy. The public key released by such a certification authority must be recognized in an independent manner, i.e. published on a large scale (DigiSign, 2006).

For example, in Romania, there are two certification authorities, DigiSign and e-Sign both headquartered in Bucharest. These companies are certified by ADACOM, and VERISIGN is the root of this certification tree. A certificate released by the e-Sign company is presented in figure 3.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. THE MANAGEMENT OF DIGITAL CERTIFICATES

The certification authorities usually have a secure site that allows the management of certificates. The management of digital certificates comprises (DigiSign, 2007):

a) Registration in order to obtain a digital certificate:

A person who buys a digital signature from a certification authority receives a cryptographic secure device (like e-token or smart card). Any use of the secure device involves entering the pin code. At three wrong introductions of the pin code, the device locks and can be unlocked by entering the puk code.

In order to obtain a digital certificate, one has to register on a secure page of the certification authority's website. Before registration, the future signatory must send certain documents to the certification authority. These documents consist of copies of personal identification documents, as well as a statement certified by the public notary through which one declares on his own risk that he learned about the legal provisions regarding the digital signature. The next step involves the installation of secured device drivers. Once these steps are taken, the registration process can be carried out only using the cryptographic device connected to the computer. The person fills in the form in which he enters personal data. Some of these personal data, such as one's name, email address, workplace, phone number, will appear in the certificate. The future signatory must also enter a password, which is needed in case of the certificate's revocation.

b) Receiving the certificate: Within 24 hours of the registration time, the administrator of the application who manages the certificates verifies the accuracy of the registration data, comparing them with the ones contained in the papers sent by the person. In case the registration data are valid, the administrator sends to the applicant, an email which contains a link and a password that the applicant can use to download the certificate. The applicant must introduce the cryptographic device in the computer and then he can download the certificate on the secure device. After this operation, the applicant can digitally sign any document.

c) Searching the certificates: all digital certificates generated by certification authorities are public and accessible to everyone. The confidentiality of a document or email message can be assured by encrypting the email message or document with the recipient's public key. This way only the recipient can decrypt the email or document with his private key located on his secure device. That is why certification authorities offer the opportunity to search and download any certificate generated by them, to anyone who is interested.

d) Renewal of the certificate: digital certificates have a period of validity of one year. They must be renewed annually, the expired certificates having no legal power.

e) Revocation of the certificate: if a person has lost possession of the secured device and believes that the pin code necessary in order to be able to sign is compromised, he may revoke the certificate. The digital signature done with a revoked certificate no longer has any legal value.

4. CONCLUSIONS

Digital signature offers many advantages and can be used in a wide range of applications, being able to substitute physical signature in any domain.

This technology assures important time reductions, allowing the achievement of remote transactions between individuals and / or institutions and eliminates paper work.

The authors offer in this paper a systematic presentation of the infrastructure of the digital signature and of the management of digital certificates, founded on their practical experience in implementing digital signature in hospitals and territorial work inspectorates from several counties of Romania.

5. REFERENCES

Diffie, W. & Hellman, M. (1976). New directions in cryptography, IEEE Transactions on Information Theory, Vol. 22 (Nov. 1976), pp. 644-654, ISSN 0018-9448

DigiSign (2006). The trust chain, Available from: http://www.digisign.ro/products-and-services/security-services/ qualified-certificates/lant.html, Accesses: 2008-06-14

DigiSign (2007). Registration, Searching, Renewal and Revocation of certificates, The homepage of Center of Digital Certification, Available from: https://ca-services. digisign.ro/public/digitalidCenter.htm, Accessed: 2008-06-15

NIST--National Institute of Standards and Technology (1995). Announcing the Secure Hash Standard, FIPS 180-1 (1995)

NIST--National Institute of Standards and Technology (2002). Announcing the Secure Hash Standard, FIPS 180-2 (2002)

Patriciu, V.V.; Ene-Pietrosanu, M.; Bica, I.; Vaduva, C. & Voicu, N. (2001). Security of electronic commerce, Bic All, ISBN 973-571-325-X, Romania

Rivest, R.; Shamir, A. & Adleman, L. (1978). A method for obtaining digital signatures and public key cryptosystems, Communications of the ACM, Vol. 21 (Feb. 1978), pp. 120-126, ISSN 0001-0782

Romanian Parliament (2001). Law nr. 455 about digital signature, Official Gazette of Romania, No. 429 (Jul. 2001), Available from: http://www.mcti.ro/index.php?L=0&id=20 &lege=3, Accessed: 2008-06-14