Public Key Cryptography

The concept of public key cryptography is used as long as humans are expect-ing secure communications with each others. Many techniques are developed over

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thousands of years for encrypting message in order to secure the confidentiality of the message over the communication link from potential eavesdroppers [15].

Each technique used pre shared secret keys for encrypting and decrypting the mes-sage. Encrypting message (plain text) produces cipher text (non readable text) and decrypting cipher text produces the message (plain text). This technique is called symmetric encryption.

In the beginning of 1970 a new encryption technique was developed by Whitfield Diffie and Martin Hellman where two different keys were used for encrypting and decrypting [78]. The encryption key is called private key and the decryption key is called public key and both of them are called private-public key pair, and the technique is called asymmetric encryption. Private Key is the secret while public key, as the name reveal, is public i.e. it is published. The invention of this tech-nique opened a new chapter in the cryptography world.

Public key cryptography is a powerful tool that allows for authentication key dis-tribution and non-repudiation [76]. A fundamental problem raises here that how to believe that the public key belongs to the one who claims for, especially when one is accessing public key.

A public key cryptography is useful in commercial applications when the public key is tracked efficiently. This is accomplished through the so called public key infrastructure (PKI).

A brief description of some entities in PKI is given below;

Certificate

A traditional definition of certificate according to [8] is that it is a document testi-fying the truth of something. The owner is identified through picture, handwritten signature and other physical cues in certificate.

In the same way as in a paper-based certificate, in digital world, certificate is an electronic document or digital stamp testifying the identity of the holder of certifi-cate. In other words a certificate is digital document binding an entity name to a public key [76]. A certificate is issued by certification authority. A certificate con-tains all necessary security parameters for identifying the holder and proving the originality of the certificate. The issuer of the certificate guarantees the authentic-ity of the holder of certificate. As an example SSL/TLS certificate will be discussed.

SSL/TLS Certificate

SSL is originated by Netscape. The main idea behind this protocol was to establish secure channel between two entities. It has several versions where the last version according to [77] is V3.1. This protocol utilized TCP to provide reliable and secure

end-to-end secure services. It is a two layers protocol which operates on the top of TCP. Figure 2.1 illustrates the protocol stack.

Figure 2.1: SSL Protocol Stack [77]

The important concepts of SSL are connection and session. Providing service be-tween two entities according to OSI definition is transport which in this case is peer-to-peer [76]. The session is association between client and server which is created by hand shake protocol, and is secured through cryptographic means and can be established between different entities [76]. Figure 2.2 illustrates how SSL works and a deep detail is omitted here.

Figure 2.2: SSL Connection set up [75]

For securing the content (confidentiality) symmetric encryption and key exchang-ing asymmetric encryption are used. For connection’s reliability message includes integrity check i.e. hash of message is computed and signed.

TLS is predecessor of SSL. IETF established a working group to standardize SSL to be adopted in the internet environment. There are slightly technical differences between SSL and TLS which is not discussed further here.

Certification Authority (CA)

To ensure that the communication is secure using public key cryptography a third trusted party is needed to attest the authenticity of the communication parties.

This is achieved through the concept of certification authority (CA) where CA cer-tifies that public keys to be used for decrypting belong to the claimant.

A physical entity who issues such certificates which identifies the owner of a public key is called certification authority. In other words a CA certifies that a public key belongs to physical entity. This is called identity binding or public-key certificate.

Relying Parties (RP)

Relying parties are entities that are trying to identify the identity of another party using digital signature. In the real world a relying party would be a service provider who wants to identify its customer in order to provide service to the right person.

Certificate Revocation Methods

Certificate revocation is a process for changing the status of a certificate issued by CA.

An issued certificate is expected to be in use for a period of time for which it is valid for, but in some circumstances certificate validity may expire due to different reasons for example an employee is no longer working for an employer where the employer is needed to cancel his certificate [63]. This is achieved through two dif-ferent protocols, Certificate Revocation List (CRL) and Online Certificate Status Protocol (OCSP).

In CRL protocol each CA issues a data structure periodically, called Certificate Re-vocation List, and made it publicly available through repository for the end users.

The list identifying the revoked certificates is time stamped [63].

When a timely revocation of certificate is necessary OCSP is a better solution.

OCSP enables the applications to check the status of a certificate from an OCSP responder. In some cases where the information is sensitive the revocation infor-mation of a certificate must be clear before a communication can take place. OCSP gives timely revocation information of certificate than achieved through CRL [47].

These two methods contribute in finding the status of a certificate whether it is valid or revoked.