Identity-based systems resemble ordinary public-key systems, involving a private transformation and a public transformation, but users do not have explicit public keys as before. Instead, the public key is effectively replaced by (or constructed from) a user’s publicly available identity information (e.g., name and network or street address). Any publicly available information which uniquely identifies a user and can be undeniably associated with the user, may serve as the identity information.
13.25 Definition An identity-based cryptographic system (ID-based system) is an asymmetric system wherein an entity’s public identification information (unique name) plays the role of its public key, and is used as input by a trusted authority T (along with T’s private key) to compute the entity’s corresponding private key.
After computing it, T transfers the entity’s private key to the entity over a secure (authentic and private) channel. This private key is computed from not only the entity’s identity information, but must also be a function of some privileged information known only to T (T’s private key). This is necessary to prevent forgery and impersonation - it is essential that only T be able to create valid private keys corresponding to given identification information. Corresponding (authentic) publicly available system data must be incorporated in the cryptographic transformations of the ID-based system, analogous to the certification authority’s public key in certificate-based systems. Figure 13.7(b) on page 564 illustrates the design of an identity-based system. In some cases, additional system-defined public data I), must be associated with each user A in addition to its a priori identity ID a (see Remark 13.27); such systems are no longer “purely” identity-based, although neither the authenticity of D nor ID a need be explicitly verified.
13.26 Remark (authenticity> in ID-based systems) ID-based systems differ from public-key systems in that the authenticity of user-specific public data is not (and need not be) explicitly verified, as is necessary' for user public keys in certificate-based systems. The inherent redundancy of user public data in ID-based systems (derived through the dependence of the corresponding private key thereon), together with the use of authentic public system data, implicitly protects against forgeiy; if incorrect user public data is used, the cryptographic transformations simply fail. More specifically: signature verification fails, entity authentication fails, public-key encryption results in undecipherable text, and key-agreement results in parties establishing different keys, respectively, for (properly constructed) identity-based signature, authentication, encryption, and key establishment mechanisms.
The motivation behind ID-based systems is to create a cryptographic system modeling an ideal mail system wherein knowledge of a person’s name alone suffices to allow mail to be sent which that person alone can read, and to allow verification of signatures that person alone could have produced. In such an ideal cryptographic system:
- 1. users need exchange neither symmetric keys nor public keys;
- 2. public directories (files of public keys or certificates) need not be kept; and
- 3. the services of a trusted authority are needed solely during a set-up phase (during which users acquire authentic public system parameters, to be maintained).
- 13.27 Remark (ideal vs. actual ID-based systems) A drawback in many concrete proposals of ID-based systems is that the required user-specific identity data includes additional data (an integer or public data value), denoted Da in Figure 13.7(b), beyond an a priori identity ID a- For example, see Note 10.29(ii) on Feige-Fiat-Shamir identification. Ideally, Da is not required, as a primary motivation for identity-based schemes is to eliminate the need to transmit public keys, to allow truly non-interactive protocols with identity information itself sufficing as an authentic public key. The issue is less significant in signature and identification schemes where the public key of a claimant is not required until receiving a message from that claimant (in this case Da is easily provided); but hi this case, the advantage of identity-based schemes diminishes. It is more critical in key agreement and public-key encryption applications where another party’s public key is needed at the outset. See also Remark 13.31.
- 13.28 Example (ID-based system implemented using chipcards) A simplified ID-based system
based on chipcards may be run as follows. A third party T, acting as a trusted key generation system, is responsible solely for providing each user a chipcard during a set-up phase, containing that party’s ID-based private key, after carrying out a thorough identity check. If no further users need be added, T may publish the public system data and cease to exist. Users are responsible for not disclosing their private keys or losing their cards. □