ID-based cryptography (or identity based cryptography or identity based encryption) is a key authentication system in which the public key of a user is some unique information about the identity of the user (e.g. email address).

This system works by having a trusted third party who has a secret which can be combined with a user's identity information to produce the user's secret key. The third party also produces some public information which is derived from its secret.

To decrypt or sign a message the user uses their private key as with normal public key cryptography, but to verify the signature or encrypt a message only the identity information and the third party's public information is needed.

Originally when this system was developed in 1984 by Adi Shamir it could only be used for keys for digital signatures, but in 2001 the method was extended by Dan Boneh and Matthew K. Franklin to encryption/decryption through the use of Weil pairings.

Only limited work has been done in terms of formally analysing ID based cryptosystems, some of which have been recently broken (said in 2004).

Because any user's private key can be generated through the use of the third party's secret, this system has inherent key escrow. A number of variant systems have been proposed which remove the escrow including certificate-based encryption, secure key issuing cryptography and certificateless cryptography.

One of this system's major advantages is that if there are only a finite number of users, after all users have been issued with keys the third party's secret can be destroyed. This can take place because this system assumes that, once issued, keys are always valid (as this basic system lacks an method of key revocation ). The majority of derivatives of this system which have key revocation lose this advantage.

References

• Adi Shamir. Identity-Based Cryptosystems and Signature Schemes. Advances in Cryptology: Proceedings of CRYPTO 84, Lecture Notes in Computer Science, 7:47--53, 1984.
• Yevgeniy Dodis, Jonathan Katz, Shouhuai Xu, Moti Yung, Strong Key-Insulated Signature Schemes (2002).
• Dan Boneh , Matthew K. Franklin, Identity-Based Encryption from the Weil Pairing. Advances in Cryptology - Proceedings of CRYPTO 2001 (2001).