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'''Identity-based cryptography''' is a type of [[public-key cryptography]] in which a publicly known string representing an individual or organization is used as a [[public key]]. The public string could include an email address, ___domain name, or a physical IP address.
The first implementation of identity-based signatures and an email-address based [[public-key infrastructure]] (PKI) was developed by [[Adi Shamir]] in 1984,<ref>Adi Shamir, [http://www.iseca.org/modules/mydownloads/visit.php?cid=56&lid=33 Identity-Based Cryptosystems and Signature Schemes] {{Webarchive|url=https://web.archive.org/web/20200812185833/http://www.iseca.org/modules/mydownloads/visit.php?cid=56&lid=33 |date=2020-08-12 }}. ''Advances in Cryptology: Proceedings of CRYPTO 84, Lecture Notes in Computer Science'', 7:47--53, 1984</ref> which allowed users to verify [[digital signatures]] using only public information such as the user's identifier. Under Shamir's scheme, a trusted third party would deliver the private key to the user after verification of the user's identity, with verification essentially the same as that required for issuing a [[public-key certificate|certificate]] in a typical PKI.
Shamir similarly proposed [[identity-based encryption]], which appeared particularly attractive since there was no need to acquire an identity's public key prior to encryption. However, he was unable to come up with a concrete solution, and identity-based encryption remained an open problem for many years. The first practical implementations were finally devised by Sakai in 2000,<ref>
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==Limitation==
Identity-based systems have a characteristic problem in operation. Suppose Alice and Bob are users of such a system. Since the information needed to find Alice's public key is completely determined by Alice's ID and the master public key, it is not possible to revoke Alice's credentials and issue new credentials without either (a) changing Alice's ID (usually a phone number or an email address which will appear in a corporate directory); or (b) changing the master public key and re-issuing private keys to all users, including Bob.<ref name=green1>{{Cite journal |title=Forward Secure Asynchronous Messaging from Puncturable Encryption |archive-url=https://archive.
This limitation may be overcome by including a time component (e.g. the current month) in the identity.{{r|green1}}
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