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As a result, parties may encrypt messages (or verify signatures) with no prior distribution of keys between individual participants. This is extremely useful in cases where pre-distribution of authenticated keys is inconvenient or infeasible due to technical restraints. However, to decrypt or sign messages, the authorized user must obtain the appropriate private key from the PKG. A caveat of this approach is that the PKG must be highly trusted, as it is capable of generating any user's private key and may therefore decrypt (or sign) messages without authorization. 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]],<ref>Craig Gentry [http://eprint.iacr.org/2003/183.pdf Certificate-Based Encryption and the Certificate Revocation Problem] ''Advances in Cryptology - Proceedings of EUROCRYPT 2003'' (2003)</ref> [[secure key issuing cryptography]]<ref>{{cite conference | first1 = Byoungcheon | last1 = Lee | first2 = Colin | last2 = Boyd | first3 = Ed | last3 = Dawson | first4 = Kwangjo | last4 = Kim | first5 = Jeongmo | last5 = Yang | first6 = Seungjae | last6 = Yoo | citeseerx = 10.1.1.6.337 | title = Secure Key Issuing in ID-based Cryptography | conference = ACS Conferences in Research and Practice in Information Technology - Proceedings of the Second Australian Information Security Workshop-AISW 2004 | year = 2004 }}</ref> and [[certificateless cryptography]].<ref>SS Al-Riyami, KG Paterson [http://www.springerlink.com/index/4WC47ELK7FP8XWTY.pdf Certificateless Public Key Cryptography] ''Advances in Cryptology - Proceedings of ASIACRYPT 2003'' (2003)</ref>
The steps involved are depicted in this diagram:[[
==Protocol framework==
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