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{{Short description|Type of public-key encryption}}
'''Identity-based encryption''' ('''IBE'''), is an important primitive of [[identity-based cryptography]]. As such it is a type of [[public-key encryption]] in which the [[public key]] of a user is some unique information about the identity of the user (e.g. a user's email address). This means that a sender who has access to the public parameters of the system can encrypt a message using e.g. the text-value of the receiver's name or email address as a key. The receiver obtains its decryption key from a central authority, which needs to be trusted as it generates secret keys for every user.
Identity-based encryption was proposed by [[Adi Shamir]] in 1984.<ref name="iseca.org">{{cite conference
| last = Shamir | first = Adi | author-link = Adi Shamir
| editor1-last = Blakley | editor1-first = G. R.
| editor2-last = Chaum | editor2-first = David
| contribution = Identity-Based Cryptosystems and Signature Schemes
| doi = 10.1007/3-540-39568-7_5
| pages = 47–53
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Advances in Cryptology, Proceedings of CRYPTO '84, Santa Barbara, California, USA, August 19–22, 1984, Proceedings
| volume = 196
| year = 1984| doi-access = free
}}</ref> He was however only able to give an instantiation of [[Identity-based cryptography|identity-based signatures]]. Identity-based encryption remained an open problem for many years.
The [[pairing-based cryptography|pairing]]-based [[Boneh–Franklin scheme]]<ref>{{cite journal
| last1 = Boneh | first1 = Dan | author1-link = Dan Boneh
| last2 = Franklin | first2 = Matthew | author2-link = Matthew K. Franklin
| doi = 10.1137/S0097539701398521
| issue = 3
| journal = [[SIAM Journal on Computing]]
| mr = 2001745
| pages = 586–615
| title = Identity-based encryption from the Weil pairing
| volume = 32
| year = 2003}}</ref> and [[Cocks IBE scheme|Cocks's encryption scheme]]<ref>{{cite conference
| last = Cocks | first = Clifford C. | author-link = Clifford Cocks
| editor-last = Honary | editor-first = Bahram
| contribution = An identity based encryption scheme based on quadratic residues
| doi = 10.1007/3-540-45325-3_32
| pages = 360–363
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Cryptography and Coding, 8th IMA International Conference, Cirencester, UK, December 17–19, 2001, Proceedings
| volume = 2260
| year = 2001}}</ref> based on [[quadratic residue]]s both solved the IBE problem in 2001.
== Usage ==
Identity-based systems allow any party to generate a public key from a known identity value such as an ASCII string. A trusted third party, called the [[Private Key Generator]] (PKG), generates the corresponding private keys. To operate, the PKG first publishes a master public key, and retains the corresponding '''master private key''' (referred to as ''master key''). Given the master public key, any party can compute a public key corresponding to the identity by combining the master public key with the identity value. To obtain a corresponding private key, the party authorized to use the identity ''ID'' contacts the PKG, which uses the master private key to generate the private key for identity ''ID''.
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]],
| last = Gentry | first = Craig | author-link = Craig Gentry
| editor-last = Biham | editor-first = Eli
| contribution = Certificate-based encryption and the certificate revocation problem
| contribution-url = https://eprint.iacr.org/2003/183
| doi = 10.1007/3-540-39200-9_17
| pages = 272–293
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Advances in Cryptology – EUROCRYPT 2003, International Conference on the Theory and Applications of Cryptographic Techniques, Warsaw, Poland, May 4–8, 2003, Proceedings
| volume = 2656
| year = 2003| doi-access = free
}}</ref> [[secure key issuing cryptography]]<ref>{{cite conference
| last1 = Lee | first1 = Byoungcheon
| last2 = Boyd | first2 = Colin
| last3 = Dawson | first3 = Ed
| last4 = Kim | first4 = Kwangjo
| last5 = Yang | first5 = Jeongmo
| last6 = Yoo | first6 = Seungjae
| editor1-last = Hogan | editor1-first = James M.
| editor2-last = Montague | editor2-first = Paul
| editor3-last = Purvis | editor3-first = Martin K.
| editor4-last = Steketee | editor4-first = Chris
| contribution = Secure key issuing in ID-based cryptography
| contribution-url = https://crpit.scem.westernsydney.edu.au/abstracts/CRPITV32Lee.html
| pages = 69–74
| publisher = Australian Computer Society
| series = CRPIT
| title = ACSW Frontiers 2004, 2004 ACSW Workshops – the Australasian Information Security Workshop (AISW2004), the Australasian Workshop on Data Mining and Web Intelligence (DMWI2004), and the Australasian Workshop on Software Internationalisation (AWSI2004), Dunedin, New Zealand, January 2004
| volume = 32
| year = 2004}}</ref> and [[certificateless cryptography]].<ref>{{cite conference
| last1 = Al-Riyami | first1 = Sattam S.
| last2 = Paterson | first2 = Kenneth G.
| editor-last = Laih | editor-first = Chi-Sung
| contribution = Certificateless public key cryptography
| contribution-url = https://eprint.iacr.org/2003/126
| doi = 10.1007/978-3-540-40061-5_29
| pages = 452–473
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Advances in Cryptology – ASIACRYPT 2003, 9th International Conference on the Theory and Application of Cryptology and Information Security, Taipei, Taiwan, November 30 – December 4, 2003, Proceedings
| volume = 2894
| year = 2003| doi-access = free
}}</ref>
The steps involved are depicted in this diagram:[[File:Identity Based Encryption Steps.png|center|thumb|600px|ID Based Encryption: Offline and Online Steps]]
== Protocol framework ==
[[Dan Boneh]] and [[Matthew K. Franklin]] defined a set of four algorithms that form a complete IBE system:
* '''Setup''': This algorithm is run by the PKG one time for creating the whole IBE environment. The master key is kept secret and used to derive users' private keys, while the system parameters are made public. It accepts a [[security parameter]] <math>\textstyle k</math> (i.e. binary length of key material) and outputs:
# A set <math>\textstyle \mathcal{P}</math> of system parameters, including the
# a master key <math>\textstyle K_m</math>.
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* '''Decrypt''': Accepts <math>\textstyle d</math>, <math>\textstyle \mathcal{P}</math> and <math>\textstyle c \in \mathcal{C}</math> and returns <math>\textstyle m \in \mathcal{M}</math>.
=== Correctness constraint ===
In order for the whole system to work, one has to postulate that:
:<math> \forall m \in \mathcal{M}, ID \in \left\{0,1\right\}^*: \mathrm{Decrypt}\left(\mathrm{Extract}\left(\mathcal{P}, K_m, ID\right), \mathcal{P}, \mathrm{Encrypt}\left(\mathcal{P}, m, ID \right) \right) = m </math>
== Encryption schemes ==
The most efficient identity-based encryption schemes are currently based on [[Pairing|bilinear pairings]] on [[elliptic curves]], such as the [[weil pairing|Weil]] or [[Tate pairing|Tate]] pairings. The first of these schemes was developed by [[Dan Boneh]] and [[Matthew K. Franklin]] (2001), and performs [[probabilistic encryption]] of arbitrary ciphertexts using an [[ElGamal encryption|Elgamal]]-like approach. Though the [[BonehFranklinScheme|Boneh-Franklin scheme]] is [[Provable security|provably secure]], the security proof rests on relatively new assumptions about the hardness of problems in certain elliptic curve groups.
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A third approach to IBE is through the use of lattices.
=== Identity-based encryption algorithms ===
The following lists practical identity-based encryption algorithms
* [[Boneh–Franklin scheme|Boneh–Franklin]] (BF-IBE).
* [[Sakai–Kasahara scheme|Sakai–Kasahara]] (SK-IBE).<ref>{{cite
* Boneh–Boyen (BB-IBE).<ref>{{cite conference
| last1 = Boneh | first1 = Dan | author1-link = Dan Boneh
| last2 = Boyen | first2 = Xavier
| editor1-last = Cachin | editor1-first = Christian
| editor2-last = Camenisch | editor2-first = Jan
| contribution = Efficient selective-ID secure identity based encryption without random oracles
| contribution-url = https://eprint.iacr.org/2004/172
| doi = 10.1007/978-3-540-24676-3_14
| pages = 223–238
| publisher = Springer
| series = Lecture Notes in Computer Science
| title = Advances in Cryptology – EUROCRYPT 2004, International Conference on the Theory and Applications of Cryptographic Techniques, Interlaken, Switzerland, May 2–6, 2004, Proceedings
| volume = 3027
| year = 2004| doi-access = free
}}</ref>
All these algorithms have [[Provable security|security proofs]].
== Advantages ==
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== Drawbacks ==
* If a Private Key Generator (PKG) is compromised, all messages protected over the entire lifetime of the
* Because the Private Key Generator (PKG) generates private keys for users, it may decrypt and/or sign any message without authorization. This implies that
* The issue of implicit key escrow does not exist with the current [[Public key infrastructure|PKI]] system, wherein private keys are usually generated on the user's computer. Depending on the context key escrow can be seen as a positive feature (e.g., within Enterprises). A number of variant systems have been proposed which remove the escrow including [[certificate-based encryption]], [[secret sharing]], [[secure key issuing cryptography]] and [[certificateless cryptography]].
* A secure channel between a user and the Private Key Generator (PKG) is required for transmitting the private key on joining the system. Here, a [[Secure Sockets Layer|SSL]]-like connection is a common solution for a large-scale system. It is important to observe that users that hold accounts with the PKG must be able to authenticate themselves. In principle, this may be achieved through username, password or through public key pairs managed on smart cards.
* IBE solutions may rely on cryptographic techniques that are insecure against code breaking [[quantum computer]] attacks (see [[Shor's algorithm]]).
== See also ==
* [[
* [[Identity-based conditional proxy re-encryption]]
* [[Attribute-based encryption]]
== References ==
{{Reflist}}
== External links ==
* [
* [https://web.archive.org/web/20170605075501/http://www.ietf.org/rfc/rfc5091.txt RFC 5091 - the IETF RFC defining two common IBE algorithms]
* [http://www.hpl.hp.com/techreports/2003/HPL-2003-21.pdf HP Role-Based Encryption] {{Webarchive|url=https://web.archive.org/web/20031212160232/http://www.hpl.hp.com/techreports/2003/HPL-2003-21.pdf |date=2003-12-12 }}
* [https://web.archive.org/web/20090416044556/http://www.larc.usp.br/~pbarreto/pblounge.html The Pairing-Based Crypto Lounge]
* [https://web.archive.org/web/20090628190353/http://www.voltage.com/vsn/ The Voltage Security Network - IBE encryption web service]
* [https://web.archive.org/web/20080314104119/http://www.ferris.com/2006/05/30/the-total-cost-of-ownership-for-voltage-identity-based-encryption-solutions/ Analyst report on the cost of IBE versus PKI]
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