ID-based cryptography
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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. a user's email address). The first identity-based cryptosystem was a signature scheme developed by Adi Shamir in 1984, which allowed users to verify digital signatures using only public information such as the user's identity. Modern schemes include Boneh/Franklin's pairing-based encryption scheme, and Cocks's encryption scheme based on quadratic residues.
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[edit] 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. Given the master public key, any party can compute a public key corresponding to the identity i by combining the master public key with the identity value. To obtain a corresponding private key, the party authorized to use the identity i contacts the PKG, which uses the master private key to generate the private key for identity i.
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, secure key issuing cryptography and certificateless cryptography.
[edit] Encryption schemes
The most efficient identity-based encryption schemes are currently based on bilinear pairings on elliptic curves, such as the Weil or 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-like approach. Though the Boneh-Franklin scheme is provably secure, the security proof rests on relatively new assumptions about the hardness of problems in certain elliptic curve groups.
Another approach to identity-based encryption was proposed by Clifford Cocks in 2001. The Cocks IBE scheme is based on well-studied assumptions (the quadratic residuosity assumption) but encrypts messages one bit at a time with a high degree of ciphertext expansion. Thus it is highly inefficient and impractical for sending all but the shortest messages, such as a session key for use with a symmetric cipher.
[edit] Advantages
One of the major advantages of any identity-based encryption scheme 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 a method of key revocation). The majority of derivatives of this system which have key revocation lose this advantage.
[edit] 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.
- Dan Boneh, Matthew K. Franklin, Identity-Based Encryption from the Weil Pairing. Advances in Cryptology - Proceedings of CRYPTO 2001 (2001).
- Clifford Cocks, An Identity Based Encryption Scheme Based on Quadratic Residues, Proceedings of the 8th IMA International Conference on Cryptography and Coding, 2001.
