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# RSA Laboratories

## 3.5.1 What are elliptic curve cryptosystems?

Elliptic curve cryptosystems were first proposed independently by Victor Miller [Mil86] and Neal Koblitz [Kob87] in the mid-1980s. At a high level, they are analogs of existing public-key cryptosystems in which modular arithmetic is replaced by operations defined over elliptic curves (see Question 2.3.10). The elliptic curve cryptosystems that have appeared in the literature can be classified into two categories according to whether they are analogs to the RSA system or to discrete logarithm based systems.

Just as in all public-key cryptosystems, the security of elliptic curve cryptosystems relies on the underlying hard mathematical problems (see Section 2.3). It turns out that elliptic curve analogs of the RSA system are mainly of academic interest and offer no practical advantage over the RSA system, since their security is based on the same underlying problem, namely integer factorization. The situation is quite different with elliptic curve variants of discrete logarithm based systems (see Question 2.3.7). The security of such systems depends on the following hard problem: Given two points G and Y on an elliptic curve such that Y = kG (that is, Y is G added to itself k times), find the integer k. This problem is commonly referred to as the elliptic curve discrete logarithm problem.

Presently, the methods for computing general elliptic curve discrete logarithms are much less efficient than those for factoring or computing conventional discrete logarithms. As a result, shorter key sizes can be used to achieve the same security of conventional public-key cryptosystems, which might lead to better memory requirements and improved performance. One can easily construct elliptic curve encryption, signature, and key agreement schemes by making analogs of ElGamal, DSA, and Diffie-Hellman. These variants appear to offer certain implementation advantages over the original schemes, and they have recently drawn more and more attention from both the academic community and the industry.

For more information on elliptic curve cryptosystems, see the survey article [RY97] by Robshaw and Yin or the CryptoBytes article [Men95] by Alfred Menezes.

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