Computing pairings using x-coordinates only

  • Authors:
  • Steven D. Galbraith;Xibin Lin

  • Affiliations:
  • Mathematics Department, Royal Holloway University of London, Egham, UK TW20 0EX;School of Mathematics and Computational Science, Sun Yat-Sen University, Guangzhou, People's Republic of China 510275

  • Venue:
  • Designs, Codes and Cryptography
  • Year:
  • 2009

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Abstract

To reduce bandwidth in elliptic curve cryptography one can transmit only x-coordinates of points (or x-coordinates together with an extra bit). This is called point compression. For further computation using the points one can either recover the y-coordinates by taking square roots or one can use point multiplication formulae which use x-coordinates only. We consider how to efficiently use point compression in pairing-based cryptography when the embedding degree is even. We give a method to compute compressed pairings using x-coordinates only. We also show how to compute the compressed pairings using two x-coordinates and one y-coordinate. Our methods are more efficient than taking square roots when the embedding degree is small. We implemented the algorithms in the case of embedding degree 2 curves over $${\mathbb {F}_p}$$ where $${p \equiv 3}$$ (mod 4) and found that our methods can be 10---15% faster than the analogous methods using square roots.