GGH encryption scheme
The Goldreich–Goldwasser–Halevi (GGH) cryptosystem makes use of the fact that the closest vector problem can be a hard problem. It was published in 1997 and uses a trapdoor one-way function that is relying on the difficulty of lattice reduction. The idea included in this trapdoor function is that, given any basis for a lattice, it is easy to generate a vector which is close to a lattice point, for example taking a lattice point and adding a small error vector. But to return from this erroneous vector to the original lattice point a special basis is needed.
The GGH encryption scheme was cryptanalyzed in 1999 by Phong Q. Nguyen.
GGH involves a private key and a public key.
The public key is another basis of the lattice of the form .
For some chosen M, the message space consists of the vector in the range .
Given a message , error , and a public key compute
In matrix notation this is
Remember consists of integer values, and is a lattice point, so v is also a lattice point. The ciphertext is then
To decrypt the cyphertext one computes
The Babai rounding technique will be used to remove the term as long as it is small enough. Finally compute
to get the messagetext.
Let be a lattice with the basis and its inverse
Let the message be and the error vector . Then the ciphertext is
To decrypt one must compute
This is rounded to and the message is recovered with
Security of the scheme
1999 Nguyen showed at the Crypto conference that the GGH encryption scheme has a flaw in the design of the schemes. He showed that every ciphertext reveals information about the plaintext and that the problem of decryption could be turned into a special closest vector problem much easier to solve than the general CVP.
- Oded Goldreich, Shafi Goldwasser, and Shai Halevi. Public-key cryptosystems from lattice reduction problems. In CRYPTO ’97: Proceedings of the 17th Annual International Cryptology Conference on Advances in Cryptology, pages 112–131, London, UK, 1997. Springer-Verlag.
- Phong Q. Nguyen. Cryptanalysis of the Goldreich–Goldwasser–Halevi Cryptosystem from Crypto ’97. In CRYPTO ’99: Proceedings of the 19th Annual International Cryptology Conference on Advances in Cryptology, pages 288–304, London, UK, 1999. Springer-Verlag.