McEliece cryptosystem

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In cryptography, the McEliece cryptosystem is an asymmetric encryption algorithm developed in 1978 by Robert McEliece. It was the first such scheme to use randomization in the encryption process. The algorithm has never gained much acceptance in the cryptographic community, but is a candidate for "post-quantum cryptography", as it is immune to attacks using Shor's algorithm and – more generally – measuring coset states using Fourier sampling. The algorithm is based on the hardness of decoding a general linear code (which is known to be NP-hard). For a description of the private key, an error-correcting code is selected for which an efficient decoding algorithm is known, and that is able to correct t {\displaystyle t} errors. The original algorithm uses binary Goppa codes (subfield codes of algebraic geometry codes of a genus-0 curve over finite fields of characteristic 2); these codes can be efficiently decoded, thanks to an algorithm due to Patterson. The public key is derived from the private key by disguising the selected code as a general linear code. For this, the code's generator matrix G {\displaystyle G} is perturbated by two randomly selected invertible matrices S {\displaystyle S} and P {\displaystyle P} (see below). Variants of this cryptosystem exist, using different types of codes. Most of them were proven less secure; they were broken by structural decoding. McEliece with Goppa codes has resisted cryptanalysis so far. The most effective attacks known use information-set decoding algorithms. A 2008 paper describes both an attack and a fix. Another paper shows that for quantum computing, key sizes must be increased by a factor of four due to improvements in information set decoding. The McEliece cryptosystem has some advantages over, for example, RSA. The encryption and decryption are faster. For a long time, it was thought that McEliece could not be...