  The security of public-key cryptosystems is mostly based on number theoretic
problems like factorization and the discrete logarithm. There exists an
algorithm which solves these problems in polynomial time using a quantum
computer. Hence, these cryptosystems will be broken as soon as quantum
computers emerge. Code-based cryptography is an alternative which resists
quantum computers since its security is based on an NP-complete problem, namely
decoding of random linear codes. The McEliece cryptosystem is the most
prominent scheme to realize code-based cryptography. Many codeclasses were
proposed for the McEliece cryptosystem, but most of them are broken by now.
Sendrier suggested to use ordinary concatenated codes, however, he also
presented an attack on such codes. This work investigates generalized
concatenated codes to be used in the McEliece cryptosystem. We examine the
application of Sendrier's attack on generalized concatenated codes and present
alternative methods for both partly finding the code structure and recovering
the plaintext from a cryptogram. Further, we discuss modifications of the
cryptosystem making it resistant against these attacks.
