Nowadays, using cryptography for secrecy communications and storage of digital information is very essential. Since generic decoding of linear block codes is a hard problem, this fact can be used to design a cryptosystem; such a system was first designed by McEliece in 1978. The McEliece cryptosystem type is asymmetric key; its decryption key consists of a scrambling matrix, a generator matrix of a binary goppa code as private code and a binary permutation matrix; the encryption key is the product of these components, which is a generator matrix of a linear block code, named public code. In this system, the encrypted version of each plain-text block is the sum of its corresponding codeword of public code and a random intentional error pattern. The most important advantage of this cryptosystem is that, no algorithm even on quantum computer has been presented to realize a threat in polynomial time up to now; furthermore, with construction probability of quantum computers in the future, the cryptosystems based on integer factoring and discrete logarithm problems will be realized a total break and therefore the McEliece cryptosystem can be a proper alternative for such systems. But this cryptosystem suffers from low information rate and large size of the encryption key as two drawbacks, which caused less attention devoted to it with respect to other cryptosystems like RSA in practical applications. Therefore, to overcome these drawbacks, some schemes have been proposed for modification of its structure or private code. But most of these modifications except the goppa code-based Niedereiter cryptosystem reduce security level. In this thesis, after describing the McEliece cryptosystem structure and its properties, the attacks against it, will be analyzed and in favor of those work factor, a method has proposed for proper selection of system parameters. Then we show that this system has another disadvantage in practice, which is the random selection of intentional error patterns. Pseudo-random selection of these patterns can be a solution of this problem. Since the most known pseudo-random generator is the linear feedback shift register (LFSR).At first we prove that using the state of these generators as error patterns is not practical, then based on random selection of its part, we propose an algorithm to select these patterns. Also we prove that it is a proper approach. In 1998 Sun proposed some schemes for modification of the McEliece cryptosystem to protect against transmission of related plain-text blocks attack. In his schemes some parts of the plain-text map to intentional error patterns which cause increasing the information rate without changing the size of the encryption key respect to the original system; but In general these schemes are not so practical because storing its map requires a large amount of memory. Therefore a new approach for selection of this map is proposed, to improve efficiency of the McEliece cryptosystem with an acceptable memory. Keywords McEliece cryptosystem, Work factor, Attack, Intentional error patterns, Pseudo-random selection, Linear feedback shift register, Similar and related blocks.