The process of converting object code into a high level programming language similar to the original source code, so that the manner in which the software works and its API’s and data transfer protocols can be determined. Although some major software houses have sought to make decompiling a violation of license terms or a form of copyright infringement, their efforts have been largely unsuccessful. As a technical legal matter, decompiling involves making a derivative work of the object code, which is, in principle, a copy—this is referred to as “intermediate copying.” The legality of decompiling in Europe is protected in narrow and specific circumstances under Article 6 of the European Software Directive, which provides

(a) Article 6 Decompilation

1. The authorization of the rightholder shall not be required where reproduction of the code and translation of its form within the meaning of Article 4 (a) and (b) are indispensable to obtain the information necessary to achieve the interoperability of an independently created computer program with other programs, provided that the following conditions are met:

(a) these acts are performed by the licensee or by another person having a right to use a copy of a program, or on their behalf by a person authorized to do so;
(b) the information necessary to achieve interoperability has not previously been readily available to the persons referred to in subparagraph (a); and
(c) these acts are confined to the parts of the original program, which are necessary to achieve interoperability.

2. The provisions of paragraph 1 shall not permit the information obtained through its application:

(a) to be used for goals other than to achieve the interoperability of the independently created computer program;
(b) to be given to others, except when necessary for the interoperability of the independently created computer program; or
(c) to be used for the development, production, or marketing of a computer program substantially similar in its expression, or for any other act that infringes copyright.

3. In accordance with the provisions of the Berne Convention for the protection of Literary and Artistic Works, the provisions of this Article may not be interpreted in such a way as to allow its application to be used in a manner that unreasonably prejudices the right holder’s legitimate interests or conflicts with a normal exploitation of the computer program.

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