5. A method as claimed in claim 4, further characterized in that the protection words (w.sub.L) and check words (w.sub.K) are code words of a linear code.
4. A method as claimed in claim 2, further characterized in that the check words (w.sub.K) are the same as the protection words (w.sub.L) and are selected such that key words (w.sub.KL) which result from combining the protection words (w.sub.L) with the check words (w.sub.K) are in secondary classes of code words other than the code words used for encoding the data provided by the data source.