An earlier software of mine, TEXTCOMBINE-SP, posted to this group was not satisfactory due a bug in a function employed in its design and was retracted.
I am posting now its replacement TEXTCOMBINE-REV which has been carefully tested and has a fairly good performance IMHO. The following is extracted from its Prologue:
# What has been achieved by the present software can be tersely summarized as # follows, assuming the general case where the text files are sufficiently # large: # # (1) The generated byte sequences pass, via design specifications of the # software, Maurer's universal test and the autocorrelation test for all # d in the range [1, 16] as well as the ENT test with an entropy value # according to it of at least 7.99 bits per byte. The software is namely # coded such that it would give up, reporting failure, after a certain # specified maximum amount of processing has been done without finding # a solution. # # (2) An extensive expermiment of the present author done on all different # combinations, totalling 3060 in number, of 4 source materials (of size # 600 KB each) taken from 18 different books of English literature # downloaded from Project Gutenberg resulted in the following: # # (a) No case of failure was ever encountered. On the contrary, the above # mentioned processing limit, which is in terms of rounds of certain # preprocessing of source materials before they are xor-ed together, # was by far not being approached in the experiment. For details, see # Epilogue. # # (b) The worst case of entropy according to ENT in the experiment was # higher than 7.995 bits per byte and the average CPU-time was less than # 15 sec on author's PC.