Is it permissible to create a program to decrypt the files of sock-making machines and sell it, after learning the encryption method using an original program from the company?

There is no objection to creating a program of your own production that emulates another company's program, so long as it does not infringe upon intellectual property rights. The determination of what constitutes an infringement of these rights is referred to experts. Whatever does not fall under infringement is permissible, while whatever does is forbidden, because invention and authorship rights are protected by Islamic law.