The basic transformation is keyless, but all the data is required to reverse it. The tool can also throw away part of its internal key, making the data decipherable only with the key or via brute force attack. If a content publisher, Alice, wants to check for copyright violations by another party, Bob, she could be thwarted: Bob could staple Alice's file with one of his own and discard part of the key. To check for copyright violation, Alice must brute force the stapled file (possibly violating the DMCA), which protects Bob's file. The FAQ has some more detail.
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