Technically: one must have the private key to perform decryption. There are many schemes that attempt to protect the original key and derive a decryption key, o

Technically: one must have the private key to perform decryption. There are many schemes that attempt to protect the original key and derive a decryption key, or do various other things to prevent disclosure of the private side of a key pair. Practically: the point is not to "secure" anything, but to make it obvious that any infringement is due to "circumventing a protection device." Circumvention is also not allowed under DMCA, because it was obvious to the authors of the law that to view content one must indeed decrypt it.