Why encrypt epub

[balmydrizzle]

I thought epub intends to flow a public format around different reader system & they can all recognize it. But if we encrypt epub, doesn't this disobey the original purpose? How can different reader systems decrypt such epub book to read it?

I learned from internet (https://www.mobileread.com/forums/showthread.php?t=54046) that some one said he want to make money, so he needs to encrypt the epub. Besides the disobeying things I said above, encryption will be meaningless, doesn't it since suppose he wrote a special reader software which can decrypt the encrypted epub, then anybody can take a copy of this reader along with the encryped epub to someone else. Then the encryption is meaningless under such circumstance, isn't it?

I'm so confused. Can anybody enlighten me?

[Jellby]

If you create an encryption mechanism, and a program that can decrypt it, but don't tell anyone how the mechanism works and don't let anyone look at the program's source, you can then give away copies of you program and sell encrypted books. You also have to make sure that each copy of the program is different, so that an encrypted book that works for a copy doesn't work for others, and every user has to buy another book.

That's basically what DRM (digital rights management) does, and yes, it's pointless.

[balmydrizzle]

Originally Posted by Jellby

.... You also have to make sure that each copy of the program is different, so that an encrypted book that works for a copy doesn't work for others, and every user has to buy another book.

If the reader is on iPad, maybe we can read iPad's unique ID to differentiate copy of program. But what if we run the reader on Windows of a PC, how can we differentiate the copy of program?