I studied a lot of information from sources:

https://www.w3.org/TR/xmlenc-core1/#sec-Processing-Decryption

https://www.w3.org/publishing/epub3/epub-ocf.html#sec-container-metainf-encryption.xml

Do I understand correctly that encryption epub will translate everything in Base64?

If this is true, then I can translate the file into the Base64 string on the server, encrypt, and then decrypt in the application on React Native?

Sorry for my English. This is not my native language.