512-page book, bound by hand, containing the encrypted archive (ciphertext) of a 3 month conversation with an ex lover on the Signal Private Messenger app.
Signal is an open source, encrypted messaging service for instant messaging, voice, and video calls. It secures all communications with end-to-end encryption. 

When we started using Signal our relationship had already taken a bad turn. 
After breaking up I felt that these messages were the only thing left and that somehow depicted the reasons of our separation. 

As Signal's software is free and open-source given the ciphertext and the key, someone can potentially decrypt the archive and read the conversation. The title of the book is encryption key.