Can someone please explain the below paragraphs from The X3DH Key Agreement Protocol?
1st paragraph: What is deniability and how does it work here?
X3DH doesn't give either Alice or Bob a publishable cryptographic proof of the contents of their communication or the fact that they communicated.
Like in the OTR protocol , in some cases a third party that has compromised legitimate private keys from Alice or Bob could be provided a communication transcript that appears to be between Alice and Bob and that can only have been created by some other party that also has access to legitimate private keys from Alice or Bob (i.e. Alice or Bob themselves, or someone else who has compromised their private keys).
If either party is collaborating with a third party during protocol execution, they will be able to provide proof of their communication to such a third party. This limitation on "online" deniability appears to be intrinsic to the asynchronous setting.
2nd paragraph: How would this passive attack work?
Compromise of prekey private keys may enable attacks that extend into the future, such as passive calculation of SK values, and impersonation of arbitrary other parties to the compromised party ("key-compromise impersonation"). These attacks are possible until the compromised party replaces his compromised prekeys on the server (in the case of passive attack); or deletes his compromised signed prekey's private key (in the case of key-compromise impersonation).