Compression for K-PKE

[jnsiemer]

Description

This PR implements...

• compression for K-PKE
• requires any PK-Encryption scheme for dec to get ownership of the decrypted ciphertext.

Checklist:

• I have performed a self-review of my own code
  • The code provides good readability and maintainability s.t. it fulfills best practices like talking code, modularity, ...
    • The chosen implementation is not more complex than it has to be
  • My code should work as intended and no side effects occur (e.g. memory leaks)
  • The doc comments fit our style guide