source: http://www.cosic.esat.kuleuven.be/publications/thesis-152.pdf ,page 32
Note that key recovery implies invertibility of the cryptographic primitive (because due to the Kerckhoffs’ principle, the corresponding decryption routine is available). The inverse is not always true. PR-CPA secure ⇒ KR-CPA secure.
What does “invertibility of the cryptographic primitive” mean?