Proxy Reencryption: Make tranformation key public

[andreea.alexandru]

@bhavinmoriya There might have been some misunderstanding regarding “public”: The re-encryption key can be known to the proxy party, which is given the ciphertext to re-encrypt. However, the re-encryption key should not be known to the owner of the secret key of the resulting ciphertext, because it can use them to extract information about the original secret key. See also the response here.