decrypt_session_key does not exist but is referred #565

/// Decrypts a plaintext using RSA and the padding scheme from PKCS#1 v1.5.                                          
///                                                                                                                  
/// If an `rng` is passed, it uses RSA blinding to avoid timing side-channel attacks.                                
///                                                                                                                  
/// Note that whether this function returns an error or not discloses secret                                         
/// information. If an attacker can cause this function to run repeatedly and                                        
/// learn whether each instance returned an error then they can decrypt and                                          
/// forge signatures as if they had the private key. See                                                             
/// `decrypt_session_key` for a way of solving this problem.