Tests whether the key is consistent for encryption; whether encrypting and then decrypting gives to the original plaintext. 
   22    PK_Encryptor_EME encryptor(key, padding);
    23    PK_Decryptor_EME decryptor(key, padding);
    29    if(encryptor.maximum_input_size() == 0)
    32    SecureVector<byte> plaintext =
    33       rng.random_vec(encryptor.maximum_input_size() - 1);
    35    SecureVector<byte> ciphertext = encryptor.encrypt(plaintext, rng);
    36    if(ciphertext == plaintext)
    39    SecureVector<byte> decrypted = decryptor.decrypt(ciphertext);
    41    return (plaintext == decrypted);