Leakage of information in power line communication networks is a threat to privacy and security both in smart grids and in-home applications. A way to enhance security is to encode the transmitted information with a secret key. Relying on the channel properties, it is possible to generate a common key at the two communication ends without transmitting it through the broadcast channel. Since the key is generated locally, it is intrinsically secure from a possible eavesdropper. Most of the existing physical layer key generation techniques have been developed for symmetric channels. However, the power line channel is in general not symmetric, but just reciprocal. Therefore, in this paper, we propose two novel methods that exploit the reciprocity of the power line channel to generate common information at the two intended users. This information is processed through different quantization techniques to generate secret keys. To assess the security of the generated keys, we analyze the spatial correlation of the power line channels and verify the low correlation of the possible eavesdropping channels. The two proposed methods are tested on a measurement dataset. The results show that the information leaked to possible eavesdroppers has very low correlation to any secret key.