NordSec camera-ready version

This is the camera-ready version of the paper, as accepted in NordSec. The published version is available from SpringerLink. Its BibTeX entry is:

@inproceedings{OTPKX,
  title={Towards Perfectly Secure and Deniable Communication Using an NFC-Based 
    Key-Exchange Scheme},
  author={Bosk, Daniel and Kjellqvist, Martin and Buchegger, Sonja},
  booktitle={Secure IT Systems},
  editor={Buchegger, Sonja and Dam, Mads},
  series={Lecture Notes in Computer Science},
  volume={9417},
  pages={72--87},
  year={2015},
  publisher={Springer International Publishing},
  isbn={978-3-319-26501-8},
  doi={10.1007/978-3-319-26502-5_6},
  URL={http://dx.doi.org/10.1007/978-3-319-26502-5_6},
  keywords={Deniability; Deniable encryption; Authenticated encryption; Perfect 
    secrecy; Off-the-record; Key-exchange; Nearfield communication; 
    Surveillance},
}