3. The method of claim 1 , further comprising: by the mobile electronic device, generating a randomness value, and sending the randomness value to the computational tag; and by the computational tag, using the randomness value to generate the encrypted key.
4. The method of claim 3 , wherein generating a randomness value comprises: recording a pattern of fluctuations of the mobile electronic device over a period of time; and generating the randomness value using the pattern of fluctuations.