i have some experience with cryptography in the banking sectors, mostly in block ciphers like DES 3DES etc. im interested in prepaid electricity meters as it is a number that is generated from a central system which is disconnected from the meter its self.
This would mean they are using a symmetric algorithm using the device meter number and the "recharge" value to make it unique. so i presume there would be a secret key stored on the device of which they have the clear component in order to create the recharge token. similar to an HSM and its LMK. The tokens are 16 digits which in its self would make brute force difficult as you dont know the transformations performed on the input.
i was thinking about how ciphers where "broken" during the mid 1950's, how would someone analyse cyphertext without knowledge of the algorithm or the clear text content ?
just in case anyone thinks i'm going to try and do it. i'm not,its a question based on curiosity