prepaid meters that rely on a disconnected system

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

• Why would it "mean they are using a" symmetric algorithm? $\;$ – user991 May 21 '15 at 7:12
• an assumption, im open to ideas. it cant be public/ private and asymetric dosnt fit the architecture of one server sending to many clients – user2168435 May 21 '15 at 7:16
• Why doesn't asymmetric "fit the architecture of one server sending to many clients"? $\hspace{1.29 in}$ – user991 May 21 '15 at 7:21
• en.wikipedia.org/wiki/Public-key_cryptography so the server would have to maintain public keys for all of the meters ? – user2168435 May 21 '15 at 7:21
• @user2168435 I fail to see how that article supports your point. Read the article Ricky linked on digital signatures -- encryption is irrelevant here anyway, because there's no reason for the amount added to be secret (what matters is that no one can fake an amount). – cpast May 21 '15 at 7:23