Security & Cryptography II - Exercises

General Information

If not mentioned otherwise all the tasks refer to the exercises section of the script

Tasks for 26.4.2024

• I guess you have heard about satellite-based Internet access.
  How can you (mis)use such satellite-based Internet connectivity for anonymous file sharing, assuming that the encryption applied on the satellite link (used for download direction) is broken?
• Think about the relationship between implicit addresses and encryption. In the lecture it was mentioned, that asymmetric as well as symmetric encryption can be used to create invisible implicit addresses. Does it also work the other way around: How can one build an asymmetric encryption system using invisible public implicit addresses? How can one build a symmetric encryption system using invisible private implicit addresses? (Note: Use only the general properties of the addressing scheme to create the encryption scheme - not specific features of a specific implementation of the addressing scheme, e.g. do not base your solution on the assumption that some keys are exchanged between the communication partners because encryption is used to implement the addressing scheme and therefore these keys can be used for encryption  - because using encryption to implement invisible implicit addressing is just one way of doing it, but this can not be generalised.)
• Private Message Service: Do we need to encrypt the query vectors? Do we need to encrypt the local sums sent by the servers? If yes, why?
• Calculate a small example for the private message service (use 3 database servers, 4 records per database server)
• Private Message Service: What do you think about the following suggestion: To confuse the servers the participant randomly generates an additional request vector, sends it to an additional server, receives its answer and ignores this. None of the used s + 1 servers know if it is used as a normal or as an additional, ignored server.
• Can you imagine communication services where “Requesting and Overlaying/Private Message Services” are necessary because “Broadcast” is not applicable?
• Please prepare exercise 5-5 from the script.