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National Security Service's functioning

Let's assume that the NSS has the encrypted version of the suspect (national or international) message, sent by Alice to Bob.

The NSS and the TA carry out the following, to obtain the clear version of the suspect message :

1. the NSS isolates the stamp of the suspect message, Stp
2. the NSS generates a random number, Rnd, based on the present date and time (1)
3. the NSS sends 4 different encrypted blocks to the TA :
   • the first block contains the random number, Rnd, encrypted with the SessionKey, K_ses, from the NSS to the TA
   • the second block contains the password of the NSS, first encrypted with the random number, Rnd, then with the SessionKey, K_ses (2)
   • the third block contains the stamp of the suspected message, Stp, first encrypted with the random number, Rnd, then with the SessionKey, K_ses
   • the fourth block contains the "message signature" of the suspect message, first encrypted with the random number, Rnd, then with the SessionKey, K_ses (3)
4. the TA records the incoming encrypted request from the NSS (4)
5. based on the stamp of the suspect message, Stp, the TA reconstructs the SessionKey from Alice to Bob, K_sesAB
6. the TA sends 3 encrypted blocks to the NSS
   • the first block contains the SessionKey from Alice to Bob, K_sesAB, first encrypted with the random number, Rnd, then with the inverse SessionKey from the NSS to the TA, K^-1_ses
   • the second block contains the stamp of the suspect message, Stp, first encrypted with SessionKey from Alice to Bob, K_sesAB, then with the inverse SessionKey from the NSS to the TA, K^-1_ses
   • the third block contains the "message signature" of the suspect message, first encrypted with the random number, Rnd, then with the inverse SessionKey from the NSS to the TA, K^-1_ses
7. the hardware of the NSS, first deciphers the third block received from the TA and compares it with the "message signature" of the suspect message. If there is no corroboration, the hardware of the NSS does not decipher the suspect message.

It is important to note at this stage :

• that nobody, except the NSS, has access to the ID numbers of the people involved in suspect messages (all calculations are done inside the hardware).
• that the NSS's management team has no access to the SessionKeys. They are reconstructed inside the hardware and are not accessible to other machines or people.

(1) Without this random number, all demands for information concerning messages involving the same sender and receiver, will lead to the same encrypted result (see third block of message send to TA). With the help of this random number we avoid this situation.

(2) The password was given to the NSS by the TA during the installation of the NSS. Without this password, the TA will not give any information concerning SessionKeys between end-users.

(3) Without specifying a "message signature", the NSS could decipher all the messages sent from Alice to Bob. Now he is only allowed to decipher the suspect message.

(4) Normally the NSS may only request information from the TA, in strict legally defined situations, but technically, nothing prevents the NSS making all the requests it wants. Therefore the TA records all demands issued by the NSS for later control purposes.