Privacy preserving authorisation in pervasive environments

1. A method of using a network element used by a user to authorise the user to a service provider in a communications network for accessing a service or group of services by generating and using a service credential associated with verifiable constraints, the method comprising:

generating a random value for use as a the user credential;

partially blinding the verifiable constraints and user credential to generate a partially blinded credential, the verifiable constraints being kept in clear and the user credential being blinded;

sending the partially blinded credential towards an authentication server;

receiving a partially blinded signature from the authentication server;

unblinding the partially blinded signature to generate a user access signature;

generating and storing an authorisation credential using the user access signature together with the user credential and a access profile;

generating and storing an initial service credential from the authorisation credential;

generating an initial access request message including the initial service credential;

sending the access request message towards a service provider; and

receiving authorisation from the service provider to access the service.

2. The method of claim 1 , further comprising:

generating a symmetric authentication key;

encrypting at least part of the access request message with the authentication key before it is sent to the service provider;

encrypting the authentication key with a public key of the authentication server; and

sending the encrypted authentication key towards the authentication server.

3. The method of claim 1 , further comprising:

generating subsequent access request messages associated with the same authorisation credential, each including an unencrypted subsequent service credential; and

sending the subsequent access request messages towards the service provider.

4. A method of using an authentication server to authorise a user to a service provider for accessing a service or group of services by generating and authenticating a service credential associated with verifiable constraints, the method comprising:

receiving a partially blinded credential from the user, the partially blinded credential generated from the verifiable constraints kept in clear and a blinded user credential;

generating a partially blinded signature from the partially blinded credential, the partially blinded signature, when unblinded, being verifiably associated with the user credential and the verifiable constraints;

sending the partially blinded signature towards the user;

receiving from a service provider a message originating from the user, the message including at least an initial service credential;

recovering an authorisation credential from the message, the authorisation credential comprising a user access signature and the user credential and the verifiable constraints;

verifying that the user access signature is a valid signature for the user credential and the verifiable constraints of the authorisation credential and that the verifiable constraints in the authorization credential are fulfilled;

authorising the user; and

sending an acceptance message to the service provider.

5. The method of claim 4 , further comprising:

receiving subsequent access request messages associated with the same authorisation credential, each including an unencrypted subsequent service credential;

identifying an association between the initial and subsequent service credentials and the authorisation credential; and

using the subsequent service credential to identify the authorisation credential.

6. The method of claim 5 , wherein:

updating the subsequent service credential using a cryptographic function each time an access request message is received from the user to produce the service credential for identifying the authorisation credential when the next access request message related to the same authorisation credential is received.

7. The method of claim 4 , further comprising:

encrypting the received access request message by a symmetric authentication key;

receiving an encrypted version of the authentication key;

decrypting the authentication key; and

using the authentication key to decrypt the access request message so as to recover the authorisation credential.

8. The method of claim 7 , further comprising storing the authentication key for use in decrypting subsequent access request messages associated with the same authorisation credential.

9. The method of claim 5 , further comprising storing an indexed access counter and incrementing the indexed access counter each time an access request message is received from the user.

10. The method of claim 4 , further comprising authenticating the user using Generic Bootstrapping Architecture.

11. The method of claim 10 , further comprising authenticating the user on the basis of a SIM, USIM or ISIM.

12. The method of claim 7 , wherein the authentication key is encrypted with a public key of the authentication server, the method further comprising decrypting the authentication key with a private key of the authentication server.