Chen, Chunhua

Mitchell, Chris J

Tang, Shaohua

Royal Holloway Research Online

Extending Trusted Computing as a Security ServiceChunhua Chen*, Chris Mitchell and Shaohua Tang*Information Security Group, Royal Holloway, University of LondonAbstractWe extend the Trusted Computing (TC) security infrastructure in a Generic Authentication Architecture (GAA)-like framework to enable the provision of security services, such as key establishment, to network applications.  BackgroundCore work Architectural Overview of TP-GAAGeneric Authentication Architecture: Specify the architecture and components of TC GAA.We make the TC security infrastructure play the role of the cellular authentication infrastructure in the GAA framework, and hence extend the TC security infrastructure to provide a security service, which we call TC GAA. Specify the interfaces and protocols between components.• Specify bootstrapping procedure of TC GAA, BSF Server Bootstrapping procedure:NAF Server ResponseB-TID, NAF-Id*SK, lifetime of SK, etc1354Bootstrapping Server Function (BSF):Architecture Elements Bootstrapping procedure1. TP  BSF: Request for bootstrapping master session key.2. BSF  TP: RBSF.3. TP: Generates a new temporary asymmetric encryption key pair (T(public) and T(private)) and certify the public key T(public) with an identity of T(public) chosen by TP user, namely, IdTP .4. TP  BSF: RBSF||IdBSF|| T(public)||IdTP||STP(RBSF||IdBSF||T(public)||IdTP).5. BSF: Retrieves Cert and verifies it.1. TP: Derives an application-specific symmetric key SK as follow:SK = KDF(MK, RBSF, IdTP, NAF-Id)where KDF is a key derivation function.  IdTP is the identity of T(pub), and NAF-Id consists of the Fully Qualified Domain Name (FQDN) of the intended NAF and the identifier of the application protocol.2. TP  NAF: B-TID and msg.  msg is the application request data secured using SK.3. NAF  BSF: B-TID and NAF-ID.Note that it is assumed that a secure channel has been set up by some means between NAF and BSF.• Standardised by 3GPP and 3GPP 2. • A general framework that extends the cellular authentication infrastructure (includes UMTS and GSM) to enable the provision of security services to network applications.• Consists of two procedures, GAA bootstrapping and use of bootstrapped keys.Trusted Computing (TC) Security Infrastructure:A Trusted Platform (TP) compliant with the Trusted Computing Group (TCG) specifications is a computing platform with a tamper-proof and built-in Trusted Platform Module (TPM). Properties of TPM:• Protected capabilities, such as random number generation, asymmetric key generation, digital signing, encryption capabilities, etc.• A unique Endorsement Key (EK) pair and a set of derived keys, such as an Attestation Identity Key (AIK).• Other properties:TPM, associated keys, protected capabilities, and the underlying including an authenticated key agreement protocol.• Specify the derivation of an application-specific session key.• Specify use of bootstrapped key of TC GAA.TCG compliant Trusted platform (TP):Network Application Function (NAF):• A new component, that acts as Trusted Third Party.• Has a certified public key pair for entity authentication.• The server functionality of each GAA-aware network application.• Assumed to have some means to set up a secure channel with BSF (e.g. as provided by SSL/TLS tunnel).• The Endorsement Key for encryption.• Has a certified public key pair for entity authentication (e.g. AIK).• Protected capabilities as described in the protocol.Bootstrapping procedure of TC GAA  is used to bootstrap a new symmetric master session key between BSF and TP. It is an authenticated key agreement protocol specified as below:Use of bootstrapped key• An authenticated key agreement protocol.• Bootstrap a symmetric master session key MK.• An identifier of MK, namely B-TID.Trusted Platform (TP)B-TID, msg2Derivation of SK in BSF and TP:SK = KDF(MK, NAF-Id, … …).*NAF-Id consists of the Fully Qualified Domain Name (FQDN) of the intended NAF and the identifier of the application protocol.TP6. BSF: Verifies STP(RBSF||IdBSF||T(public)||IdTP).7. BSF: Verifies RBSF to ensure the message is fresh and verifies that the message was intended for BSF.8. Assuming the signature from TP verifies correctly, the values of RBSF and IdBSF are expected, then BSF extracts T(public).10. BSF: Generates a symmetric key MK as master session key, and set lifetime of MK according to local policy. Generates an identifier B-TID of MK which consists of RBSF and the domain name of BSF. 10. BSF  TP : ET(public)(MK)||SBSF(ET(public)(MK))11. TP: Retrieves CertBSF and verifies it.12. TP: Verifies SBSF(ET(public)(MK)) .13. TP: Decrypts ET(public)(MK) to get MK.Contact information4. BSF: Derives SK = KDF(MK, RBSF, IdTP, NAF-Id), and sets lifetime of SK according local policy.5. BSF  NAF: SK, lifetime of SK, etc.6. NAF: Responses to the request using SK, if SK is valid..Name: Chunhua ChenAddress: Mcrea 106, Information Security Group, Royal Holloway, University of London, Egham, TW20 0EX Surrey, UKT: 01784 443696E: Chunhua.chen@rhul.ac.ukPublic Key Infrastructure (PKI) comprise a security infrastructure.Figure 1: a trusted platform module.Image source: http://img.tomshardware.com/us/2008/02/11/how_hardware_based_security_protects_pcs/tpmchip.jpgNotation Chunhua Chen is a Phd student under supervison of Prof. Shaohua Tang in School of Computer Science and Engineering, South China University of Technology. This work was performed during a visit to the Information Security Group at Royal Holloway, University of London, sponsored by the Chinese Scholarship Council.• CertX :  A public key certification of entity X.• MK    :  A symmetric master session key.• SK    :   An application-specific symmetric session key.• RX :  A random number issued by entity X.• EK(Z) :  Encryption of data Z using the key K.• H(Z)  :  A one-way hash function on data Z.• SX(Z) :  The digital signature of data Z computed using entity X’s                   private signature transformation.• X(public) : The public asymmetric key of X.• X(private)  :  The private asymmetric of X.• IdX : The identity of X.• X||Y     : The concatenation of data items X and Y in that order.• X Y : Z :Indicate that the message Z is sent by X to Y.Steps 2 and 4 of the above protocol conform to the two pass unilateral authentication protocol described in clause 5.1.2 of ISO/IEC 9798-3:1998 where T(pub) serves as the nonce which is generated in every run. The key agreement part of the protocol is a key transfer protocol.After the procedure, BSF and TP share RBSF, IdTP, B-TID, MK.