DAA-Related APIs in TPM 2.0 Revisited

In TPM 2.0, a single signature primitive is proposed to support various signature schemes including Direct Anonymous Attestation (DAA), U-Prove and Schnorr signature. This signature primitive is implemented by several APIs which can be utilized as a static Diffie-Hellman (SDH) oracle. In this paper, we measure the practical impact of the SDH oracle in TPM 2.0 and show the security strength of these signature schemes can be weakened by 13-bit. We propose a novel property of DAA called forward anonymity and show how to utilize these DAA-related APIs to break forward anonymity. Then we propose new APIs which not only remove the SDH oracle but also support the forward anonymity, thus significantly improve the security of DAA and the other signature schemes supported by TPM 2.0. We prove the security of our new APIs under the discrete logarithm assumption in the random oracle model. We prove that the proposed DAA schemes satisfied the forward anonymity property using the new APIs under the Decision Diffie-Hellman assumption. Our new APIs are almost as efficient as the original APIs in TPM 2.0 specification and can support LRSW-DAA and SDH-DAA together with U-Prove as the original APIs.