Paper 2018/1128

Direct Anonymous Attestation with Optimal TPM Signing Efficiency

Kang Yang and Liqun Chen and Zhenfeng Zhang and Chris Newton and Bo Yang and Li Xi

Abstract

Direct Anonymous Attestation (DAA) is an anonymous signature scheme, which is designed to allow the Trusted Platform Module (TPM), a small chip embedded in a host computer, to attest to the state of the host system, while preserving the privacy of the user. DAA provides two signature modes: fully anonymous signatures and pseudonymous signatures. To generate a DAA signature, the calculations are divided between the TPM and the host. One goal for designing new DAA schemes is to reduce the signing burden on the TPM as much as possible, since the TPM has only limited resources when compared to the host and the computational overhead of the TPM dominates the whole signing performance. In an optimal DAA scheme, the signing workload on the TPM will be no more than that required for a normal signature. DAA has developed about fifteen years, but no scheme has achieved this optimal signing efficiency for both signature modes. In this paper, we propose the first DAA scheme which achieves this optimal TPM signing efficiency for both signature modes. In particular, the TPM takes only a single exponentiation in a prime-order group when generating a DAA signature. Additionally, this single exponentiation can be precomputed, which enables our scheme to achieve fast online signing time. Our DAA scheme is provably secure under the DDH, DBDH and q-SDH assumptions in the Universally Composable (UC) security model. Our scheme can be implemented using the existing TPM 2.0 commands, and thus is compatible with the TPM 2.0 specification. There are three important use cases for DAA: quoting platform configuration register values, certifying a key and signing a message. We have implemented and benchmarked the commands needed for these use cases on an Infineon TPM 2.0 chip. Based on these benchmark results, our scheme is about twice as fast as the existing DAA schemes supported by TPM 2.0 in terms of signing efficiency. In addition, our DAA scheme supports selective attribute disclosure, which can satisfy more application requirements. We also extend our DAA scheme to support signature-based revocation and to guarantee privacy against subverted TPMs. The two extended DAA schemes keep the TPM signing efficiency optimal for both signature modes, and outperform existing related schemes in terms of signing performance.