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Chu et al. (ASIACCS 2012) proposed group signature with time-bound keys (GS-TBK), where each signing key is associated with expiry time $\tau$ τ . In addition, to prove membership of the group, a signer needs to prove that the expiry time has not passed, i.e., $t t τ , where $t$ t is the current time. A signer whose expiry time has passed is automatically revoked, and this revocation is called natural revocation. Signers can be revoked simultaneously before their expiry times if the credential is compromised. This revocation is called premature revocation. A nice property in the Chu et al. proposal is that the size of revocation lists can be reduced compared to those of Verifier-Local Revocation (VLR) group signature schemes by assuming that natural revocation accounts for most of the signer revocations in practice, and prematurely revoked signers are only a small fraction. In this paper, we point out that the definition of traceability of Chu et al. did not capture the unforgeability of expiry time for signing keys, which guarantees that no adversary who has a signing key associated with expiry time $\tau$ τ can compute a valid signature after $\tau$ τ has passed. This situation significantly reduces the dependability of the system since legitimate signing keys may be used for providing a forged signature. We introduce a security model that captures unforgeability, and propose a secure GS-TBK scheme in the new model. Our scheme also provides constant signing costs, whereas those of the previous schemes depended on the bit-length of the time representation. Finally, we provide the implementation results. We employ Barreto-Lynn-Scott (BLS) curves with 455-bit prime order and the RELIC library, and demonstrate that our scheme is feasible in practical settings.

This paper describes techniques that enable vehicles to collect local information (such as road conditions and traffic information) and report it via road-to-vehicle communications. To exclude malicious data, the collected information is signed by each vehicle. In this communication system, the location privacy of vehicles must be maintained. However, simultaneously linkable information (such as travel routes) is also important. That is, no such linkable information can be collected when full anonymity is guaranteed using cryptographic tools such as group signatures. Similarly, continuous linkability (via pseudonyms, for example) may also cause problem from the viewpoint of privacy. In this paper, we propose a road-to-vehicle communication system with relaxed anonymity via group signatures with time-token dependent linking (GS-TDL). Briefly, a vehicle is unlinkable unless it generates multiple signatures in the same time period. We provide our experimental results (using the RELIC library on a cheap and constrained computational power device, Raspberry Pi) and simulate our system by using a traffic simulator (PTV), a radio wave propagation analysis tool (RapLab), and a network simulator (QualNet). Though a similar functionality of time-token-dependent linking was proposed by Wu et al. (“Balanced trustworthiness, safety, and privacy in vehicle-to-vehicle communications,” IEEE Trans. Veh. Technol., vol. 59, 2010), we can show an attack against the scheme where anyone can forge a valid group signature without using a secret key. In contrast, our GS-TDL scheme is provably secure. In addition to the time-dependent linking property, our GS-TDL scheme supports verifier-local revocation, where a signer (vehicle) is not involved in the revocation procedure. It is particularly worth noting that no secret key or certificate of a signer (vehicle) must be updated, whereas the security credential management system must update certificates frequently for vehicle privacy. Moreover, our technique maintains constant signing and verification costs by using the linkable part of signatures. This might be of independent interest.

Boneh and Franklin gave a naive revocation method in identity-based encryption (IBE) which imposes a huge overhead into the key generation center. Later, Boldyreva, Goyal, and Kumar proposed an elegant way of achieving an IBE with efficient revocation, called revocable IBE (RIBE). In this paper, we revisit RIBE from the viewpoint of both security models and constructions. First, we introduce a realistic threat, which we call decryption key exposure, and show that all prior RIBE constructions, except the Boneh-Franklin one, are vulnerable to decryption key exposure. Next, we propose the first scalable RIBE scheme with decryption key exposure resistance by combining the (adaptively secure) Waters IBE scheme and the (selectively secure) Boneh-Boyen IBE scheme, and show that our RIBE scheme is more efficient than all previous adaptively secure scalable RIBE schemes. In addition, we extend our interest into identity-based signatures; we introduce a new security definition of revocable identity-based signature (RIBS) with signing key exposure resistance, and propose the first scalable RIBS scheme based on the Paterson-Schuldt IBS. Finally, we provide implementation results of our schemes to adduce the feasibility of our schemes.