This paper provides theoretical foundations for the group signature primitive. We introduce strong, formal definitions for the core requirements of anonymity and traceability. We then show that these imply the large set of sometimes ambiguous existing informal requirements in the literature, thereby unifying and simplifying the requirements for this primitive. Finally we prove the existence of a construct meeting our definitions based only on the assumption that trapdoor permutations exist.

We improve the Bellare-Miner (Crypto ’99) construction of signature schemes with forward security in the random oracle model. Our scheme has significantly shorter keys and is, therefore, more practical. By using a direct proof technique not used for forward-secure schemes before, we are able to provide better security bounds for the original construction as well as for our scheme. Bellare and Miner also presented a method for constructing such schemes without the use of the random oracle. We conclude by proposing an improvement to their method and an

Abstract. Cryptographic computations (decryption, signature generation, etc.) are often performed on a relatively insecure device (e.g., a mobile device or an Internet-connected host) which cannot be trusted to maintain secrecy of the private key. We propose and investigate the notion of key-insulated security whose goal is to minimize the damage caused by secret-key exposures. In our model, the secret key(s) stored on the insecure device are refreshed at discrete time periods via interaction with a physically-secure — but computationally-limited — device which stores a “master key”. All cryptographic computations are still done on the insecure device, and the public key remains unchanged. In a (t, N)-keyinsulated scheme, an adversary who compromises the insecure device and obtains secret keys for up to t periods of his choice is unable to violate the security of the cryptosystem for any of the remaining N − t periods. Furthermore, the scheme remains secure (for all time periods) against an adversary who compromises only the physically-secure device. We focus primarily on key-insulated public-key encryption. We construct a (t, N)key-insulated encryption scheme based on any (standard) public-key encryption scheme, and give a more ef£cient construction based on the DDH assumption. The latter construction is then extended to achieve chosen-ciphertext security. 1

Abstract. We propose the first forward-secure signature scheme for which both signing and verifying are as efficient as for one of the most efficient ordinary signature schemes (Guillou-Quisquater [GQ88]), each requiring just two modular exponentiations with a short exponent. All previously proposed forward-secure signature schemes took significantly longer to sign and verify than ordinary signature schemes. Our scheme requires only fractional increases to the sizes of keys and signatures, and no additional public storage. Like the underlying [GQ88] scheme, our scheme is provably secure in the random oracle model. 1

This paper provides a comprehensive treatment of forward-security in the context of sharedkey based cryptographic primitives, as a practical means to mitigate the damage caused by key-exposure. We provide definitions of security, practical proven-secure constructions, and applications for the main primitives in this area. We identify forward-secure pseudorandom bit generators as the central primitive, providing several constructions and then showing how forward-secure message authentication schemes and symmetric encryption schemes can be built based on standard schemes for these problems coupled with forward-secure pseudorandom bit generators. We then apply forward-secure message authentication schemes to the problem of maintaining secure access logs in the presence of break-ins.

We study the problem of partial key exposure. Standard cryptographic de nitions and constructions do not guarantee any security even if a tiny fraction of the secret key is compromised. We show how to build cryptographic primitives that remain secure even when an adversary is able to learn almost all of the secret key.

Abstract. The Guillou-Quisquater (GQ) and Schnorr identification schemes are amongst the most efficient and best-known Fiat-Shamir follow-ons, but the question of whether they can be proven secure against impersonation under active attack has remained open. This paper provides such a proof for GQ based on the assumed security of RSA under one more inversion, an extension of the usual one-wayness assumption that was introduced in [5]. It also provides such a proof for the Schnorr scheme based on a corresponding discrete-log related assumption. These are the first security proofs for these schemes under assumptions related to the underlying one-way functions. Both results extend to establish security against impersonation under concurrent attack. 1

A programmable secure coprocessor platform can help solve many security problems in distributed computing. These solutions usually require that coprocessor applications be able to participate as full-fledged parties in distributed cryptographic protocols. Thus, to fully enable these solutions, a generic platform must not only provide programmability, maintenance, and configuration in the hostile field—it must also provide outbound authentication for the entities that result. A particular application on a particular untampered device must be able to prove who it is to a party on the other side of the Internet. To be effective, a secure outbound authentication service must closely mesh with the overall security architecture. Our initial architecture only sketched a rough design for this service, and did not complete it. This paper presents our research and development experience in refining and implementing this design, to provide PKI-based outbound authentication for the IBM 4758 Model 2 secure coprocessor platform. 1

Abstract. We identify and fill some gaps with regard to consistency (the extent to which false positives are produced) for public-key encryption with keyword search (PEKS). We define computational and statistical relaxations of the existing notion of perfect consistency, show that the scheme of [7] is computationally consistent, and provide a new scheme that is statistically consistent. We also provide a transform of an anonymous IBE scheme to a secure PEKS scheme that, unlike the previous one, guarantees consistency. Finally we suggest three extensions of the basic notions considered here, namely anonymous HIBE, public-key encryption with temporary keyword search, and identity-based encryption

In Crypto'99, Bellare and Miner introduced forward-secure signatures as digital signature