Secure channel

Abstract: We describe a theory of authentication and a system that implements it. Our theory is based on the notion of principal and a “speaks for” relation between principals. A simple principal either has a name or is a communication channel; a compound principal can express an adopted role or delegated authority. The theory shows how to reason about a principal's authority by deducing the other principals that it can speak for; authenticating a channel is one important application. We use the theory to explain many existing and proposed security mechanisms. In particular, we describe the system we have built. It passes principals efficiently as arguments or results of remote procedure calls, and it handles public and shared key encryption, name lookup in a large name space, groups of principals, program loading, delegation, access control, and revocation.

Abstract: The public key encryption with keyword search (PEKS) scheme, proposed by Boneh, Di Crescenzo, Ostrovsky and Persiano, enables one to search for encrypted keywords without compromising the security of the original data. In this paper, we address two important issues of a PEKS scheme, "removing secure channel" and "refreshing keywords", which have not been considered in Boneh et al.'s paper. We point out the inefficiency of the original PEKS scheme due to the use of the secure channel. We resolve this problem by constructing an efficient PEKS scheme that removes a secure channel. We then argue that care must be taken when keywords are used frequently in the PEKS scheme as this situation might contradict the security of PEKS.

Abstract: We study the question of how to generically compose symmetric encryption and authentication when building "secure channels" for the protection of communications over insecure networks. We show that any secure channels protocol designed to work with any combination of secure encryption (against chosen plaintext attacks) and secure MAC must use the encrypt-then-authenticate method. We demonstrate this by showing that the other common methods of composing encryption and authentication, including the authenticate-then-encrypt method used in SSL, are not generically secure. We show an example of an encryption function that provides (Shannon's) perfect secrecy but when combined with any MAC function under the authenticate-then-encrypt method yields a totally insecure protocol (for example, finding passwords or credit card numbers transmitted under the protection of such protocol becomes an easy task for an active attacker). The same applies to the encrypt-and-authenticate method used in SSH. On the positive side we show that the authenticate-then-encrypt method is secure if the encryption method in use is either CBC mode (with an underlying secure block cipher) or a stream cipher (that xor the data with a random or pseudorandom pad). Thus, while we show the generic security of SSL to be broken, the current practical implementations of the protocol that use the above modes of encryption are safe.

Abstract: The public key encryption with keyword search (PEKS) scheme recently proposed by Boneh, Di Crescenzo, Ostrovsky, and Persiano enables one to search encrypted keywords without compromising the security of the original data. In this paper, we address three important issues of a PEKS scheme, “refreshing keywords”, “removing secure channel”, and “processing multiple keywords”, which have not been considered in Boneh et. al.’s paper. We argue that care must be taken when keywords are used frequently in the PEKS scheme as this situation might contradict the security of PEKS. We then point out the inefficiency of the original PEKS scheme due to the use of the secure channel. We resolve this problem by constructing an efficient PEKS scheme that removes secure channel. Finally, we propose a PEKS scheme that encrypts multiple keywords efficiently.