I. Introduction
Many security mechanisms and applications on Internet today rely on the assumption of immediate end-to-end reachability. But, in the space internet this assumption never holds true, because the space internet, which is recognized as a type of Delay/disruption Tolerant Networks (DTNs) [1], is subject to a high delay and unexpected loss of connectivity. Accordingly, the conventional Public Key Infrastructure (PKI) and online negotiation-based key distributing protocols are not applicable. In this paper, we aim to design non-interactive key exchange protocols on which other security mechanisms are built, by utilizing two-channel cryptography combined with the predictable contacts of space DTNs.