  The Distributed Symmetric Key Establishment (DSKE) protocol provides secure
secret exchange (e.g., for key exchange) between two honest parties that need
not have had prior contact, and use intermediaries with whom they each securely
share confidential data. We show the composable security of the DSKE protocol
in the constructive cryptography framework of Maurer. Specifically, we prove
the security (correctness and confidentiality) and robustness of this protocol
against any computationally unbounded adversary, who additionally may have
fully compromised a bounded number of the intermediaries and can eavesdrop on
all communication. As DSKE is highly scalable in a network setting with no
distance limit, it is expected to be a cost-effective quantum-safe
cryptographic solution to safeguarding the network security against the threat
of quantum computers.
