  This paper proposes a new scheme to secure the transmissions in an untrusted
decode-and-forward (DF) relaying network. A legitimate source node, Alice,
sends her data to a legitimate destination node, Bob, with the aid of an
untrusted DF relay node, Charlie. To secure the transmissions from Charlie
during relaying time slots, each data codeword is secured using a secret-key
codeword that has been previously shared between Alice and Bob during the
perfectly secured time slots (i.e., when the channel secrecy rate is positive).
The secret-key bits exchanged between Alice and Bob are stored in a
finite-length buffer and are used to secure data transmission whenever needed.
We model the secret-key buffer as a queueing system and analyze its Markov
chain. Our numerical results show the gains of our proposed scheme relative to
benchmarks. Moreover, the proposed scheme achieves an upper bound on the secure
throughput.
