  We consider a key agreement setting where two parties observe correlated
random sources, and want to agree on a secret key via public discussions. In
order to allow the key length to adapt to the realizations of the random
sources, we allow the key to be of variable length, subject to a novel
variable-length version of the uniformity constraint based on random stopping
time. We propose simple, computationally efficient key agreement schemes under
the new constraint. The proposed scheme can be considered as the key agreement
analogue of variable-length source coding via Huffman coding, and the Knuth-Yao
random number generator.
