applying to each of the segments a threshold encryption scheme using a first key from the first set of keys and {HEAD ~ a second key} from {TAIL ~ the second set} of keys 
from
applying to each of the segments a threshold encryption scheme using a first key from the first set of keys and { HEAD ~ a second key } from { TAIL ~ the second set } of keys
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH