Secure communication

A method for deriving shared secret information between two devices (device A and device B). Device A obtains a data set DA 301A, and device B obtains a data set DB 310B. Then for each of N>1 subsets, DAi and DBi (i=1,2,...,N), respectively of DA and DB, a first value, VAi=MA(DAi) 303A, based on DAi is determined by device A, a second value, VBi=MB(DBi) 310B, based on DBi is determined by device B, and one or more messages are exchanged between devices A and B to determine whether a condition based on the values, VAi and VBi, is satisfied 305. MA and MB each comprise an entropy reducing and/or statistical function (e.g. a parity, hamming distance, mean or variance function). Devices A and B then each obtain a reduced data set DA', DB' respectively based on those subsets DAi, DBi respectively for which the condition is satisfied. The steps of the method may be repeated until one or more termination criteria are satisfied, with the data sets used in one iteration comprising the reduced data sets obtained in the preceding iteration. Upon termination, the reduced data sets may be used as a one-time pad.