An apparatus includes a memory and a processor. The memory stores a time-series of data sets, and a first version of a data structure generated from the time-series as it existed at a first time. The data structure includes a terminal node that stores a compressed representation of the time-series and a bottom level of nodes, each of which stores a hash of a data set of the time-series. The processor generates a second version of the data structure based on the time-series as it exists at a second time. The processor determines that the data stored by the terminal nodes in the first and second versions do not match. In response, the processor identifies a bottom level node for which the stored data in the first and second versions do not match, and requests validation. The processor receives a response indicating whether an intentional change was made.