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 bottom level of nodes, and subsequent levels of nodes, ending with a top level terminal node. Each bottom level node stores a hash of an assigned time-series data set. Each node of each subsequent level stores data generated from an assigned group of nodes of a previous level. The processor receives a validation request. In response, 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 terminal nodes in the first and second versions of the data structure do not match. In response, the processor generates an alert.