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There are many measurement applications in which it is not sufficient to store the measurements. It is often necessary to prove that the stored data have not been altered since the measurement was made. In a previous column , we discussed the general theory of one of these methods. This column gives the details. The method has two basic steps. Given a sequence of measurements, first find a number that is easy to derive from the sequence but which changes if the recorded measurements are altered using cryptographic hash functions. The second step is for the storage device to encrypt the "representative number," the hash value, so that the mere fact that a particular key decrypts the number shows who encrypted the number.