Technologies are generally described for generating obfuscated message data. In some examples, a method performed under control of a server may include calculating co-occurrence probabilities of a plurality of combinations, each of which includes at least two elements that are included in original message data; extracting, from the original message data, a first data set that includes at least one combination that has a first co-occurrence probability from among the plurality of combinations and a second data set that includes at least one combination that has a second co-occurrence probability from among the plurality of combinations; generating dummy data by using the first data set and the second data set; and adding the generated dummy data to the original message data to generate obfuscated message data.