We investigate decimal number representations in large data streams. When decimal numbers are encoded in bits, not all possibilities are used. This opens possibilities to establish a covert channel, i.e., to inject secret data into the data stream and transport it hidden in the mass of data, which poses a security risk, as covert channels are mostly used for criminal purposes. We present a novel covert channel approach for streams of decimal data, a field which so far has been neglected. Moreover, we sketch an application scenario and analyze the covert channel’s detectability, in particular how detectability and steganographic bandwidth can be traded against each other. The approach and proposals for detection are tested via simulation experiments.