An apparatus for data coding includes an encoder and a decoder. The encoder is configured to receive input data including one or more m-bit data groups that are associated with respective group indices, to generate a code word that includes the input data and an m-bit redundancy that depends on the data groups and on the respective group indices, and to send the code word over a channel. The decoder is connected to the channel and is configured to produce a syndrome that equals zero when the code word is error-free, and when the code word contains a single error caused by the channel, is indicative of an erroneous group in which the single error occurred, and of a location of the single error within the erroneous group, and to recover the input data by correcting the single error at the location in the erroneous group.