Message authentication code based on error correcting code

A method of processing a message to determine a tag value from the message and from a key according to a message authentication code. The method including the steps of selecting one of a plurality of symbols, the plurality of symbols forming a codeword encoding a data item derived from the message, the codeword encoding the data item according to an error correcting code, wherein said key determines which one of said plurality of symbols is selected; and determining the tag value to be the selected symbol.