Methods, systems, and apparatus, including computer programs encoded on computer storage media, for secure data transmission using natural language. One of the methods includes: obtaining sensitive information for a user; obtaining a natural language key for the user, wherein the natural language key for the user includes one or more natural language tokens; generating decoding data for the sensitive information for the user, wherein generating the decoding data comprises: for each place in the sensitive information for the user: assigning a respective one of the natural language tokens in the natural language key for the user to the value at the place, and generating one or more respective dummy natural language tokens for each value of the respective set of possible values for the place other than the value at the place; and providing the decoding data for use in decoding the natural language key into the sensitive information.