A method and apparatus for the digital signing of a message to be signed, the message to be signed is transmitted via a communication network to a mobile radio telephone to be used as a signing unit. A message to be signed is transmitted from a transmitter to a receiver and then from the receiver to the mobile radio telephone via a telephone network. The mobile radio telephone user indicates that the message to be signed should be signed, and the mobile radio telephone generates a signed message. The signed message is then transmitted from the mobile radio telephone to the receiver and from the receiver to an addressee.