A method is disclosed that enables the transmission of a digital message along with a corresponding information signal, such as audio or video. The supplemental information contained in digital messages can be used for a variety of purposes, such as enabling or enhancing packet authentication. In particular, a telecommunications device that is processing an information signal from its user, such as a speech signal, encrypts the information signal by performing a bitwise exclusive-or of an encryption key stream with the information signal stream. The device, such as a telecommunications endpoint, then intersperses the bits of the digital message throughout the encrypted signal in place of those bits overwritten, in a process referred to as “watermarking.” The endpoint then transmits the interspersed digital message bits as part of a composite signal that also comprises the encrypted information bits. No additional bits are appended to the packet to be transmitted, thereby addressing compatibility issues.