  In this paper we describe a new, lightweight security mechanism for PSTN-VoIP
cooperation that is based on two information hiding techniques: digital
watermarking and steganography. Proposed scheme is especially suitable for
PSTN-IP-PSTN (toll-by-passing) scenario which nowadays is very popular
application of IP Telephony systems. With the use of this mechanism we
authenticate end-to-end transmitted voice between PSTN users. Additionally we
improve IP part traffic security (both media stream and VoIP signalling
messages). Exemplary scenario is presented for SIP signalling protocol along
with SIP-T extension and H.248/Megaco protocol.
