Abstract: The transmission system according to the invention comprises a transmitter (10) and a receiver (14). The transmitter can transmit data packets, e.g. IP packets, to the receiver. These data packets comprise a header (24), e.g. an ESP header, and an encrypted data field (26). The data fields (26) can be decrypted in the receiver (14) by means of a changing decryption key (48). The header (24) comprises information (32) indicating a change of the decryption key (48). This information (32) may be included in the SPI field of an ESP header. In this way the receiver (14) can determine exactly when to start using a new decryption key (48).

Abstract: The transmission system comprises a transmitter (10) and a receiver (14). The transmitter (10) can transmit a multiplex signal (12), e.g. an MPEG-2 transport stream, to the receiver (14). This multiplex signal (12) comprises first and second sections which include, respectively, encrypted data packets and decryption packets. The first and second sections are linked together by an identical part of said first and second sections. The decryption packets comprise decryption keys (48). The data packets can be decrypted by means of the decryption keys (48), which are retransmitted a number of times. The first and second sections further comprise a header (20), which includes information indicating a change of the decryption key being used. The receiver (14) comprises filter means (16) for filtering out second and further occurrences of the same decryption key (48) on the basis of this information.