Data encryption

A method of decrypting a data stream, wherein the data stream comprises encrypted encoded packets of data, and the encrypted encoded packets of data include packets having a first importance level, and packets having a second importance level, the second importance level being lower than the first importance level, wherein the method comprises:

• decrypting a first encrypted encoded packet of data having the first importance level to generate a first decrypted packet;
• using data from the first encrypted encoded packet of data in decrypting a second packet of data having the first importance level to generate a second decrypted packet;
• and
• using data from the first encrypted encoded packet of data in decrypting a first packet of data having the second importance level to generate a third decrypted packet.

An encryption system for encrypting a data stream, wherein the data stream comprises encoded packets of data, and the packets of data include packets having a first importance level, and packets having a second importance level, the second importance level being lower than the first importance level, wherein the system comprises:

• a first encryption unit for encrypting a first packet of data having the first importance level to generate a first encrypted packet;
• a second encryption unit for encrypting a second packet of data having the first importance level to generate a second encrypted packet, wherein the step of encrypting the second packet of data having the first importance level uses the first encrypted packet;
• and
• a third encryption unit for encrypting a first packet of data having the second importance level to generate a third encrypted packet, wherein the step of encrypting the first packet of data having the second importance level uses the first encrypted packet.

A decryption system for decrypting a data stream, wherein the data stream comprises encrypted encoded packets of data, and the encrypted encoded packets of data include packets having a first importance level, and packets having a second importance level, the second importance level being lower than the first importance level, wherein the system comprises:

• a first decryption unit for decrypting a first encrypted encoded packet of data having the first importance level to generate a first decrypted packet;
• a second decryption unit for using data from the first encrypted encoded packet of data in decrypting a second packet of data having the first importance level to generate a second decrypted packet;
• and
• a third decryption unit for using data from the first encrypted encoded packet of data in decrypting a first packet of data having the second importance level to generate a third decrypted packet.