Secure communication
Encryption is a method in which data is rendered hard to read by an unauthorized party. Since encryption methods are created to extremely hard to break, many communication methods either use deliberately weaker encryption than possible, or have backdoors inserted to permit rapid decryption. In some cases government authorities have required backdoors be installed in secret. Many methods of encryption are also subject to "man in the middle" attack whereby a third party who can 'see' the establishment of the secure communication is made privy to the encryption method, this would apply for example to interception of computer use at an ISP. Provided it is correctly programmed, sufficiently powerful, and the keys not intercepted, encryption would usually be considered secure. The article on key size examines the key requirements for certain degrees of encryption security.
Packet capture appliance
Some packet capture appliances encrypt the captured data before saving it to disk, while others do not. Considering the breadth of information that travels on a network or internet connection and that at least a portion of it could be considered sensitive, encryption is a good idea for most situations as a measure to keep the captured data secure. Encryption is also a critical element of authentication of data for the purposes of data/network forensics.