 the case of devices communicating with each other from a distance, the encryption key that the transmitter intends to use must be somehow conveyed to the receiver in advance of any transmission. This poses some logistical security problems, which are thor- oughly discussed in the references at the end of the chapter. For the moment, we will assume that the transmitting party manages to convey the encryption key to the receiver in a safe and timely fashion. The receiver, knowing the encryption algorithm, will be able to use the same key as its decryption key. Once it applies it to the received ciphertext, it will safely and reliably produce the original plaintext. The security of the arrangement (assuming that both the algorithm and the key are considered cryptographically secure) requires that no unauthorized third party knows the key. Symmetric-key encryption is also called private-key encryption because of the secrecy required to protect the encryp- tion/decryption key. In some private encryption schemes, the keys are not symmetric. A simple exam- ple would be a case where the same algorithm but two different keys (for example, K and K2) are agreed upon, where K1 is used to encrypt traffic originating from party A and destined for party B, and K2 is used to encrypt the traffic originating from party B and destined for party A. In this case, party A encrypts with K1 and decrypts with K, whereas party B encrypts with K2 and decrypts with K. Different algorithms can also be used for different directions of traffic, but systems synchroniza- tion issues usually become more difficult to manage. Symmetric encryption is computationally fast, meaning that it can occur in real time on content that is unknown previously (such as live telephone conversation, streaming video transmission, high- speed data link, and SO on). All securely transmitted live traffic today is encrypted using private/ symmetric encryption algorithms. This does not necessarily include the offline-secured material such as a piece of e-mail (or other