The difficulties of key management are discussed below. Because given enough time a key will be discovered, the long term viability of an encryption requires that the key be changed periodically. The stronger the encryption, the less frequently keys must be changed to prevent attack. The dissemination of keys is of critical importance, since it is both cumbersome and a major source of vulnerability. Key use must be synchronized so that both sender and receiver are using the same key for a communication.
Messages intended for groups require the same key for all group members.
Keys are also usually changed periodically in case the key was inadvertently divulged. The major differentiator between encryption methods is that of public vs. private key. In a traditional private key system, the encryption and decryption keys are identical and must be kept secret. Each pair of communicating partners or groups must have a secret key. The encryption key is known to anyone who wants it and is generally available from a well-known location to prevent spoofing.