Session key
Second, many otherwise good encryption algorithms require that keys be distributed securely before encryption can be used. All symmetric secret key algorithms have this undesirable property. There are other algorithms which don't require secure distribution of secret keys, but they are too slow to be practical for encrypting long messages (see Public-key cryptography). By using one of these "asymmetric" algorithms to distribute an encrypted secret key for another, faster, symmetric-key algorithm, it's possible to improve overall performance considerably.