Part 1: General - Computer Security Resource Center - National ...

March, 2007
a. When a symmetric key is used only for securing communications, the period of time
from the originator’s application of protection to the recipient’s processing is negligible.
In this case, the key is authorized for either purpose during the entire cryptoperiod, i.e.,
the originator usage period and the recipient usage period are the same.
b. When a symmetric key is used to protect stored information, the originator usage period
(when the originator applies cryptographic protection to stored information) may end
much earlier than the recipient usage period (when the stored information is processed).
In this case, the cryptoperiod begins at the initial time authorized for the application of
protection with the key, and ends with the latest time authorized for processing using that
key. In general, the recipient usage period for stored information will continue beyond
the originator usage period, so that the stored information may be authenticated or
decrypted at a later time.
c. When a symmetric key is used to protect stored information, the recipient usage period
may start after the beginning of the originator usage period as shown in Figure 1. For
example, information may be encrypted before being stored on a compact disk. At some
later time, the key may be distributed in order to decrypt and recover the information.
Figure 1: Symmetric key cryptoperiod (Example C)
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