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

March, 2007
guidance regarding specific key types. Examples of cryptoperiod issues associated with public
key cryptography include:
1. The cryptoperiod of a private key transport key may be longer than the cryptoperiod of
the associated public key (i.e., the public key transport key). The public key is used for a
fixed period of time to encrypt keying material. That period of time may be indicated by
the expiration date on a public key certificate. The private key will need to be retained as
long as there is a need to recover (i.e., decrypt) the key(s) encrypted by the public key
(often after the public key has been destroyed).
2. In contrast, the cryptoperiod of a private authentication key that is used to sign challenge
information is basically the same as the cryptoperiod of the associated public key (i.e.,
the public authentication key). That is, when the private key will not be used to sign
challenges, the public key is no longer needed.
3. If a private signature key is used to generate digital signatures as a proof-of-origin, the
cryptoperiod of the private key may be significantly shorter than the cryptoperiod of the
associated public signature verification key. In this case, the private key is usually
intended for use for a fixed period of time, after which time the key owner shall destroy 12
the private key. The public key may be available for a longer period of time for verifying
signatures. However, other factors such as the strength of the signing algorithm, the value
of the signature, and the likelihood of forgery should be considered.
5.3.5 Symmetric Key Usage Periods and Cryptoperiods
For symmetric keys, a single key is used for both applying the protection (e.g., encrypting or
computing a MAC) and processing the protected information (e.g., decrypting the encrypted
information or verifying a MAC). The period of time during which cryptographic protection may
be applied to data is called the originator usage period, and the period of time during which the
protected information is processed is called the recipient usage period. A symmetric key shall
not be used to provide protection after the end of the originator usage period. The recipient usage
period may extend beyond the originator usage period. This permits all information that has been
protected by the originator to be processed by the recipient before the processing key must be
deactivated. However in many cases, the originator and recipient usage periods are the same. The
cryptoperiod of a symmetric key is the period of time from the beginning of the originator usage
period to the end of the recipient usage period.
Note that in some cases predetermined cryptoperiods may not be adequate for the security life of
the protected data. If the required security life exceeds the cryptoperiod, then the protection will
need to be reapplied using a new key.
Examples of the use of the usage periods include:
12 A simple deletion of the keying material might not completely obliterate the information. For example, erasing the
information might require overwriting that information multiple times with other non-related information, such as
random bits, or all zero or one bits. Keys stored in memory for a long time can become “burned in”. This can be
mitigated by splitting the key into components that are frequently updated (see [DiCrescenzo]).
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