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

8. Symmetric and Asymmetric RNG keys:
March, 2007
a. Type Considerations: Symmetric and asymmetric RNG keys are used in deterministic
random number generation functions. The Approved RNGs control key changes (e.g.,
during reseeding).
b. Cryptoperiod: Assuming the use of Approved RNGs, the cryptoperiod of symmetric
and asymmetric RNG keys is determined by the design of the RNG.
9. Symmetric master key:
a. Type Considerations: A symmetric master key may be used multiple times to derive
other keys using a (one-way) key derivation function (see Section 8.2.4). Therefore, the
cryptoperiod consists of only an originator usage period for this key type. A suitable
cryptoperiod depends on the nature and use of the keys derived from the master key and
on considerations provided earlier in Section 5.3. The cryptoperiod of a key derived from
a master key could be relatively short, e.g., a single use, communication session, or
transaction. Alternatively, the master key could be used over a longer period of time to
derive (or re-derive) multiple keys for the same or different purposes. The cryptoperiod
of the derived keys depends on their use (e.g., as symmetric encryption or authentication
keys).
b. Cryptoperiod: An appropriate cryptoperiod for the symmetric master key might be 1
year, depending on its usage environment and the sensitivity/criticality of the information
protected by the derived keys and the number of keys derived from the master key.
10. Private key transport key:
a. Type Considerations: A private key transport key may be used multiple times. Due to
the potential need to decrypt keys some time after they have been encrypted for transport,
the cryptoperiod of the private key transport key may be longer than the cryptoperiod of
the associated public key. The cryptoperiod of the private key is the length of time during
which any keys encrypted by the associated key transport public key need to be
decrypted.
b. Cryptoperiod: Given 1) the use of FIPS-Approved algorithms and key sizes, 2) the
volume of information that may be protected by keys encrypted under the associated
public transport key, and 3) an expectation that the security of the key storage and use
environment will increase as the sensitivity and/or criticality of the processes for which
the key provides protection increases; a maximum cryptoperiod of about 2 years is
recommended. In certain applications (e.g., email), where received messages are stored
and decrypted at a later time, the cryptoperiod of the private key transport key may
exceed the cryptoperiod of the public key transport key.
11. Public key transport key:
a. Type Considerations: The cryptoperiod for the public key transport key is that period
of time during which the public key may be used to actually apply the encryption
operation to the keys that will be protected. Public key transport keys can be public
knowledge. The driving factor in establishing the public key transport key cryptoperiod is
the cryptoperiod of the associated private key transport key. As indicated in the private
key transport key discussion, due to the potential need to decrypt keys some time after
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