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

March, 2007
2. The application in which the key will be used (e.g., interactive communications, file
storage),
3. Whether the key is "owned" by the local entity (e.g., a private key) or by another entity
(e.g., the other entity's public key) or is shared (e.g., a symmetric data encryption key
shared by two entities),
4. The role of the entity in a communication (e.g., sender of receiver), and
5. The algorithm or computation in which the key will be used (e.g., does the entity have the
necessary information to perform a given computation if the key were to be recovered) 28 .
The factors involved in a decision for or against key recovery should be carefully assessed. The
trade-offs are concerned with continuity of operations versus the risk of possibly exposing the
keying material and the information it protects if control of the keying material is lost. If it is
determined that a key needs to be recovered, and the key is still active (i.e., the cryptoperiod of
the key has not expired), then the key may be replaced in order to limit the exposure of the data
protected by that key (see Section 8.2.3).
Issues associated with key recovery and discussions about whether or not different types of
cryptographic material need to be recoverable are provided in Appendix B.
8.2.3 Key Change Function
Key change is the replacement of a key with another key that performs the same function as the
original key. There are several reasons for changing a key.
1. The key may have been compromised.
2. The key’s cryptoperiod may be nearing expiration.
3. It may be desirable to limit the amount of data protected with any given key.
A key may be replaced by re-keying or by key update.
8.2.3.1 Re-keying
If the new key is generated in a manner that is entirely independent of the “value” of the old key,
the process is known as re-keying. This replacement shall be accomplished using one of the key
establishment methods discussed in Section 8.1.5. Re-keying is used when a key has been
compromised (provided that the re-keying scheme itself is not compromised) or when the
cryptoperiod is nearing expiration.
8.2.3.2 Key Update Function
If the “value” of the new key is dependent on the value of the old key, the process is known as
key update (i.e., the current key is modified to create a new key). This shall be accomplished by
applying a non-reversible function to the old key and other data. Unlike re-keying, key update
does not require the exchange of any new information between the entities that previously shared
the old key. For example, the two entities may agree to update their shared key on the first day of
each month. Since a non-reversible function is used in the update process, previous keys are
28
This could be the case when performing a key establishment process for some key establishment schemes (see SP
800-56).
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