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

B.3.5 Symmetric Key Wrapping Keys
March, 2007
A symmetric key wrapping key is used to wrap (i.e., encrypt) keying material that is to be
protected and may be used to protect multiple sets of keying material. The protected keying
material is then transmitted or stored or both.
If a symmetric key wrapping key is used only to transmit keying material, and the key wrapping
key becomes unavailable (e.g., is lost or corrupted), it may be possible to either resend the key
wrapping key, or to establish a new key wrapping key and use it to resend the keying material. If
this is possible within a reasonable timeframe, backup of the key wrapping key is not necessary.
If the key wrapping key cannot be resent or a new key wrapping key cannot be readily obtained,
backup of the key wrapping key should be considered. The archive of a key wrapping key that is
only used to transmit keying material is not necessary.
If a symmetric key wrapping key is only used to protect keying material in storage, then the key
wrapping key should be backed up or archived for as long as the keying material may need to be
accessed. However, at some time the strength of the key wrapping may be reduced or lost
completely. The key wrapping algorithm may no longer offer adequate security or the key
wrapping key may have been compromised. If the wrapping algorithm has been withdrawn or
the cryptoperiod of the key wrapping key has expired, the cryptographic protection shall be
regarded as minimal. Appropriate storage systems are being developed that employ
cryptographic time stamps to store sensitive data beyond the normal security life of the original
wrapping algorithm or its key wrapping keys.
If a symmetric key wrapping key is used for the protection of keying material during both
transmission and subsequent storage, then the key wrapping key should be backed up or
archived for as long as the keying material may need to be accessed.
B.3.6 Random Number Generation Keys
A key used for deterministic random bit generation shall not be backed up or archived. If this
key is lost or modified, it shall be replaced with a new key.
B.3.7 Symmetric Master Keys
A symmetric master key is normally used to derive one or more other keys. It shall not be used
for any other purpose.
The determination as to whether or not a symmetric master key needs to be backed up or
archived depends on a number of factors:
1. How easy is it to establish a new symmetric master key? If the master key is distributed
manually (e.g., in smart cards or in hard copy by receipted mail), the master key should
be backed up or archived. If a new master key can be easily and quickly established using
electronic key establishment protocols, then the backup or archiving of the master key
may not be desirable, depending on the application.
2. Are the derived keys recoverable without the use of the symmetric master key? If the
derived keys do not need to be backed up or archived (e.g., because of their use) or
recovery of the derived keys does not depend on reconstruction from the master key (e.g.,
the derived keys are stored in an encrypted form), then the backup or archiving of the
master key may not be desirable. If the derived keys need to be backed up or archived,
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