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

March, 2007
All records of the entity and the entity's associations shall be marked to indicate that the entity is
no longer a member of the security domain, but the records should not be deleted. To reduce
confusion and unavoidable human errors, identification information associated with the deregistered
entity should not be re-used (at least for a period of time). For example, if a “John
Wilson” retires and is de-registered on Friday, the identification information assigned to his son
“John Wilson”, who is hired the following Monday, should be different.
8.3.3 Key De-registration Function
Registered keying material may be associated with the identity of a key owner, owner attributes
(e.g., email address), or role or authorization information. When the keying material is no longer
needed, or the associated information becomes invalid, the keying material should be deregistered
(i.e., all records of the keying material and its associations should be marked to
indicate that the key is no longer in use).
When a cryptographic key is compromised, that key and any associated keying material should
be de-registered. In a PKI, key de-registration is commonly achieved by including the certificate
in a list of revoked certificates (i.e., a CRL). Where the PKI uses online status mechanisms (e.g.,
the Online Certificate Status Protocol [RFC2560]), de-registration is achieved by informing the
appropriate certificate status server(s). For example, when a private key is compromised, the
corresponding public key certificate should be revoked. Certificate revocation because of a key
compromise indicates that the binding between the owner and the key is no longer to be trusted.
Keying material should be de-registered when the attributes associated with an entity are
modified. For example, if an entity's email address is associated with a public key, and the
entity's address changes, the keying material should be de-registered to indicate that the
associated attributes have become invalid. Unlike the case of key compromise, the entity could
safely re-register the public key after modifying the entity's attributes through the user
registration process (see Section 8.1.1).
8.3.4 Key Destruction Function
When copies of cryptographic keys are made, care should be taken to provide for their eventual
destruction. All copies of the private or symmetric key shall be destroyed as soon as no longer
required (e.g., for archival or reconstruction activity) in order to minimize the risk of a
compromise. Any media on which unencrypted keying material requiring confidentiality
protection is stored shall be erased in a manner that removes all traces of the keying material so
that it cannot be recovered by either physical or electronic means 30 . Public keys may be retained
or destroyed, as desired.
8.3.5 Key Revocation Function
It is sometimes necessary to remove keying material from use prior to the end of its normal
cryptoperiod for reasons that include key compromise, removal of an entity from an
organization, etc. This process is known as key revocation and is used to explicitly revoke a
30 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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