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

March, 2007
1. Private key used to sign a root certificate in a public key infrastructure
2. Symmetric key transport key shared by a large number of users
3. Private asymmetric key transport key shared by a large number of users
4. Master key used in the generation of keys by a large number of users
5. Symmetric data encryption key used to encrypt data in a large distributed database
6. Symmetric key shared by a large number of communications network participants
7. Key used to protect a large number of stored keys
8. A certification authority’s (CA’s) private key
In all of these cases, a large number of holders would need to be immediately notified of the
compromise. Inclusion of the key identifier on a Compromised Key List (CKL) or Certificate
Revocation List (CRL) to be published at a later date would not be sufficient. This means that a
list of holders would need to be maintained and a means for communicating news of the
compromise to the holders would be required. News of the compromise and the replacement of
keys should be sent only to the affected entities so as not to encourage others to exploit the
situation.
In all of these cases, a secure path for replacing the compromised keys is required. In order to
permit rapid restoration of service, an electronic (e.g., over-the-air) replacement path is preferred
(see Section 8.2.3). In some cases, however, there may be no practical alternative to manual
distribution (e.g., compromise of a root CA’s private key). Contingency distribution of alternate
keys may help restore service rapidly in some circumstances (e.g., compromise of a widely held
symmetric key), but the possibility of simultaneous compromise of operational and contingency
keys would need to be considered.
Damage assessment can be extraordinarily complex, particularly in cases such as the
compromise and replacement of CA private keys, widely used transport keys, and keys used by
many users of large distributed databases.
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