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

March, 2007
altered from their original contents), i.e., non-repudiation and the authenticity of the
information is in question.
The unauthorized disclosure of a private signature key means that the integrity and nonrepudiation
qualities of all data signed by that key are suspect. An unauthorized party in
possession of the private key could sign false information and make it appear to be valid.
In cases where it can be shown that the signed data was protected by other mechanisms
(e.g., physical security) from a time before the compromise, the signature may still have
some value. For example, if a signed message was received on day 1 and it was later
determined that the private signing key was compromised on day 15, the receiver may
still have confidence that the message is valid because it was maintained in the receiver’s
possession. Note that cryptographic time stamping may also provide protection for
messages signed before the private signature key was compromised. However, the
security provided by these other mechanisms is now critical to the security of the
signature. In addition the non-repudiation may be questioned, since the private signature
key may have been disclosed to the message receiver who then altered the message in
some way.
2. A compromise of the integrity of a key means that the key is incorrect - either that the
key has been modified (either deliberately or accidentally), or that another key has been
substituted; this includes a deletion (non-availability) of the key. The compromise of a
key used to provide integrity 17 calls into question the integrity of all information
protected by the key. This information could have been provided by, or changed by, an
unauthorized entity.
3. A compromise of a key’s usage or application association means that the key could be
used for the wrong purpose (e.g., key establishment instead of digital signatures) or for
the wrong application, and could result in the compromise of information protected by
the key.
4. A compromise of a key’s association with the owner or other entity means that the
identity of the other entity cannot be assured (i.e., one doesn’t know who the other entity
really is) or that information cannot be processed correctly (e.g., encrypted or decrypted
with the correct key).
5. A compromise of a key’s association with other information means that there is no
association at all, or the association is with the wrong “information”. This could cause the
cryptographic services to fail, information to be lost, or the security of the information to
be compromised.
Certain protective measures may be taken in order to minimize the likelihood or consequences of
a key compromise. The following procedures are usually involved:
a. Limiting the amount of time a symmetric or private key is in plaintext form.
b. Preventing humans from viewing plaintext symmetric and private keys.
17 As opposed to the integrity of a key that could, for example, be used for encryption.
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