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

March, 2007
symmetric authentication key rather than reusing the “lost” key is also acceptable; a new
MAC would need to be computed on the information using the new authentication key.
Otherwise, the symmetric authentication key should be backed up. Archiving the
authentication key is not appropriate if the MAC and the authenticated information are
not subsequently stored, since the use of the key for both applying and checking the
MAC would be discontinued at the end of the key's cryptoperiod. If the MAC and the
authenticated information are subsequently stored, then the symmetric authentication key
should be backed up or archived for as long as the integrity and source of the information
needs to be determined. However, at some time the cryptographic strength of the MAC
may be reduced or lost completely. If the MAC algorithm has been withdrawn or the
cryptoperiod of the MAC key has expired, the MAC shall be regarded as an error
detection code rather than a cryptographic integrity check.
In case 4, the symmetric authentication key should be backed up or archived for as long
as the integrity and source of the information needs to be determined. However, at some
time the cryptographic strength of the MAC may be reduced or lost completely. If the
MAC algorithm has been withdrawn or the cryptoperiod of the MAC key has expired, the
MAC shall be regarded as an error detection code rather than a cryptographic integrity
check.
The symmetric authentication key may be stored in backup storage for the cryptoperiod of the
key, and in archive storage until no longer required. If the authentication key is recovered by
reconstruction, the “base” key (e.g., the master key for a key derivation method) may be stored in
normal operational storage or backup storage for the cryptoperiod of the key, and in archive
storage until no longer required.
B.3.3 Authentication Key Pairs
A public authentication key is used by a receiving entity to obtain the assurance of both the
identity of the entity that originated information and the integrity of the information. The
associated private authentication key is used by the originating entity to provide this assurance to
a receiving entity by computing a digital signature on the information. This key pair may not
provide non-repudiation.
B.3.3.1 Public Authentication Keys
It is appropriate to store a public authentication key in either backup or archive storage for as
long as required to verify the authenticity of the data that was authenticated by the associated
private authentication key. However, at some time the cryptographic strength of the signature
may be reduced or lost completely. The signature algorithm may no longer offer adequate
security or the private authentication key may have been compromised. If the signature algorithm
has been withdrawn or the cryptoperiod of the private authentication key has expired, signature
shall be regarded as an error detection code rather than a digital signature. Appropriate storage
systems are being developed that employ cryptographic time stamps to store sensitive data
beyond the normal security life of the original signature algorithm or its keys.
In the case of a public key that has been certified (e.g., by a Certificate Authority), saving the
public key certificate would be an appropriate form of storing the public key; backup or archive
storage may be provided by the infrastructure (e.g., by a certificate repository). The public key
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