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

March, 2007
may be stored in backup storage until the end of the private key’s cryptoperiod, and may be
stored in archive storage as long as required.
B.3.3.2 Private Authentication Keys
When the private key is used only for the authentication of transmitted data, whether or not the
authenticated data is subsequently stored, the private authentication key need not be backed up if
a new key pair can be generated and distributed in accordance with Section 8.1.5.1 in a timely
manner. However, if a new key pair cannot be generated quickly, the private key should be
stored in backup storage during the cryptoperiod of the private key. The private key shall not be
stored in archive storage.
When the private authentication key is used to protect stored information only, the authentication
private key should not be backed up if a new key pair can be generated. However, if a new key
pair cannot be generated, the private key should be stored in backup storage during the
cryptoperiod of the private key. The private key shall not be stored in archive storage.
B.3.4 Symmetric Data Encryption Keys
A symmetric data encryption key is used to protect the confidentiality of stored or transmitted
information or both.
Information that is stored needs to be readily available as long as the information is encrypted
(i.e., for as long as the information may need to be recovered); this includes information that is
both transmitted and stored using the same key. The decryption key (which was also used to
encrypt the information) needs to be available during the period of time that the information may
need to be recovered. Therefore, the key should be backed up or archived during this period.
However, at some time the strength of the cryptography may be reduced or lost completely. The
encryption algorithm may no longer offer adequate security or the symmetric key may have been
compromised. If the encryption algorithm has been withdrawn or the cryptoperiod of the
symmetric 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 encryption algorithm or its keys.
In order to allow key recovery, the symmetric data encryption key should be stored in backup
storage during the cryptoperiod of the key, and stored in archive storage after the end of the
key’s cryptoperiod, if required. In many cases, the key is protected and stored with the encrypted
data. The key is wrapped by an archive encryption key or by a symmetric key wrapping key that
is wrapped by a physically protected archive encryption key.
A symmetric data encryption key that is used for transmission only is used by an originating
entity to encrypt information, and by the receiving entity to decrypt the information immediately
upon receipt. If the data encryption key is lost or corrupted, and a new data encryption key can
be easily obtained by the originating and receiving entities, then the key need not be backed up.
However, if the key cannot be easily replaced by a new key, then the key should be backed up if
the information to be exchanged is of sufficient importance. The data encryption key should not
be archived when used for transmission only.
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