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

March, 2007
and the method of key recovery requires reconstruction of the derived key from the
master key, then the master key should be backed up or archived.
B.3.8 Key Transport Key Pairs
A key transport key pair may be used to transport keying material from an originating entity to a
receiving entity during communications, or to protect keying material while in storage. The
originating entity in a communication (or the entity initiating the storage of the keying material)
uses the public key to encrypt the keying material; the receiving entity (or the entity retrieving
the stored keying material) uses the private key to decrypt the encrypted keying material.
B.3.8.1 Private Key Transport Keys
If a key transport key pair is only used during communications, then the private key transport
key does not need to be backed up if a replacement key pair can be generated and distributed in a
timely fashion. Alternatively, one or more additional key pairs could be made available (i.e.,
already generated and distributed). Otherwise, the private key should be backed up. The private
key transport key may be archived.
If the transport key pair is used only during storage, then the private key transport key should be
backed up or archived for as long as the protected keying material may need to be accessed.
However, at some time the strength of the cryptography may be reduced or lost completely. The
key transport algorithm may no longer offer adequate security or the private key transport key
may have been compromised. If the key transport algorithm has been withdrawn or the
cryptoperiod of the private key transport 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
key transport algorithm or its keys.
If the transport key pair is used during both communications and storage of keying material, then
the private key transport key should be backed up or archived for as long as the protected keying
material may need to be accessed. However, at some time the strength of the cryptography may
be reduced or lost completely. If the key transport algorithm has been withdrawn or the
cryptoperiod of the private key transport key has expired, the cryptographic protection shall be
regarded as minimal.
B.3.8.2 Public Key Transport Keys
If the sending entity (the originating entity in a communications) loses the public key transport
key or determines that the key has been corrupted, the key can be reacquired from the key pair
owner or by obtaining the public key certificate containing the public key (if the public key was
certified).
If the entity that applies the cryptographic protection to keying material that is to be stored
determines that the public key transport key has been lost or corrupted, the entity may recover in
one of the following ways:
1. If the public key has been certified and is stored elsewhere within the infrastructure, then
the certificate can be requested.
2. If some other entity knows the public key (e.g., the other entity is the actual owner of the
key pair), the key can be requested from this other entity.
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