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

3. If the private key is known, then the public key can be recomputed.
4. A new key pair can be generated.
B.3.9 Symmetric Key Agreement Keys
March, 2007
Symmetric key agreement keys are used to establish keying material (e.g., symmetric key
wrapping keys, symmetric data encryption keys, or symmetric authentication keys). Each key
agreement key is shared between two or more entities. If these keys are distributed manually
(e.g., in a key loading device or by receipted mail), then the symmetric key agreement key
should be backed up. If an electronic means is available for quickly establishing new keys (e.g.,
a key transport mechanism can be used to establish a new symmetric key agreement key), then a
symmetric key agreement key need not be backed up. Symmetric key agreement keys shall not
be archived.
B.3.10 Static Key Agreement Key Pairs
Static key agreement key pairs are used to establish shared secrets between entities, often in
conjunction with ephemeral key pairs (see [SP800-56]). Each entity uses their private key
agreement key(s), the other entity's public key agreement key(s) and possibly their own public
key agreement key(s) to determine the shared secret. The shared secret is subsequently used to
derive shared keying material. Note that in some key agreement schemes, one or more of the
entities may not have a static key agreement pair (see [SP800-56]).
B.3.10.1 Private Static Key Agreement Keys
If the private static key agreement key cannot be replaced in a timely manner, or if it needs to be
retained in order to recover encrypted stored data, then the private key should be backed up in
order to continue operations. The private key should not be archived unless needed for
reconstruction of the keying material.
B.3.10.2 Public Static Key Agreement Keys
If an entity determines that the public static key agreement key is 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.
3. If the private key is known, then the public key can be recomputed.
4. If the entity is the owner of the key pair, a new key pair can be generated and distributed.
If none of these alternatives are possible, then the public static key agreement key should be
backed up. The public key should not be archived unless needed for reconstruction of the keying
material.
B.3.11 Ephemeral Key Pairs
Ephemeral key agreement keys are generated and distributed during a single key agreement
process (e.g., at the beginning of a communication session) and are not reused. These key pairs
are used to establish a shared secret (often in combination with static key pairs); the shared secret
is subsequently used to derive shared keying material. Not all key agreement schemes use
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