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

March, 2007
The implementation and associated key management must provide adequate security to prevent
any feasible attack from being successful.
6.2.1 Protection Mechanisms for Cryptographic Information in Transit
Cryptographic information in transit may be keying material being distributed in order to obtain
a cryptographic service (e.g., establish a key that will be used to provide confidentiality) (see
Section 8.1.5), or cryptographic information being backed up or archived for possible use or
recovery in the future (see Sections 8.2.2 and 8.3.1). This may be accomplished manually (i.e.,
via trusted courier), in an automated fashion (i.e., electronic communication protocols) or by
some combination of manual and automated methods. For some protocols, the protections are
provided by the protocol; in other cases, the protection for the keying material is provided
directly on the keying material. It is the responsibility of the originating entity to apply protection
mechanisms, and the responsibility of the recipient to undo or check the mechanisms used.
6.2.1.1 Availability
Since communications may be garbled, intentionally altered, or destroyed, the availability of
cryptographic information after transit cannot be assured using cryptographic methods. However,
availability can be supported by redundant or multiple channels, store and forward (deleting only
after confirmation of receipt) systems, error correction codes, and other non-cryptographic
mechanisms. Retransmission should only be used as a fall-back mechanism, since it imposes
requirements on entities outside of the communications system.
6.2.1.2 Integrity
Integrity involves both the prevention and detection of modifications to information. When
modifications are detected, measures may be taken to restore the information to its unaltered
form. Cryptographic mechanisms are often used to detect unauthorized modifications. The
integrity of cryptographic information during transit shall be protected using one or more of the
following mechanisms:
1. Manual method (physical protection is provided):
(a) An integrity mechanism comparable to a CRC (e.g., CRC, MAC or digital signature)
is used on the information, and the resulting code (e.g., CRC, MAC or digital
signature) is provided to the recipient. Note: A CRC may be used instead of MAC or
digital signature, since the physical protection is intended to protect against
intentional modifications.
-OR-
(b) The keying material is used to perform the intended cryptographic operation. If the
received information does not conform to the expected format, or the data is
inconsistent in the context of the application, then the keying material may have been
corrupted.
2. Electronic distribution via communication protocols (provided by the user or by the
communication protocol):
(a) A cryptographic integrity mechanism (e.g., a MAC or digital signature) is used on the
information, and the resulting code (e.g., MAC or digital signature) is provided to the
recipient. Note that a CRC is not Approved for this purpose. The integrity mechanism
79