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Printable Version: RFC1244.PDF
RFC 1244 Site Security Handbook July 1991
it was verified to be in an "integral" state. The value of
information integrity to a site will vary. For example, it is
more important for military and government installations to
prevent the "disclosure" of classified information, whether it is
right or wrong. A bank, on the other hand, is far more concerned
with whether the account information maintained for its customers
is complete and accurate.
Numerous computer system mechanisms, as well as procedural
controls, have an influence on the integrity of system
information. Traditional access control mechanisms maintain
controls over who can access system information. These mechanisms
alone are not sufficient in some cases to provide the degree of
integrity required. Some other mechanisms are briefly discussed
below.
It should be noted that there are other aspects to maintaining
system integrity besides these mechanisms, such as two-person
controls, and integrity validation procedures. These are beyond
the scope of this document.
3.9.4.1 Checksums
Easily the simplest mechanism, a simple checksum routine can
compute a value for a system file and compare it with the last
known value. If the two are equal, the file is probably
unchanged. If not, the file has been changed by some unknown
means.
Though it is the easiest to implement, the checksum scheme
suffers from a serious failing in that it is not very
sophisticated and a determined attacker could easily add enough
characters to the file to eventually obtain the correct value.
A specific type of checksum, called a CRC checksum, is
considerably more robust than a simple checksum. It is only
slightly more difficult to implement and provides a better
degree of catching errors. It too, however, suffers from the
possibility of compromise by an attacker.
Checksums may be used to detect the altering of information.
However, they do not actively guard against changes being made.
For this, other mechanisms such as access controls and
encryption should be used.
Site Security Policy Handbook Working Group [Page 38]