the context of communications across a network, the following attacks can be
Disclosure: Release of message contents to any person
or process not
possess- ing the appropriate cryptographic key.
Traffic analysis: Discovery
of the pattern of traffic between parties. In a connection-oriented
application, the frequency and duration of connections could be determined. In either a connection-oriented or connectionless environ- ment, the number and length of messages between
parties could be determined.
Masquerade: Insertion of messages into the network
from a fraudulent source. This includes the creation of messages by
an opponent that are purported to come from an authorized entity. Also included
are fraudulent acknowledg- ments of message receipt or nonreceipt by someone
other than the message recipient.
Content modification: Changes to the contents of a message, including insertion,
deletion, transposition, and modification.
Sequence modification: Any modification to a sequence of messages
including insertion, deletion, and reordering.
Timing modification: Delay
or replay of messages. In a connection-oriented application, an entire session
or sequence of messages could be a replay of some previous valid session, or individual messages in the sequence could be delayed or replayed. In a connectionless application, an individual message
(e.g., data- gram) could be delayed
Source repudiation: Denial of transmission of message
Destination repudiation: Denial of receipt of message by destination.
Measures to deal with the first two attacks are in the realm of message confi- dentiality and are dealt with in
Part One. Measures to deal with items (3)
in the foregoing list are generally regarded as
message authentication. Mechanisms for dealing
specifically with item (7) come under the heading of digital
signatures. Generally, a digital signature
technique will also counter some or all of
the attacks listed under items (3) through (6). Dealing with item (8) may
require a combination of the use of digital signatures and a protocol
designed to counter
summary, message authentication is a procedure to verify that received messages come
from the alleged source and have not been altered. Message authentication may also verify
sequencing and timeliness. A digital signature is an authentication
technique that also includes measures to counter repudiation by the source.