The protocols require computation of a message digest computed over the SNMPv2 message prepended by the secret for the relevant party. By virtue of this mechanism and assumptions 1 and 2, the protocols realize goal 1.
Normally, the inclusion of the message digest value with the digested message would not be sufficient to guarantee data integrity, since the digest value can be modified in addition to the message while it is enroute. However, since not all of the digested message is included in the transmission to the destination, it is not possible to substitute both a message and a digest value while enroute to a destination.
Strictly speaking, the specified strategy for data integrity does not detect a SNMPv2 message modification which appends extraneous material to the end of such messages. However, owing to the representation of SNMPv2 messages as ASN.1 values, such modifications cannot - consistent with goal 1 - result in unauthorized management operations.
The data integrity mechanism specified in this memo protects only against unauthorized modification of individual SNMPv2 messages. A more general data integrity service that affords protection against the threat of message stream modification is not realized by this mechanism, although limited protection against reordering, delay, and duplication of messages within a message stream are provided by other mechanisms of the protocol.