After the KRB_AP_REQ/KRB_AP_REP exchange has occurred, the client and server share an encryption key which can be used by the application. The "true session key" to be used for KRB_PRIV, KRB_SAFE, or other application-specific uses may be chosen by the application based on the subkeys in the KRB_AP_REP message and the authenticator (Implementations of the protocol may wish to provide routines to choose subkeys based on session keys and random numbers and to orchestrate a negotiated key to be returned in the KRB_AP_REP message.). In some cases, the use of this session key will be implicit in the protocol; in others the method of use must be chosen from a several alternatives. We leave the protocol negotiations of how to use the key (e.g., selecting an encryption or checksum type) to the application programmer; the Kerberos protocol does not constrain the implementation options.
With both the one-way and mutual authentication exchanges, the peers should take care not to send sensitive information to each other without proper assurances. In particular, applications that require privacy or integrity should use the KRB_AP_REP or KRB_ERROR responses from the server to client to assure both client and server of their peer's identity. If an application protocol requires privacy of its messages, it can use the KRB_PRIV message (section 3.5). The KRB_SAFE message (section 3.4) can be used to assure integrity.