A variety of error cases may occur and be detected in the course of processing a received PEM message. The specific actions to be taken in response to such conditions are local matters, varying as functions of user preferences and the type of user interface provided by a particular PEM implementation, but certain general recommendations are appropriate. Syntactically invalid PEM messages should be flagged as such, preferably with collection of diagnostic information to support debugging of incompatibilities or other failures. RFC 1422 defines specific error processing requirements relevant to the certificate-based key management mechanisms defined therein.
Syntactically valid PEM messages which yield MIC failures raise special concern, as they may result from attempted attacks or forged messages. As such, it is unsuitable to display their contents to recipient users without first indicating the fact that the contents' authenticity and integrity cannot be guaranteed and then receiving positive user confirmation of such a warning. MIC-CLEAR messages (discussed in Section 4.6.1.1.3 of this RFC) raise special concerns, as MIC failures on such messages may occur for a broader range of benign causes than are applicable to other PEM message types.