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On some HP printers, when I try to import a certificate to enable HTTPS for the printer's Embedded Web Server (EWS), I get the error message "The format of the file is invalid". This happens both (1) when I have created the CSR using openssl and (2) when I use the "Create a Certificate Request" option in EWS.
Updating to the latest firmware does not help.
Example model: HP Color LaserJet MFP M477fnw
This failure occurs when the certificate file includes the full certificate chain to the root certificate. Apparently, the affected printers cannot process a chain of root and intermediate certificates.
When you submit your certificate signing request (CSR) to your certificate authority (CA), whether that be internal PKI or a commercial CA, request that it omit the certificate chain from the certificate. Then the printer will be able to successfully import it.
I suspect that it might be possible to remove the certificate chain using a tool like openssl (so you wouldn't have to make a special request to your CA), but that is beyond my expertise.
