What format of certificate encoding is incompatible with ChromeOS devices?

ChromeOS natively supports importing certificates in text-based formats like PEM (.pem, .crt, .cer) and the PKCS#12 (.p12, .pfx) format, which bundles a certificate with its private key. The Distinguished Encoding Rules (DER) format, which is a binary encoding for certificates, is not listed as a directly supported format for import through the Google Admin console or the ChromeOS user interface for network or certificate authority configurations. Administrators must typically convert DER-encoded certificates to a supported format like PEM before deployment.

A. PEM (Privacy-Enhanced Mail) is a text-based format and is explicitly listed as a supported certificate format for ChromeOS.
B. CER is a common file extension for certificates, which ChromeOS supports, typically when they are encoded in the PEM format.
D. CRT is another common file extension for certificates, which ChromeOS supports, typically when they are encoded in the PEM format.

1. Google Chrome Enterprise Help. (n.d.). Manage client certificates on Chrome devices. Google Support. Retrieved from https://support.google.com/chrome/a/answer/9645280?hl=en. (This article states, "The supported format is PKCS #12 (.p12 or .pfx)... or PEM (.pem, .crt, or .cer) which contains only the certificate.")

2. Google Chrome Enterprise Help. (n.d.). Set up TLS (or SSL) inspection on ChromeOS devices. Google Support. Retrieved from https://support.google.com/chrome/a/answer/6342300?hl=en. (Under "Step 1: Set up a hostname," the instructions for adding a certificate authority specify using a PEM, CRT, or CER file.)