Using the Cloudflare Origin Certificate in the Google Application Engine

A relevant question was published in 2016 (

The cloudflare documentation is dated April 2019 ( It says

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but it is difficult to say whether this section has been reviewed recently.

I am not sure if the situation has changed since.

I decided to try it. Here is the screen capture of the Google App Engine interface:

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I have downloaded the certificate of origin and the private key. I have downloaded the cert. And then convert the key by running

openssl rsa -in priv_key.pem -out priv_key.rsa.pem

When I clicked on upload, I had the following error:

The certificate data is invalid. Make sure that the private key and the public certificate match

I'm pretty sure they're the same since I just downloaded them a few minutes ago.

My questions are:

1) Can GAE accept the Cloudflare Certificate of Origin?

2) Have I forgotten a step when I prepare the certificate of origin for GAE?