Hi all
My email provider offers pgp support on a roundcube server. Therefore you can upload your public and/or private key.

Am I right to think it isn't secure at all to upload a private key, even if it's the only way to decrypt received (and encrypted messages) ?

Yes.

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Ser Olmy has correctly answered the question.

But I cannot but tremble at the very thought of people seeing uploading a private key as an option. Where you write “the only way to decrypt received and encrypted messages” – and as there may be people reading this thread – I have to add that a service which necessitates that a private key be “uploaded” has disqualified itself as being a “a service” in the first place.

This should not exist and It should not be necessary to ask the question.

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I believe that the correct option would be to upload your public key only. This will allow anyone to send an encrypted message to you which only you could then decrypt.

You would need to upload a private key only if you intended to use their "web-mail screen" to review your incoming mail. Ordinarily, I do not do this – I use a dedicated email client which can gather the mail from every e-mail account that I have and present it all to me at once. This email client has a PGP plug-in to handle encrypted traffic, referencing a private key that is only stored on my machine.

I am also rather surprised that this service does not know about "PGP key servers" and does not use them. I think of this as a deficiency.

The status of the key server network (and all kinds of key server networks) would interest me. In view of recent exchanges I must assume, that we lack reliable services. Unfortunately I do not understand much about the new exigences, nor the deficiencies of the old key servers. New ones appear to not meet the requirements. That is all I know... the rest is technical gibberish.

GnuPG has changed so much in recent years that what I use nowadays is a mostly unknown software to me.

I was quite surprised to see the offer of uploading a private key. This make sens when you rely on the webmail to decrypt. What would worry me is that I have not seen anything that may be an additional layer of protecting a private key (no export, some encryption of the key itself or whatever).
While the server is vulnerable (aren't we all ?), it looks quite stupid to put a private key at such risk.

If you intend to use a web page to access your private, encrypted mail, then it is of course necessary that you must upload your private key to the service. (As well, perhaps, as the public key.) This would be the only reasonable – but, necessary – reason for you to upload a private key.

Yes, it is certainly possible for a private key to be encrypted with a passphrase, and in this scenario you would want to do that. I am not familiar enough with this mail-service to know whether they support this. (Of course they should ...)

"Key servers" are an accepted source of various users' public keys. If you want to send a private message to someone, you interrogate the key-server system to find the necessary public key and add it to your keychain. PGP®/GPG-aware mail client software will often simply do this for you, behind the scenes.

A properly designed crypto-aware email service should not allow you to upload a private key that isn't passphrase-protected. It should prompt you each time for the passphrase, and "on their honor" never keep it. I don't know if the designers of this service did that.

A private key (or even a public one ...) should never be anywhere "public" ... e.g., on a laptop in any airport anywhere, where it might be stolen ... unless it is passphrase-encrypted. (This is especially important in the case of VPN ... which should n-e-v-e-r(!) rely on "mere passwords = PSKs.")