From what I understood, was that having a S/MIME certificate allow me to sign emails, and any recipient of my signed email would be able to encrypt emails send to me.

Is that correct? or does the recipient of my signed email also needs their own S/MIME certificate in order to encrypt emails to me?

Thanks in advance

  • It seems that, the second party (recipient) also needs its own certificate in order to encrypt the reply. Apple Mail, iPhone and Thunderbird throwing an error and asking for setting up a personal certificate for the recipients email address. Thunderbird error: "You need to set up one or more personal certificates before you can use this security feature", iPhone error: "You can't send encrypted messages because an encryption identity for (recipient@mail) could not be found". In my opinion both parties needs S/MIME certificates in order to communicate securely. Is that correct?
    – igigi
    Jan 30 '19 at 14:18

While it is technically correct that you can encrypt content for a recipient key that is contained in an S/MIME certificate without having a certificate of your own, this is not the same thing as sending an S/MIME email.

While the RFC allows for Enveloped only S/MIMES it does state:

It is important to note that sending enveloped but not signed messages does not provide for data integrity. It is possible to replace ciphertext in such a way that the processed message will still be valid, but the meaning can be altered.

In production mail clients, S/MIME messages are hence always signed - whether or not they are encrypted. While you do not need a key pair and certificate for the encryption part of S/MIME, you do need one for signing that encrypted message.

In my opinion both parties needs S/MIME certificates in order to communicate securely. Is that correct?

Yes, and for good reason: S/MIME is not only about confidentiality but also about authenticity; when an E-Mail is not signed properly, authenticity is out of the window.

  • Strictly spoken, an encrypted, but not signed email is still valid and true S/MIME. It's just not the recommended implementation for the reasons you list.
    – not2savvy
    Jan 30 '19 at 17:00
  • I do acknowledge that the rfc does allow that;)
    – Tobi Nary
    Jan 30 '19 at 17:01
  • Thank you very much for the clarification. Best regrads
    – igigi
    Jan 30 '19 at 20:51

A signed email should contain your public certificate to allow the recipient to control that the message has not be tampered. The public key included in the certificate is enough to encrypt a message that only you could decode with the private key.

That being said, a certificate also includes usages for the key. If the certificate declares to be only valid for signing, a strict application could refuse to use it for encryption. An advanced user could probably circumvent that limitation, but casual users could not.

In addition, not all mail readers allow encryption. If the recipient uses one that does not, they will not be able to encrypt the response.

The only thing I can confirm is that a user certificate will not be involved to encrypt the message: it is only used for signing.

  • Thank yo for your extensive answer. The certificate in question is from instantssl. The information on their webpage states "Digitally sign and encrypt to authenticate and secure emails". I can sign mail (Apple Mail) The email arrives signed and verified, but the recipient has no option to encrypt the reply. Therefore, I was wondering if the second party also needs an certificate
    – igigi
    Jan 30 '19 at 13:32
  • Usage is "Digital Signature, Key Encipherment". Then why would the recipient not have the option to encrypt the email reply? Is there anything else one should consider?
    – igigi
    Jan 30 '19 at 13:51
  • ...because the recipient when trying to encrypt the mail from iphone is getting an error "You can't send encrypted messages because an encryption identity for (recipient email adress) could not be found" Seem like the second party would need their own certificate, maybe not as you stated for encryption but for signing the encrypted message. Is that correct?
    – igigi
    Jan 30 '19 at 13:59

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