Support of S/MIME Signing

[theexiile1305]

Is your feature request related to a problem? Please describe.

I would like to send signed mails with a valid S/MIME certificate and for this reason I have already looked into the go-mail middleware.

Describe the solution you'd like

After consultation with @wneessen the implementation of the S/MIME feature in the core of go-mail seems to make more sense than within a go-mail middleware.

Describe alternatives you've considered

The issue wneessen/go-mail-middleware#49 can be seen as the start and reference for the discussion

Additional context

• An S/MIME message consists of several parts
• The ContentType of the message should be set to Content-Type: multipart/signed; protocol=”application/pkcs7-signature“; micalg=sha-256; boundary=”----=_Part_0_1161688972.1725478215506“ with reference to the net message and signature parts
• For example, the actual net body is Test Content and should be adopted in the first part with Content-Type and Content-Transfer-Encoding
• The subsequent part, i.e. the signature, would need the following headers, followed by the signature in base64 encoding.
  • Content-Type: application/pkcs7-signature; name="smime.p7s"; smime-type=signed-data
  • Content-Transfer-Encoding: base64
  • Content-Disposition: attachment; filename="smime.p7s"
  • Content-Description: S/MIME Cryptographic Signature

A valid S/MIME mail could look like this:

Date: Wed, 5 Sep 2024 21:30:15 +0200
Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-256; boundary="----=_Part_0_1161688972.1725478215506"
From: <mail@example.com>
To: <mail@example.com>
Subject: Test Subject
Return-Path: accounts-test@hm.edu
MIME-Version: 1.0

------=_Part_0_1161688972.1725478215506
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit

Test Content
------=_Part_0_1161688972.1725478215506
Content-Type: application/pkcs7-signature; name="smime.p7s"; smime-type=signed-data
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="smime.p7s"
Content-Description: S/MIME Cryptographic Signature

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------=_Part_0_1161688972.1725478215506--

[wneessen]

Looking forward to your contribution!