After some research without any answer to my question, if it\'s possible and doe

Question

After some research without any answer to my question, if it's possible and does it make sense to sign an E-mail with PGP and s/mime (?), I'm hoping to get an answer here.

I know that, PGP and S/MIME fulfil the same purpose. They create the signature with the same cryptographic utilities, but manifest a different format. The question about the differences of this two tools were already asked here: How does PGP differ from S/MIME?.

Far more clients are compatible with S/MIME out of the box. Is it possible to provide a PGP and a S/MIME signature per E-Mail, so I can satisfy PGP and S/MIME users?

Explanation / Answer

You can sign with both PGP and S/MIME. Contrary to Oliver Schmidt's response, it is possible to create an external signature using PGP (you can sign any file with a PGP key, not just text - a executable (.exe) is not a good candidate for inline signatures). This is referred to as "PGP/MIME".

An external signature on a PGP file will be *.sig and S/MIME will typically be in the format of *.p7s for the PKCS#7 format. If you are using external signature it should not matter the order since you are not using both (as long as mail clients support it). Encapsulating them as suggested in another answer should technically work since the PGP user can ignore the S/MIME attachment.

The big issue on what to use will depend upon what your recipient is expecting and what their email client is capable of doing with (or what external tools they have). Some older clients may not support external PGP signature.

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