Can't use phpmailer to sign with S/MIME, getting "signing error: "

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PHP

OpenSSL

I'm using phpmailer to try to send signed emails (S/MIME).

I used this excellent tutorial by John Dalesandro to create a Self-Signed S/MIME Certificate and package it in PKCS12 Format.

I then followed the instructions in PHPMailer S/MIME signing to make cert.crt, cert.key & certchain.pem from my PKCS12 package.

When I sign with the code below I get "Signing Error:" :

$mail->sign(
  '/mypath/cert.crt', 
  '/mypath/cert.key', 
  'mypassword',
  '/mypath/certchain.pem'
);

The emails are sent normally when I don't try to sign the email.

Since there is no explanation at all in my error code, I don't understand what I'm doing wrong.

What I have tried:

<pre>$mail->sign(
  '/mypath/cert.crt', 
  '/mypath/cert.key', 
  'mypassword',
  '/mypath/certchain.pem'
);