(defun smime-decrypt-region (b e keyfile &optional from)
"Decrypt S/MIME message in region between B and E with key in KEYFILE.
+Optional FROM specifies sender's mail address.
On success, replaces region with decrypted data and return non-nil.
Any details (stderr on success, stdout and stderr on error) are left
in the buffer specified by `smime-details-buffer'."