It seems that Safari is not handling correctly some special characters when using mod_lti (external tool).
The problem can easily be reproduce by adding an external tool with a title containing the character: è (which is not è).
When checking the data sent in the POST, I can see è instead of è resulting in a "invalid signature" error.
The problem only appears in Safari, the external tool is working correctly with other browsers.
So, Safari bug ? Or mod_lti bug ?
PS: I tried to add this problem in the issue tracker but I'm not allowed to.