A few people seem to have run into NSXMLParser error 111 before, but it's not defined in the constants. This answer seems to have mistaken 111 with 11: NSXMLParserErrorDomain 111
As far as I can tell, I have no illegal characters in my final xml:
<?xml version="1.0" encoding="utf-16"?><wsse:BinarySecurityToken wsu:Id="uuid:383b6148-1c27-45ab-963b-30e14af8154e" ValueType="http://schemas.xmlsoap.org/ws/2009/11/swt-token-profile-1.0" EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd">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</wsse:BinarySecurityToken>
I've also tried changing the encoding to utf-8, but it made no difference. What causes a parser to fail with error 111? Is the parser not set up correctly, or is the XML killing it?
Turns out I was simply passing in an entirely wrong string. The XML chunk came from a larger JSON structure, which I then processed down to get the only XML part; yet when inited the parser, I used the wrong string to create the NSData. So make sure you aren't mixing variables up. I'm still not sure why error 111 isn't defined in the documentation, though.