"""
Learning Tools Interoperability (LTI) module.


Resources
---------

Theoretical background and detailed specifications of LTI can be found on:

    http://www.imsglobal.org/LTI/v1p1p1/ltiIMGv1p1p1.html

This module is based on the version 1.1.1 of the LTI specifications by the
IMS Global authority. For authentication, it uses OAuth1.

When responding back to the LTI tool provider, we must issue a correct
response. Types of responses and their message payload is available at:

    Table A1.2 Interpretation of the 'CodeMajor/severity' matrix.
    http://www.imsglobal.org/gws/gwsv1p0/imsgws_wsdlBindv1p0.html

A resource to test the LTI protocol (PHP realization):

    http://www.imsglobal.org/developers/LTI/test/v1p1/lms.php

We have also begun to add support for LTI 1.2/2.0.  We will keep this
docstring in synch with what support is available.  The first LTI 2.0
feature to be supported is the REST API results service, see specification
at
http://www.imsglobal.org/lti/ltiv2p0/uml/purl.imsglobal.org/vocab/lis/v2/outcomes/Result/service.html

What is supported:
------------------

1.) Display of simple LTI in iframe or a new window.
2.) Multiple LTI components on a single page.
3.) The use of multiple LTI providers per course.
4.) Use of advanced LTI component that provides back a grade.
    A) LTI 1.1.1 XML endpoint
        a.) The LTI provider sends back a grade to a specified URL.
        b.) Currently only action "update" is supported. "Read", and "delete"
            actions initially weren't required.
    B) LTI 2.0 Result Service JSON REST endpoint
       (http://www.imsglobal.org/lti/ltiv2p0/uml/purl.imsglobal.org/vocab/lis/v2/outcomes/Result/service.html)
        a.) Discovery of all such LTI http endpoints for a course.  External tools GET from this discovery
            endpoint and receive URLs for interacting with individual grading units.
            (see lms/djangoapps/courseware/views.py:get_course_lti_endpoints)
        b.) GET, PUT and DELETE in LTI Result JSON binding
            (http://www.imsglobal.org/lti/ltiv2p0/mediatype/application/vnd/ims/lis/v2/result+json/index.html)
            for a provider to synchronize grades into edx-platform.  Reading, Setting, and Deleteing
            Numeric grades between 0 and 1 and text + basic HTML feedback comments are supported, via
            GET / PUT / DELETE HTTP methods respectively
"""

import logging
import oauthlib.oauth1
from oauthlib.oauth1.rfc5849 import signature
import hashlib
import base64
import urllib
import textwrap
import bleach
from lxml import etree
from webob import Response
import mock
from xml.sax.saxutils import escape

from xmodule.editing_module import MetadataOnlyEditingDescriptor
from xmodule.raw_module import EmptyDataRawDescriptor
from xmodule.x_module import XModule, module_attr
from xmodule.course_module import CourseDescriptor
from xmodule.lti_2_util import LTI20ModuleMixin, LTIError
from pkg_resources import resource_string
from xblock.core import String, Scope, List, XBlock
from xblock.fields import Boolean, Float

log = logging.getLogger(__name__)

# Make '_' a no-op so we can scrape strings
_ = lambda text: text

DOCS_ANCHOR_TAG = (
    "<a target='_blank'"
    "href='http://edx.readthedocs.org/projects/ca/en/latest/exercises_tools/lti_component.html'>"
    "the edX LTI documentation</a>"
)


class LTIFields(object):
    """
    Fields to define and obtain LTI tool from provider are set here,
    except credentials, which should be set in course settings::

    `lti_id` is id to connect tool with credentials in course settings. It should not contain :: (double semicolon)
    `launch_url` is launch URL of tool.
    `custom_parameters` are additional parameters to navigate to proper book and book page.

    For example, for Vitalsource provider, `launch_url` should be
    *https://bc-staging.vitalsource.com/books/book*,
    and to get to proper book and book page, you should set custom parameters as::

        vbid=put_book_id_here
        book_location=page/put_page_number_here

    Default non-empty URL for `launch_url` is needed due to oauthlib demand (URL scheme should be presented)::

    https://github.com/idan/oauthlib/blob/master/oauthlib/oauth1/rfc5849/signature.py#L136
    """
    display_name = String(
        display_name=_("Display Name"),
        help=_(
            "Enter the name that students see for this component.  "
            "Analytics reports may also use the display name to identify this component."
        ),
        scope=Scope.settings,
        default="LTI",
    )
    lti_id = String(
        display_name=_("LTI ID"),
        help=_(
            "Enter the LTI ID for the external LTI provider.  "
            "This value must be the same LTI ID that you entered in the "
            "LTI Passports setting on the Advanced Settings page."
            "<br />See " + DOCS_ANCHOR_TAG + " for more details on this setting."
        ),
        default='',
        scope=Scope.settings
    )
    launch_url = String(
        display_name=_("LTI URL"),
        help=_(
            "Enter the URL of the external tool that this component launches. "
            "This setting is only used when Hide External Tool is set to False."
            "<br />See " + DOCS_ANCHOR_TAG + " for more details on this setting."
        ),
        default='http://www.example.com',
        scope=Scope.settings)
    custom_parameters = List(
        display_name=_("Custom Parameters"),
        help=_(
            "Add the key/value pair for any custom parameters, such as the page your e-book should open to or "
            "the background color for this component."
            "<br />See " + DOCS_ANCHOR_TAG + " for more details on this setting."
        ),
        scope=Scope.settings)
    open_in_a_new_page = Boolean(
        display_name=_("Open in New Page"),
        help=_(
            "Select True if you want students to click a link that opens the LTI tool in a new window. "
            "Select False if you want the LTI content to open in an IFrame in the current page. "
            "This setting is only used when Hide External Tool is set to False.  "
        ),
        default=True,
        scope=Scope.settings
    )
    has_score = Boolean(
        display_name=_("Scored"),
        help=_(
            "Select True if this component will receive a numerical score from the external LTI system."
        ),
        default=False,
        scope=Scope.settings
    )
    weight = Float(
        display_name=_("Weight"),
        help=_(
            "Enter the number of points possible for this component.  "
            "The default value is 1.0.  "
            "This setting is only used when Scored is set to True."
        ),
        default=1.0,
        scope=Scope.settings,
        values={"min": 0},
    )
    module_score = Float(
        help=_("The score kept in the xblock KVS -- duplicate of the published score in django DB"),
        default=None,
        scope=Scope.user_state
    )
    score_comment = String(
        help=_("Comment as returned from grader, LTI2.0 spec"),
        default="",
        scope=Scope.user_state
    )
    hide_launch = Boolean(
        display_name=_("Hide External Tool"),
        help=_(
            "Select True if you want to use this component as a placeholder for syncing with an external grading  "
            "system rather than launch an external tool.  "
            "This setting hides the Launch button and any IFrames for this component."
        ),
        default=False,
        scope=Scope.settings
    )


class LTIModule(LTIFields, LTI20ModuleMixin, XModule):
    """
    Module provides LTI integration to course.

    Except usual Xmodule structure it proceeds with OAuth signing.
    How it works::

    1. Get credentials from course settings.

    2.  There is minimal set of parameters need to be signed (presented for Vitalsource)::

            user_id
            oauth_callback
            lis_outcome_service_url
            lis_result_sourcedid
            launch_presentation_return_url
            lti_message_type
            lti_version
            roles
            *+ all custom parameters*

        These parameters should be encoded and signed by *OAuth1* together with
        `launch_url` and *POST* request type.

    3. Signing proceeds with client key/secret pair obtained from course settings.
        That pair should be obtained from LTI provider and set into course settings by course author.
        After that signature and other OAuth data are generated.

        OAuth data which is generated after signing is usual::

            oauth_callback
            oauth_nonce
            oauth_consumer_key
            oauth_signature_method
            oauth_timestamp
            oauth_version


    4. All that data is passed to form and sent to LTI provider server by browser via
        autosubmit via JavaScript.

        Form example::

            <form
                action="${launch_url}"
                name="ltiLaunchForm-${element_id}"
                class="ltiLaunchForm"
                method="post"
                target="ltiLaunchFrame-${element_id}"
                encType="application/x-www-form-urlencoded"
            >
                <input name="launch_presentation_return_url" value="" />
                <input name="lis_outcome_service_url" value="" />
                <input name="lis_result_sourcedid" value="" />
                <input name="lti_message_type" value="basic-lti-launch-request" />
                <input name="lti_version" value="LTI-1p0" />
                <input name="oauth_callback" value="about:blank" />
                <input name="oauth_consumer_key" value="${oauth_consumer_key}" />
                <input name="oauth_nonce" value="${oauth_nonce}" />
                <input name="oauth_signature_method" value="HMAC-SHA1" />
                <input name="oauth_timestamp" value="${oauth_timestamp}" />
                <input name="oauth_version" value="1.0" />
                <input name="user_id" value="${user_id}" />
                <input name="role" value="student" />
                <input name="oauth_signature" value="${oauth_signature}" />

                <input name="custom_1" value="${custom_param_1_value}" />
                <input name="custom_2" value="${custom_param_2_value}" />
                <input name="custom_..." value="${custom_param_..._value}" />

                <input type="submit" value="Press to Launch" />
            </form>

    5. LTI provider has same secret key and it signs data string via *OAuth1* and compares signatures.

        If signatures are correct, LTI provider redirects iframe source to LTI tool web page,
        and LTI tool is rendered to iframe inside course.

        Otherwise error message from LTI provider is generated.
    """

    css = {'scss': [resource_string(__name__, 'css/lti/lti.scss')]}

    def get_input_fields(self):
        # LTI provides a list of default parameters that might be passed as
        # part of the POST data. These parameters should not be prefixed.
        # Likewise, The creator of an LTI link can add custom key/value parameters
        # to a launch which are to be included with the launch of the LTI link.
        # In this case, we will automatically add `custom_` prefix before this parameters.
        # See http://www.imsglobal.org/LTI/v1p1p1/ltiIMGv1p1p1.html#_Toc316828520
        PARAMETERS = [
            "lti_message_type",
            "lti_version",
            "resource_link_title",
            "resource_link_description",
            "user_image",
            "lis_person_name_given",
            "lis_person_name_family",
            "lis_person_name_full",
            "lis_person_contact_email_primary",
            "lis_person_sourcedid",
            "role_scope_mentor",
            "context_type",
            "context_title",
            "context_label",
            "launch_presentation_locale",
            "launch_presentation_document_target",
            "launch_presentation_css_url",
            "launch_presentation_width",
            "launch_presentation_height",
            "launch_presentation_return_url",
            "tool_consumer_info_product_family_code",
            "tool_consumer_info_version",
            "tool_consumer_instance_guid",
            "tool_consumer_instance_name",
            "tool_consumer_instance_description",
            "tool_consumer_instance_url",
            "tool_consumer_instance_contact_email",
        ]

        client_key, client_secret = self.get_client_key_secret()

        # parsing custom parameters to dict
        custom_parameters = {}
        for custom_parameter in self.custom_parameters:
            try:
                param_name, param_value = [p.strip() for p in custom_parameter.split('=', 1)]
            except ValueError:
                _ = self.runtime.service(self, "i18n").ugettext
                msg = _('Could not parse custom parameter: {custom_parameter}. Should be "x=y" string.').format(
                    custom_parameter="{0!r}".format(custom_parameter)
                )
                raise LTIError(msg)

            # LTI specs: 'custom_' should be prepended before each custom parameter, as pointed in link above.
            if param_name not in PARAMETERS:
                param_name = 'custom_' + param_name

            custom_parameters[unicode(param_name)] = unicode(param_value)

        return self.oauth_params(
            custom_parameters,
            client_key,
            client_secret,
        )

    def get_context(self):
        """
        Returns a context.
        """
        # use bleach defaults. see https://github.com/jsocol/bleach/blob/master/bleach/__init__.py
        # ALLOWED_TAGS are
        # ['a', 'abbr', 'acronym', 'b', 'blockquote', 'code', 'em', 'i', 'li', 'ol',  'strong', 'ul']
        #
        # ALLOWED_ATTRIBUTES are
        #     'a': ['href', 'title'],
        #     'abbr': ['title'],
        #     'acronym': ['title'],
        #
        # This lets all plaintext through.
        sanitized_comment = bleach.clean(self.score_comment)

        return {
            'input_fields': self.get_input_fields(),

            # These parameters do not participate in OAuth signing.
            'launch_url': self.launch_url.strip(),
            'element_id': self.location.html_id(),
            'element_class': self.category,
            'open_in_a_new_page': self.open_in_a_new_page,
            'display_name': self.display_name,
            'form_url': self.runtime.handler_url(self, 'preview_handler').rstrip('/?'),
            'hide_launch': self.hide_launch,
            'has_score': self.has_score,
            'weight': self.weight,
            'module_score': self.module_score,
            'comment': sanitized_comment,
        }

    def get_html(self):
        """
        Renders parameters to template.
        """
        return self.system.render_template('lti.html', self.get_context())

    @XBlock.handler
    def preview_handler(self, _, __):
        """
        This is called to get context with new oauth params to iframe.
        """
        template = self.system.render_template('lti_form.html', self.get_context())
        return Response(template, content_type='text/html')

    def get_user_id(self):
        user_id = self.runtime.anonymous_student_id
        assert user_id is not None
        return unicode(urllib.quote(user_id))

    def get_outcome_service_url(self, service_name="grade_handler"):
        """
        Return URL for storing grades.

        To test LTI on sandbox we must use http scheme.

        While testing locally and on Jenkins, mock_lti_server use http.referer
        to obtain scheme, so it is ok to have http(s) anyway.

        The scheme logic is handled in lms/lib/xblock/runtime.py
        """
        return self.runtime.handler_url(self, service_name, thirdparty=True).rstrip('/?')

    def get_resource_link_id(self):
        """
        This is an opaque unique identifier that the TC guarantees will be unique
        within the TC for every placement of the link.

        If the tool / activity is placed multiple times in the same context,
        each of those placements will be distinct.

        This value will also change if the item is exported from one system or
        context and imported into another system or context.

        This parameter is required.

        Example:  u'edx.org-i4x-2-3-lti-31de800015cf4afb973356dbe81496df'

        Hostname, edx.org,
        makes resource_link_id change on import to another system.

        Last part of location, location.name - 31de800015cf4afb973356dbe81496df,
        is random hash, updated by course_id,
        this makes resource_link_id unique inside single course.

        First part of location is tag-org-course-category, i4x-2-3-lti.

        Location.name itself does not change on import to another course,
        but org and course_id change.

        So together with org and course_id in a form of
        i4x-2-3-lti-31de800015cf4afb973356dbe81496df this part of resource_link_id:
        makes resource_link_id to be unique among courses inside same system.
        """
        return unicode(urllib.quote("{}-{}".format(self.system.hostname, self.location.html_id())))

    def get_lis_result_sourcedid(self):
        """
        This field contains an identifier that indicates the LIS Result Identifier (if any)
        associated with this launch.  This field identifies a unique row and column within the
        TC gradebook.  This field is unique for every combination of context_id / resource_link_id / user_id.
        This value may change for a particular resource_link_id / user_id  from one launch to the next.
        The TP should only retain the most recent value for this field for a particular resource_link_id / user_id.
        This field is generally optional, but is required for grading.
        """
        return "{context}:{resource_link}:{user_id}".format(
            context=urllib.quote(self.context_id),
            resource_link=self.get_resource_link_id(),
            user_id=self.get_user_id()
        )

    def get_course(self):
        """
        Return course by course id.
        """
        return self.descriptor.runtime.modulestore.get_course(self.course_id)

    @property
    def context_id(self):
        """
        Return context_id.

        context_id is an opaque identifier that uniquely identifies the context (e.g., a course)
        that contains the link being launched.
        """
        return self.course_id.to_deprecated_string()

    @property
    def role(self):
        """
        Get system user role and convert it to LTI role.
        """
        roles = {
            'student': u'Student',
            'staff': u'Administrator',
            'instructor': u'Instructor',
        }
        return roles.get(self.system.get_user_role(), u'Student')

    def oauth_params(self, custom_parameters, client_key, client_secret):
        """
        Signs request and returns signature and OAuth parameters.

        `custom_paramters` is dict of parsed `custom_parameter` field
        `client_key` and `client_secret` are LTI tool credentials.

        Also *anonymous student id* is passed to template and therefore to LTI provider.
        """

        client = oauthlib.oauth1.Client(
            client_key=unicode(client_key),
            client_secret=unicode(client_secret)
        )

        # Must have parameters for correct signing from LTI:
        body = {
            u'user_id': self.get_user_id(),
            u'oauth_callback': u'about:blank',
            u'launch_presentation_return_url': '',
            u'lti_message_type': u'basic-lti-launch-request',
            u'lti_version': 'LTI-1p0',
            u'roles': self.role,

            # Parameters required for grading:
            u'resource_link_id': self.get_resource_link_id(),
            u'lis_result_sourcedid': self.get_lis_result_sourcedid(),

            u'context_id': self.context_id,
        }

        if self.has_score:
            body.update({
                u'lis_outcome_service_url': self.get_outcome_service_url()
            })

        # Appending custom parameter for signing.
        body.update(custom_parameters)

        headers = {
            # This is needed for body encoding:
            'Content-Type': 'application/x-www-form-urlencoded',
        }

        try:
            __, headers, __ = client.sign(
                unicode(self.launch_url.strip()),
                http_method=u'POST',
                body=body,
                headers=headers)
        except ValueError:  # Scheme not in url.
            # https://github.com/idan/oauthlib/blob/master/oauthlib/oauth1/rfc5849/signature.py#L136
            # Stubbing headers for now:
            headers = {
                u'Content-Type': u'application/x-www-form-urlencoded',
                u'Authorization': u'OAuth oauth_nonce="80966668944732164491378916897", \
oauth_timestamp="1378916897", oauth_version="1.0", oauth_signature_method="HMAC-SHA1", \
oauth_consumer_key="", oauth_signature="frVp4JuvT1mVXlxktiAUjQ7%2F1cw%3D"'}

        params = headers['Authorization']
        # Parse headers to pass to template as part of context:
        params = dict([param.strip().replace('"', '').split('=') for param in params.split(',')])

        params[u'oauth_nonce'] = params[u'OAuth oauth_nonce']
        del params[u'OAuth oauth_nonce']

        # oauthlib encodes signature with
        # 'Content-Type': 'application/x-www-form-urlencoded'
        # so '='' becomes '%3D'.
        # We send form via browser, so browser will encode it again,
        # So we need to decode signature back:
        params[u'oauth_signature'] = urllib.unquote(params[u'oauth_signature']).decode('utf8')

        # Add LTI parameters to OAuth parameters for sending in form.
        params.update(body)
        return params

    def max_score(self):
        return self.weight if self.has_score else None

    @XBlock.handler
    def grade_handler(self, request, suffix):  # pylint: disable=unused-argument
        """
        This is called by courseware.module_render, to handle an AJAX call.

        Used only for grading. Returns XML response.

        Example of request body from LTI provider::

        <?xml version = "1.0" encoding = "UTF-8"?>
            <imsx_POXEnvelopeRequest xmlns = "some_link (may be not required)">
              <imsx_POXHeader>
                <imsx_POXRequestHeaderInfo>
                  <imsx_version>V1.0</imsx_version>
                  <imsx_messageIdentifier>528243ba5241b</imsx_messageIdentifier>
                </imsx_POXRequestHeaderInfo>
              </imsx_POXHeader>
              <imsx_POXBody>
                <replaceResultRequest>
                  <resultRecord>
                    <sourcedGUID>
                      <sourcedId>feb-123-456-2929::28883</sourcedId>
                    </sourcedGUID>
                    <result>
                      <resultScore>
                        <language>en-us</language>
                        <textString>0.4</textString>
                      </resultScore>
                    </result>
                  </resultRecord>
                </replaceResultRequest>
              </imsx_POXBody>
            </imsx_POXEnvelopeRequest>

        Example of correct/incorrect answer XML body:: see response_xml_template.
        """
        response_xml_template = textwrap.dedent("""\
            <?xml version="1.0" encoding="UTF-8"?>
            <imsx_POXEnvelopeResponse xmlns = "http://www.imsglobal.org/services/ltiv1p1/xsd/imsoms_v1p0">
                <imsx_POXHeader>
                    <imsx_POXResponseHeaderInfo>
                        <imsx_version>V1.0</imsx_version>
                        <imsx_messageIdentifier>{imsx_messageIdentifier}</imsx_messageIdentifier>
                        <imsx_statusInfo>
                            <imsx_codeMajor>{imsx_codeMajor}</imsx_codeMajor>
                            <imsx_severity>status</imsx_severity>
                            <imsx_description>{imsx_description}</imsx_description>
                            <imsx_messageRefIdentifier>
                            </imsx_messageRefIdentifier>
                        </imsx_statusInfo>
                    </imsx_POXResponseHeaderInfo>
                </imsx_POXHeader>
                <imsx_POXBody>{response}</imsx_POXBody>
            </imsx_POXEnvelopeResponse>
        """)
        # Returns when `action` is unsupported.
        # Supported actions:
        #   - replaceResultRequest.
        unsupported_values = {
            'imsx_codeMajor': 'unsupported',
            'imsx_description': 'Target does not support the requested operation.',
            'imsx_messageIdentifier': 'unknown',
            'response': ''
        }
        # Returns if:
        #   - score is out of range;
        #   - can't parse response from TP;
        #   - can't verify OAuth signing or OAuth signing is incorrect.
        failure_values = {
            'imsx_codeMajor': 'failure',
            'imsx_description': 'The request has failed.',
            'imsx_messageIdentifier': 'unknown',
            'response': ''
        }

        try:
            imsx_messageIdentifier, sourcedId, score, action = self.parse_grade_xml_body(request.body)
        except Exception as e:
            error_message = "Request body XML parsing error: " + escape(e.message)
            log.debug("[LTI]: " + error_message)
            failure_values['imsx_description'] = error_message
            return Response(response_xml_template.format(**failure_values), content_type="application/xml")

        # Verify OAuth signing.
        try:
            self.verify_oauth_body_sign(request)
        except (ValueError, LTIError) as e:
            failure_values['imsx_messageIdentifier'] = escape(imsx_messageIdentifier)
            error_message = "OAuth verification error: " + escape(e.message)
            failure_values['imsx_description'] = error_message
            log.debug("[LTI]: " + error_message)
            return Response(response_xml_template.format(**failure_values), content_type="application/xml")

        real_user = self.system.get_real_user(urllib.unquote(sourcedId.split(':')[-1]))
        if not real_user:  # that means we can't save to database, as we do not have real user id.
            failure_values['imsx_messageIdentifier'] = escape(imsx_messageIdentifier)
            failure_values['imsx_description'] = "User not found."
            return Response(response_xml_template.format(**failure_values), content_type="application/xml")

        if action == 'replaceResultRequest':
            self.set_user_module_score(real_user, score, self.max_score())

            values = {
                'imsx_codeMajor': 'success',
                'imsx_description': 'Score for {sourced_id} is now {score}'.format(sourced_id=sourcedId, score=score),
                'imsx_messageIdentifier': escape(imsx_messageIdentifier),
                'response': '<replaceResultResponse/>'
            }
            log.debug("[LTI]: Grade is saved.")
            return Response(response_xml_template.format(**values), content_type="application/xml")

        unsupported_values['imsx_messageIdentifier'] = escape(imsx_messageIdentifier)
        log.debug("[LTI]: Incorrect action.")
        return Response(response_xml_template.format(**unsupported_values), content_type='application/xml')


    @classmethod
    def parse_grade_xml_body(cls, body):
        """
        Parses XML from request.body and returns parsed data

        XML body should contain nsmap with namespace, that is specified in LTI specs.

        Returns tuple: imsx_messageIdentifier, sourcedId, score, action

        Raises Exception if can't parse.
        """
        lti_spec_namespace = "http://www.imsglobal.org/services/ltiv1p1/xsd/imsoms_v1p0"
        namespaces = {'def': lti_spec_namespace}

        data = body.strip().encode('utf-8')
        parser = etree.XMLParser(ns_clean=True, recover=True, encoding='utf-8')
        root = etree.fromstring(data, parser=parser)

        imsx_messageIdentifier = root.xpath("//def:imsx_messageIdentifier", namespaces=namespaces)[0].text
        sourcedId = root.xpath("//def:sourcedId", namespaces=namespaces)[0].text
        score = root.xpath("//def:textString", namespaces=namespaces)[0].text
        action = root.xpath("//def:imsx_POXBody", namespaces=namespaces)[0].getchildren()[0].tag.replace('{'+lti_spec_namespace+'}', '')
        # Raise exception if score is not float or not in range 0.0-1.0 regarding spec.
        score = float(score)
        if not 0 <= score <= 1:
            raise LTIError('score value outside the permitted range of 0-1.')

        return imsx_messageIdentifier, sourcedId, score, action

    def verify_oauth_body_sign(self, request, content_type='application/x-www-form-urlencoded'):
        """
        Verify grade request from LTI provider using OAuth body signing.

        Uses http://oauth.googlecode.com/svn/spec/ext/body_hash/1.0/oauth-bodyhash.html::

            This specification extends the OAuth signature to include integrity checks on HTTP request bodies
            with content types other than application/x-www-form-urlencoded.

        Arguments:
            request: DjangoWebobRequest.

        Raises:
            LTIError if request is incorrect.
        """

        client_key, client_secret = self.get_client_key_secret()
        headers = {
            'Authorization': unicode(request.headers.get('Authorization')),
            'Content-Type': content_type,
        }

        sha1 = hashlib.sha1()
        sha1.update(request.body)
        oauth_body_hash = base64.b64encode(sha1.digest())
        oauth_params = signature.collect_parameters(headers=headers, exclude_oauth_signature=False)
        oauth_headers = dict(oauth_params)
        oauth_signature = oauth_headers.pop('oauth_signature')
        mock_request = mock.Mock(
            uri=unicode(urllib.unquote(request.url)),
            http_method=unicode(request.method),
            params=oauth_headers.items(),
            signature=oauth_signature
        )

        if oauth_body_hash != oauth_headers.get('oauth_body_hash'):
            raise LTIError("OAuth body hash verification is failed.")

        if not signature.verify_hmac_sha1(mock_request, client_secret):
            raise LTIError("OAuth signature verification is failed.")

    def get_client_key_secret(self):
        """
        Obtains client_key and client_secret credentials from current course.
        """
        course = self.get_course()
        for lti_passport in course.lti_passports:
            try:
                lti_id, key, secret = [i.strip() for i in lti_passport.split(':')]
            except ValueError:
                _ = self.runtime.service(self, "i18n").ugettext
                msg = _('Could not parse LTI passport: {lti_passport}. Should be "id:key:secret" string.').format(
                    lti_passport='{0!r}'.format(lti_passport)
                )
                raise LTIError(msg)

            if lti_id == self.lti_id.strip():
                return key, secret
        return '', ''

class LTIDescriptor(LTIFields, MetadataOnlyEditingDescriptor, EmptyDataRawDescriptor):
    """
    Descriptor for LTI Xmodule.
    """
    module_class = LTIModule
    grade_handler = module_attr('grade_handler')
    preview_handler = module_attr('preview_handler')
    lti_2_0_result_rest_handler = module_attr('lti_2_0_result_rest_handler')
    clear_user_module_score = module_attr('clear_user_module_score')
    get_outcome_service_url = module_attr('get_outcome_service_url')
