External SSO unable to get signature match

SSO
1

Hi All,

We are trying to get discourse working with an external SSO endpoint but can’t seem to get the signature to match.

The logs show:

[2020-12-23 21:10:50 +0000] [7] [DEBUG] 2d9c3e07e2e7551e51af471ca3b50e83d06411b2c75e443f96def5b6a257d141
[2020-12-23 21:10:50 +0000] [7] [DEBUG] ad02fdc5bd85adce7722bc264352cca9b931392735ed72474966c4461f188b8c
[2020-12-23 21:10:50 +0000] [7] [DEBUG] compare_digest is different

What are we doing wrong? Does the hmac preparation look okay?

endpoint:
@app.route(/discourse)
def discourse():
    try:
        cookie = request.cookies['__session']
    except KeyError:
        logging.error("Session cookie was not present")
        return redirect(login_url)
    
    try:
        decoded_token = verify_session_cookie(cookie, check_revoked=True)
    except ValueError:
        return {"message": "Session cookie was present but not a valid string or empty"}, 400
    except ExpiredSessionCookieError:
        return {"message": "Session cookie was present but expired."}, 400
    
    try:
        uid = decoded_token['uid']
    except KeyError:
        return {"message": "The user id could not be read from the decoded token"}, 500
    
    try:
        payload = request.args['sso']
    except KeyError:
        return {"message": "Payload parameter was not present"}, 400
    
    try:
        sig = request.args['sig']
    except KeyError:
        return {"message": "Sig parameter was not present"}, 400
    
    try:
        nonce = validate_sso_and_generate_nonce(payload, sig, sso_secret)
    except DiscourseError as e:
        current_app.logger.error("sso validation failed: " + str(e))
        return redirect(login_url)
    
    user = get_user(uid)
    sso_url = generate_sso_redirect_url(forum_url, nonce, sso_secret, user.email, uid, user.display_name, user.display_name)
    current_app.logger.debug("Discourse endpoint complete")
    return redirect(sso_url)

Where we generate the signature:

def validate_sso_and_generate_nonce(payload, signature, secret):
    """
        payload: provided by Discourse HTTP call to your SSO endpoint as sso GET param
        signature: provided by Discourse HTTP call to your SSO endpoint as sig GET param
        secret: the secret key you entered into Discourse sso secret
        return value: The nonce used by discourse to validate the redirect URL
    """
    if None in [payload, signature]:
        raise DiscourseError('No SSO payload or signature.')

if not secret:
    raise DiscourseError('Invalid secret..')

if not payload:
    raise DiscourseError('Invalid payload..')

payload = bytes(parse.unquote(payload), encoding='utf-8')
decoded = base64.decodestring(payload).decode('utf-8')
if len(payload) == 0 or 'nonce' not in decoded:
    raise DiscourseError('Invalid payload..')

h = hmac.new(base64.b64encode(bytes(secret, encoding='utf-8')), payload, digestmod=hashlib.sha256)
this_signature = h.hexdigest()

current_app.logger.debug(this_signature)
current_app.logger.debug(signature)

#if this_signature != signature:
#    raise DiscourseError('Payload does not match signature.')

if not hmac.compare_digest(this_signature, signature):
    current_app.logger.debug("compare_digest is different")

nonce = decoded.split('=')[1]

return nonce
2 Likes
2

Have you been able to get this to work? If not, you might be able to see where things are going wrong by having a look at this code:

https://github.com/bennylope/pydiscourse/blob/master/pydiscourse/sso.py#L39

2 Likes
3

Whilst there are some differences in the preparation of the payload for the HMAC function this code fails with the same issue that the signatures don’t match.