Quickbooks IPP: oAuth Signature Invalid

Question

I am trying to connect to QuickBooks Online, but when I try to request a token from https://oauth.intuit.com/oauth/v1/get_request_token I am getting a signaure_invalid message. My signature matches what what googles signature generator makes: http://oauth.googlecode.com/svn/code/javascript/example/signature.html

So I am really at a loss for what QuickBooks is expecting and I am not providing. I have tried including the oauth_callback in the signature, but this did not make a difference. Does anyone have some pointers for where are I am wrong?

Here are my test app details:

ConsumerKey: qyprd46Is0FZ3v1tuE4unkw3iq6cUB 

oauth_timestamp: 1398355877 
oauth_nonce: X3e3aflZMeKPDwMI 
oauth_callback: /qbGetOAuthToken 
oauth_signature_method: HMAC-SHA1 

Signature Base String: POST&https%3A%2F%2Foauth.intuit.com%2Foauth%2Fv1%2Fget_request_token&oauth_consumer_key%3Dqyprd46Is0FZ3v1tuE4unkw3iq6cUB%26oauth_nonce%3DX3e3aflZMeKPDwMI%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1398355877%26oauth_version%3D1.0

Authorization Header: Authorization: OAuth oauth_signature_method="HMAC-SHA1",oauth_nonce="X3e3aflZMeKPDwMI",oauth_timestamp="1398355877",oauth_consumer_key="qyprd46Is0FZ3v1tuE4unkw3iq6cUB",oauth_version="1.0",oauth_signature="SJNPyQU7yuhcVvoLRUgyzw9KTlA%3D",oauth_callback="%2FqbGetOAuthToken"

Answer 1

For everyone's benefit- The developer resolved this issue by twice encoding the callback URL. How properly create oauth signature base string?