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Problem creating java Tool Consumer

Problem creating java Tool Consumer

In the past, I've successfully created a java LTI Tool Provider (using Blackboard's BLTI-Sandwich) which has been working nicely for a few years.   
 
Now I'm trying to create a java-based LTI Tool Consumer. 
I've tried both the BLTI-Sandwich and the imsglobal libraries, but both throw the same 'signature_invalid' exception on the TP. 
 
Hopefully, I'm missing something simple. 
 
 // Tool Consumer code snippet - imsglobal version
 Map<String,String> parameters = new TreeMap<String,String>();
 parameters.put(BasicLTIConstants.RESOURCE_LINK_ID, "12345678");
 parameters.put(BasicLTIConstants.USER_ID,userid);
 parameters.put(BasicLTIConstants.LIS_PERSON_SOURCEDID,userid);
 parameters.put(BasicLTIConstants.CONTEXT_ID,"S3294476");
 parameters.put(BasicLTIConstants.LTI_VERSION,BasicLTIConstants.LTI_VERSION_1);
 parameters.put(BasicLTIConstants.LTI_MESSAGE_TYPE,BasicLTIConstants.LTI_MESSAGE_TYPE_BASICLTILAUNCHREQUEST);
 // ..  just showing required. remainder snipped for brevity
 
org.imsglobal.lti.launch.LtiSigner ltiSigner = new org.imsglobal.lti.launch.LtiOauthSigner();
 
try {
    Map<String, String> signedParameters = ltiSigner.signParameters(parameters, "ConsumerKey", mysecret, launchsite, "POST");
    String output = BasicLTIUtil.postLaunchHTML(signedParameters,launchsite,false);
PrintWriter out = response.getWriter();
out.println(output);
}
catch (Exception e) { e.printStackTrace();
 
This is based on sample TC code snippet from: https://github.com/IMSGlobal/basiclti-util-java, which may not tell the whole story...  
 
The error on the TP webserver is:
net.oauth.OAuthProblemException: signature_invalid
        at net.oauth.signature.OAuthSignatureMethod.validate(OAuthSignatureMethod.java:65)
        at net.oauth.SimpleOAuthValidator.validateSignature(SimpleOAuthValidator.java:103)
        at net.oauth.SimpleOAuthValidator.validateMessage(SimpleOAuthValidator.java:68)
        at blackboard.blti.provider.BLTIProvider.isValid(BLTIProvider.java:137)
        at ca.ubc.med.auth.LTIAuthenticator.authenticate(LTIAuthenticator.java:53)
...
Any guidance would be appreciated, including where else to ask this question.
Thanks, Bob