I am attempting to connect to an external application using Execute Anonymous in the Developer Console.
The application is using 2-Legged oAuth so I am encoding the signature in RSA-SHA1. Despite this, I seem to keep getting the result "Invalid Signature". Can anyone tell me if I am missing essential formatting in my code below for using oAuth?
Any help is greatly appreciated. I have also modified the Consumer Key and Private Key from thier true values, but maintained thier Size and special characters for a reference.

Http h = new Http();
String consumer_key='A1BF9UW7HS67STUOW8HUMKWODSJUOS';
                    
//Encoded PKCS8 Private Key
//Composed of Private Key Generated by OpenSSL
String secret_key='MIICXAIBAAKBgQDF9IiOjP0ZQcqGQ+Ut8K4Ug4vWe7' +
    'M6TRF+ruvVv1CtMOfZVV3kvtJPaNAD/bBs8W5wVFD+' +
    'tg+YhKL2nxkEKssdDNuyIOUa9OOQPbEBIjM7IJ8bRh' +
    '3kycneA0GPuNQIB3n5leCeCZtg56OR2WC8udc2FklQ' +
    'Jpvm0GWy+mvXC+DaewIDAQABAoGAce4+lgTROrMh88' +
    '0JCVugy1G/3UYfyxLeb26hVFPASmE635DRrLgbrjFL' +
    'qp4ZnvJwFa/1PxUWvso/3L9Or5ZNlmerHkdOdQhS3N' +
    'aUxDlB0RsCV7kwRVhaJbwsBKL07bYHqHCB3BE97xoN' +
    'Ak4ptsfEXYALeRnegSfpeq72fZGnYnECQQDrjPrfjH' +
    '9n5C60AfYgotQW6F52kHB6posgzltuTWh8UMaZUVD+' +
    'Td+mcDYIIlJnmG1qyL+v3Eu+CbBLDhY8HuXqNAbQCQ' +
    'RQkwEl1wGbrnyLeaKNH9t1yYmYwm93K+Vu9jAkEA1y' +
    'QCNs1A+IzbiUacbq1nzrstSzOdBCnLOeWBZRByqeYa' +
    'r6awJBANN5TcPYlli6/ME/a11Pjo6jZq5ZGaqR+nX6' +
    'JAeFJikDDPn/0yUicyV8d1OWJnZn8vxacvKIngsH6A' +
    'IgiM+iJMkiYnpFOtQScTj3S7PNcfNVAlz5mXuwvppA' +
    'pupU9KEU0h5iDRSXx/8UyUgHpcOWyKLewPp9FAnQyW' +
    'AF1+U0HSECQBVzu8CVaK7keNdOhHSgIsvCEdjzzA1P' +
    'd4uHbwGLObtCcxz2nwymQD4GZ8UMDZWLnX3/V2JbvQ' +
    'rlyKWHiMC6r5M=';

Long tmp=(System.now().getTime()/1000);

String encrypt_string = 'GET&https%3A%2F%2Fapi.xero.com%2Fapi.xro%2F2.0%2FContacts' +
                        '&oauth_consumer_key%3D' + consumer_key +
                        '%26oauth_nonce%3Dfef295f8188b8686fc922a66ec4fed2b' +
                        '%26oauth_signature_method%3DRSA-SHA1' +
                        '%26oauth_timestamp%3D' + tmp +
                        '%26oauth_token%3D' + consumer_key +
                        '%26oauth_version%3D1.0';


String algorithmName = 'RSA-SHA1';
String key =  secret_key;

// Decode the key from base64 to binary
Blob privateKey = EncodingUtil.base64Decode(key);
Blob input = Blob.valueOf(consumer_key);
Blob signature = Crypto.sign(algorithmName, input, privateKey);

String signatureA = EncodingUtil.base64Encode(signature);
String signatureB = EncodingUtil.urlEncode(signatureA, 'UTF-8');
System.debug('~~~ '+ signatureA);

// Try to get access token
HttpRequest req = new HttpRequest();
req.setEndpoint('https://api.xero.com/api.xro/2.0/Contacts' +
                        '?oauth_consumer_key=' + consumer_key +
                        '&oauth_nonce=fef295f8188b8686fc922a66ec4fed2b' +
                        '&oauth_signature=' + signatureB +
                        '&oauth_signature_method=RSA-SHA1' + 
                        '&oauth_timestamp=' + tmp +
                        '&oauth_token=' + consumer_key +               
                        '&oauth_version=1.0');
req.setMethod('GET');

// Send the request, and return a response
HttpResponse res = h.send(req);
System.debug('~~~ '+res.getBody());