How to use Basic128 (encryption/encoding) in my signature?

I am calling soap API's with X.509 certificate.

And receiving an error that i have to use Basic128 in the signature.

Here is my Signature:

def the_signature
        raise MissingCertificate, "Expected a private_key for signing" unless certs.private_key
        signed_info = at_xpath(@document, "//Envelope/Header/Security/Signature/SignedInfo")
        signed_info = signed_info ? canonicalize(signed_info) : ""
        signature = certs.private_key.sign(OpenSSL::Digest::SHA1.new, signed_info)
        Base64.encode64(signature).gsub("\n", '') 
end

How can i use Basic128 to solve the above problem?