Looking at the Ruby code for rpc_client.rb shows the signature being calculated using the secret key and the encoded parameter string. The signature parameter is added after the calculation.

  normalized     = normalize(params)
  canonicalized  = canonicalize(normalized)
  string_to_sign = "#{method}&#{encode('/')}&#{encode(canonicalized)}"
  key            = self.access_key_secret + '&'
  signature      = Base64.encode64(OpenSSL::HMAC.digest('sha1', key, s
+tring_to_sign)).strip
  normalized.push(['Signature', encode(signature)])
[download]

Does that sound to you as if the other side is expecting a random string?

Consider actually reading the documentation of the API instead of trying to send random data.