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)])
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