defp signature(signing_key, string_to_sign) do
        hmac_sha256(signing_key, string_to_sign)
        |> bytes_to_string
    end

    defp signing_key(secret_key, date, region) do
        hmac_sha256("AWS4#{secret_key}", date)
        |> hmac_sha256(region)
        |> hmac_sha256(@service)
        |> hmac_sha256(@aws_request)
    end

    def hmac_sha256(key, data) do
        :crypto.hmac(:sha256, key, data)
    end

    def bytes_to_string(bytes) do
        Base.encode16(bytes, case: :lower)
    end

defp policy(key, mimetype, credential, date, expiration_window \ 60) do
        %{
            expiration: now_plus(expiration_window),
            conditions: [
                %{bucket: bucket_name},
                ["starts-with", "$key", key],
                %{acl: "public-read"},
                %{success_action_status: "201"},
                ["starts-with", "$Content-Type", mimetype],
                %{"x-amz-credential": credential},
                %{"x-amz-algorithm": "AWS4-HMAC-SHA256"},
                %{"x-amz-date": date}
            ]
        }
        |> Poison.encode!
        |> Base.encode64
    end

defp credential(date) do
        credential(aws_config[:access_key_id], date)
    end

    defp credential(key, date) do
        key <> "/" <> date <> "/" <> region() <> "/" <> @service <> "/" <> @aws_request
end

makeMultiPart : UploadSignatureModel -> File -> Http.Body
makeMultiPart uploadSignature file =
    Http.multipartBody
        [ Http.stringPart "key" uploadSignature.key
        , Http.stringPart "acl" uploadSignature.acl
        , Http.stringPart "success_action_status" "201"
        , Http.stringPart "Content-Type" uploadSignature.content_type
        , Http.stringPart "X-Amz-Credential" uploadSignature.credential
        , Http.stringPart "X-Amz-Algorithm" "AWS4-HMAC-SHA256"
        , Http.stringPart "Policy" uploadSignature.policy
        , Http.stringPart "Signature" uploadSignature.signature
        , Http.stringPart "AWSAccessKeyId" uploadSignature.aws_access_key_id
        , Http.filePart "file" file
        ]