# File lib/fog/aws/storage.rb, line 690
          def sign_chunk(data, previous_signature)
            string_to_sign = "AWS4-HMAC-SHA256-PAYLOAD\n\#{date.to_iso8601_basic}\n\#{signer.credential_scope(date)}\n\#{previous_signature}\n\#{OpenSSL::Digest::SHA256.hexdigest('')}\n\#{OpenSSL::Digest::SHA256.hexdigest(data)}\n"
            hmac = signer.derived_hmac(date)
            hmac.sign(string_to_sign.strip).unpack('H*').first
          end