def signing_key
      digest = "SHA256"
      kDate = OpenSSL::HMAC.digest(digest, "AWS4" + credentials.aws_secret, request_datestamp)
      kRegion = OpenSSL::HMAC.digest(digest, kDate, region)
      kService = OpenSSL::HMAC.digest(digest, kRegion, service)
      OpenSSL::HMAC.digest(digest, kService, "aws4_request")
    end
def signing_key
      digest = "SHA256"
      kDate = OpenSSL::HMAC.digest(digest, "AWS4" + credentials.aws_secret, request_datestamp)
      kRegion = OpenSSL::HMAC.digest(digest, kDate, region)
      kService = OpenSSL::HMAC.digest(digest, kRegion, service)
      OpenSSL::HMAC.digest(digest, kService, "aws4_request")
    end
Calculate the signing key for task 3.