class Aws::S3::PresignedPost

def signature(datetime, string_to_sign)
  k_secret = @credentials.secret_access_key
  k_date = hmac('AWS4' + k_secret, datetime[0,8])
  k_region = hmac(k_date, @bucket_region)
  k_service = hmac(k_region, 's3')
  k_credentials = hmac(k_service, 'aws4_request')
  hexhmac(k_credentials, string_to_sign)
end