func sumHMAC(key []byte, data []byte) []byte {
	hash := hmac.New(sha256.New, key)
	hash.Write(data)
	return hash.Sum(nil)
}

// extractSignedHeaders extract signed headers from Authorization header
func extractSignedHeaders(signedHeaders []string, r *http.Request) (http.Header, APIErrorCode) {
	reqHeaders := r.Header
	reqQueries := r.Form
	// find whether "host" is part of list of signed headers.

	cred, _, s3Err := checkKeyValid(r, pSignValues.Credential.accessKey)
	if s3Err != ErrNone {
		return s3Err
	}

	// Extract all the signed headers along with its values.
	extractedSignedHeaders, errCode := extractSignedHeaders(pSignValues.SignedHeaders, r)
	if errCode != ErrNone {
		return errCode
	}

	// Check if the metadata headers are equal with signedheaders
	errMetaCode := checkMetaHeaders(extractedSignedHeaders, r)

	if payload != req.Header.Get(xhttp.AmzContentSha256) {
		return cred, "", "", time.Time{}, ErrContentSHA256Mismatch
	}

	// Extract all the signed headers along with its values.
	extractedSignedHeaders, errCode := extractSignedHeaders(signV4Values.SignedHeaders, r)
	if errCode != ErrNone {
		return cred, "", "", time.Time{}, errCode
	}

	cred, _, errCode = checkKeyValid(r, signV4Values.Credential.accessKey)
	if errCode != ErrNone {