return fmt.Errorf("Invalid according to Policy: Policy Condition failed: [%s, %s, %s]", op, policy.Key, policy.Value)
			}
		}
		delete(mustFindInPolicy, formCanonicalName)
	}

	// For SignV2 - Signature/AWSAccessKeyId fields do not need to be in the policy
	if _, ok := formValues[xhttp.AmzSignatureV2]; ok {
		delete(mustFindInPolicy, xhttp.AmzSignatureV2)
		for k := range mustFindInPolicy {

	return hex.EncodeToString(sumHMAC(signingKey, []byte(stringToSign)))
}

// Check to see if Policy is signed correctly.
func doesPolicySignatureMatch(formValues http.Header) (auth.Credentials, APIErrorCode) {
	// For SignV2 - Signature field will be valid
	if _, ok := formValues[xhttp.AmzSignatureV2]; ok {
		return doesPolicySignatureV2Match(formValues)
	}
	return doesPolicySignatureV4Match(formValues)
}

	req.URL.RawQuery += "&Signature=" + url.QueryEscape(signature)
	return nil
}

// Sign given request using Signature V2.
func signRequestV2(req *http.Request, accessKey, secretKey string) error {
	signer.SignV2(*req, accessKey, secretKey, false)
	return nil
}

// Sign given request using Signature V4.
func signRequestV4(req *http.Request, accessKey, secretKey string) error {
	// Get hashed payload.