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)
}

// compareSignatureV4 returns true if and only if both signatures
// are equal. The signatures are expected to be HEX encoded strings

const (
	signV2Algorithm = "AWS"
)

// AWS S3 Signature V2 calculation rule is give here:
// http://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html#RESTAuthenticationStringToSign
func doesPolicySignatureV2Match(formValues http.Header) (auth.Credentials, APIErrorCode) {
	accessKey := formValues.Get(xhttp.AmzAccessKeyID)

	r := &http.Request{Header: formValues}
	cred, _, s3Err := checkKeyValid(r, accessKey)