jwt.go
func PeekAlgorithmAndIssuer(buf []byte) (Algorithm, string, error) {
	var err error

	// get algorithm
	alg, err := PeekAlgorithm(buf)
	if err != nil {
		return NONE, "", err
	}

	// get issuer
	issuer, err := PeekPayloadField(buf, "iss")
	if err != nil {
		return NONE, "", err
	}

	return alg, issuer, nil
}
func PeekAlgorithmAndIssuer(buf []byte) (Algorithm, string, error) {
	var err error

	// get algorithm
	alg, err := PeekAlgorithm(buf)
	if err != nil {
		return NONE, "", err
	}

	// get issuer
	issuer, err := PeekPayloadField(buf, "iss")
	if err != nil {
		return NONE, "", err
	}

	return alg, issuer, nil
}
// PeekAlgorithmAndIssuer extracts the signing algorithm listed in the "alg"
// field and the issuer from the "iss" field of the serialized JWT buf's header
// and payload, attempting to unmarshal alg to Algorithm and iss to a string.
// An error will be returned if the Algorithm or Issuer fields are not
// specified in the JWT header and payload, or are otherwise invalid.