func NewChecksumWithType(alg ChecksumType, value string) *Checksum {
	if !alg.IsSet() {
		return nil
	}
	bvalue, err := base64.StdEncoding.DecodeString(value)
	if err != nil {
		return nil
	}
	c := Checksum{Type: alg, Encoded: value, Raw: bvalue}
	if !c.Valid() {
		return nil
	}
	return &c
}