protected Provider getProvider() {
        return null;
    }
When subclassing this, becare to ensure that your provider is
thread-safe (not likely) or return a new provider with each call.

@return provider to use to get the MessageDigest from the algorithm name.
Default is to return null.
tika-core/src/main/java/org/apache/tika/parser/digest/InputStreamDigester.java