private String getMetadataKey() {
        return TikaCoreProperties.TIKA_META_PREFIX + "digest" +
                TikaCoreProperties.NAMESPACE_PREFIX_DELIMITER + algorithmKeyName;
    }
@param is           InputStream to digest. Best to use a TikaInputStream because
                    of potential need to spool to disk.  InputStream must
                    support mark/reset.
@param metadata     metadata in which to store the digest information
@param parseContext ParseContext -- not actually used yet, but there for future expansion
@throws IOException on IO problem or IllegalArgumentException if algorithm couldn't be found
tika-core/src/main/java/org/apache/tika/parser/digest/InputStreamDigester.java