With things like GDPR on the horizon, the desire to hash certain data fields at the Elasticsearch level seems like a very attractive feature to natively support. It would be ideal if whatever did was available to Ingest and hopefully Painless.

Just exposing an instance of MessageDigest is likely to be very inefficient since it is not thread safe. Most systems expect SHA-256 these days rather than MD5 or SHA-1.

Workaround

It's possible to whitelist the MessageDigest class in Painless via a whitelist plugin. Alternatively, you could write a separate plugin and whitelist that code, or implement a custom Ingest processor plugin.