How should the Digest field look?

Steps to reproduce the problem

1. Read the blog article at https://blog.joinmastodon.org/2018/06/how-to-implement-a-basic-activitypub-server/
2. Try to send a POST request to a Mastodon instance.

Expected behaviour

The POST request should be accepted by the server (code 202)

Actual behaviour

The server answers an error 401

Specifications

The blog post is missing the information about the Digest field.

See

mastodon/app/controllers/concerns/signature_verification.rb

Line 131 in c3aef49

raise SignatureVerificationError, 'Mastodon requires the Digest header to be signed when doing a POST request' if request.post? && !signed_headers.include?('digest')

How should the Digest field look?

It's written in those lines:

The digest field should be added to the "signed string" like that (in Clojure): https://codeberg.org/green-coder/the-pub/src/branch/made-in-taiwan/src/the_pub/ring_handler.clj#L95-L98

The field digest should be added in the signature field like that (in Clojure): https://codeberg.org/green-coder/the-pub/src/branch/made-in-taiwan/src/the_pub/ring_handler.clj#L103

This commit diff sums up the information missing in the blog, which I found in the Mastodon source code:
https://codeberg.org/green-coder/the-pub/commit/1da6f95efa1918efa9e9eedba929df46f591b559

Can/will old blog posts be updated?
See PR for security.md: mastodon/documentation#1082
And issue filed: mastodon/documentation#1079 (I did not check issues in mastodon/mastodon before filing it as it's a separate repo and I didn't investigate where blog posts are defined)