DKIM2 library and tools
dkim2sign signs a message.
dkim2verify verifies all the Dkim2 signatures in a message.
dkim2explain unpacks all the Dkim2-Signature and
Message-Instance headers, displaying their fields in a human
readable format.
dkim2history generates previous versions of a message, from
the history stored in Message-Instance headers.
Pre-built binaries are available at github releases.
Download and unpack the appropriate .zip or .tar.gz file for your OS and architecture.
The Windows binaries are not signed.
The library seems to generate and parse messages correctly.
Message-instance recipes that return a "we can't reconstruct this" result aren't returned to the caller in any useful way. I'm waiting to see how the requirements change there before firming up the API.