Existing, deprecated header style:
Digest: SHA256=abc,SHA512=xyz

Newer, IETF HTTPBIS (RFC track) header style:
Content-Digest: SHA256=:abc:,SHA512=:xyz:

The difference is that the header key is changed, and the base64 values are enclosed with colons (per Structured Headers).

Would be nice if we could have an option to use the newer header style.