[judofyr]

It’s encoded using the spec that binary data in headers should be enclosed by colons: https://www.rfc-editor.org/rfc/rfc8941.html#name-byte-sequen...

[Y-bar]

Oh, thanks, it looked like a string such as a hash or base64 encoded data, not binary. Don’t think I have ever seen a use case for binary data like this in a header before.