Most of these attack vectors have been known for 10 years, and yet researchers keep finding bugs in major implementations to this day. Here's one from last week: https://portswigger.net/research/the-fragile-lock

> How would you digitally sign a Json document and embed the signature in the document?

You would not, because that's exactly how you get these bugs. Fortunately serialization mechanisms, whether JSON or Protobuf or XML or anything else, turn structured data into strings of bytes, and signature schemes operate on strings of bytes, so you'll have a great time signing data _after_ serializing it.

-----BEGIN PGP SIGNED MESSAGE-----

Hash: SHA1

> How would you digitally sign a Json document and embed the signature in the document?

Embedding a signature into the same file is easy enough.

-----BEGIN PGP SIGNATURE-----

Version: GnuPG v0.9.7 (GNU/Linux)

iEYEARECAAYFAjdYCQoACgkQJ9S6ULt1dqz6IwCfQ7wP6i/i8HhbcOSKF4ELyQB1

oCoAoOuqpRqEzr4kOkQqHRLE/b8/Rw2k =y6kj

-----END PGP SIGNATURE-----

If one has a reproducible JSON serializer, then one can add a signature to any JSON object via serializing the object, signing that and then adding the resulting signature to the original object.

This avoids JSON-inside-JSOn and allows to pretty-print the original object with the signature.

Other answers are good. One more that you could do is put the JSON document inside a container (A zip archive for example). Then your document can effectively be

    invoice.inv (zip archive)
    └- payload.json
    └- signature.asc

It's effectively what the Java jar is.

> How would you digitally sign a Json document and embed the signature in the document?

Presumably the same way you accomplish the thing in xml:

    { “signature”: “…”, “payload”: … }