[Someone]

> in other words, encode the JSON being signed as a string. This would then ensure that, even if the "outer" JSON is parsed and re-encoded, the string is unmodified. It'll even survive weird parsing and re-encoding, which the regex replacement option might not (unless it's tolerant of whitespace changes).

Would it be guaranteed to survive even standard parsing?

It wouldn’t surprise me at all, for example, if there are json parsers out there that, on reading, map “\u0009" and “\t" to the same string, so that they can only round-trip one of those strings. Similarly, there’s the pair of “\uabcd” and “\uABCD”. There probably are others.

[LegionMammal978]

Presumably when receiving the object, you'd first unescape the string (which should yield a unique output unless you have big parser bugs), check the UTF-8 bytes of the unescaped string against the signature, and only then decode the unescaped string as the inner JSON object. It shouldn't matter how exactly the string is escaped, as long as it can be unescaped successfully.