Semantic constraints and authentication In some cases (such as XML or JSON files), the bytes being encrypted have very specific semantic requirements, such that a random byte sequence will be rejected with very high probability.

In such circumstances, is it possible to do without an authentication tag, and reject forged messages at a subsequent stage?  This obviously requires that the cipher mode of operation meets certain requirements about non-malleability; to my knowledge XTS mode qualifies.
In some cases (such as XML or JSON files), the bytes being encrypted have very specific semantic requirements, such that a random byte sequence will be rejected with very high probability.


In the case of XML or JSON, I don't believe that's actually the case.

For example (example was the first JSON example I googled, simplified):

{
    "glossary": {
        "title": "example glossary",
        "GlossDiv": {
            "GlossList": {
                "GlossEntry": {
                    "GlossTerm": "Standard Generalized Markup Language",                
                 }
            }
        }
    }
}


}

If the modification was, say, to 16 bytes in the string "Standard Generalized Markup Language" (or "example glossary"), changing that to a random 16 byte string (which is the appropriate modification model for XTS), with nontrivial probability all the bytes will be within the JSON string alphabet (and not "), and hence that change would not be detected, as the modified string would still be valid JSON.