This is barely stronger than a cantrip like Message, so making it a level 1 spell (like Identify, Illusory script, Alarm, Detect Thoughts) is fine.
Freezing the contents creates some odd situations, and you don't seem to gain much from it. I would just make the spell indicate that the contents have been altered, by glowing or some similar way. This also adds some intrigue - you know something is altered, but what? Of course the logical thing is to ask the original sender to re-send, but that is not always practical.
Showing your name and likeness is a weird feature - how would you know that the letter is not made by a higher-level version of the spell that can embed someone else's likeness? Or what if you change your name and disguise yourself when casting it? What happens to the signature when you're much older - does it still show your younger face, or does it update (which makes it a live status tracker)? What happens when the author is dead?
You can avoid all these questions if you make the spell tell you if two given enchanted documents were written by the same author. This means that if you want to securely correspond with someone, you must first leave them a signed autograph.
The dispel detector also seems unnecessary for the purpose of secure communication. First of all it doesn't apply to dead authors, second the dispelling is of more interest to the recipient, who can already tell it's been dispelled. It makes the spell more useful as a dispel-alarm, since you could just make lots of signed notes of unimportant content, and leave them lying around locations with magical traps to instantly know when someone is casting dispel. Or the letter being destroyed by fireball etc. I would take this one out.
Lastly, this sounds more like a divination spell than an enchantment spell, since it deals with manipulating information, rather than manipulating the minds of creatures. The word "enchant" aside, enchantment spells in DnD are usually things like mind control.
exclude non spell casters and low level spell casters (for a ruling class that values you based on what level of spells you can cast)
Why not do this:
- Instead of identify, the signed documents are also checked by casting Sign Letter
- Although Sign Letter is level 1, you can cast it using any spell slot. When checking a signed letter, you must cast Sign Letter using the same level (or higher) as what was used by the author.
This gives you more room to work with the "magic level as status" concept, and also allows fun plot hooks like ancient arch mages hiding secret messages (among piles of decoys) behind level 9 Sign Letters.
So to summarize, it would be something like:
Casting time: 10 minutes
Components: V, M (10gp of special ink)
You touch up to 10 sheets of paper to mark them with an imprint of your soul - your "signature", and mark them with a symbol of your choice. If the sheets already have other signatures, your symbol and signature will be added to the sheet alongside the others.
You can cast Sign Letter to check the signatures on up to 10 sheets. This will make the symbols glow in different colors:
- Signatures from the same person will have the same color. Identical twins, clones or other copies of a person will have different signatures (and different colors), but the same person after being resurrected or reincarnated will have the same signature.
- If the contents have been altered in any way after the signature was applied, the contents of the sheet will glow as well. This includes changes, removal or additions, except for additional signatures.
You can cast Sign Letter using a higher level spell slot. When checking a signature, the spell slot you use must be of the same level (or higher) as what was used to create it.
Depending on the level of your campaign, I'd consider making it possible to forge someone's signature by using Wish, if you know their True Name.