How much of a bump would it need. Or just make it hardened ( old Champions PnP term). Basically resist penetration better. Then the other side will improve their armor (Parma) piercing abilities 
Maybe it is me, but I think the answer to these questions would be much wider if you posted them in the general Ars forum and not in an inside subtheme of Bibracte 
Parma x20 would be starting to go in the right direction.
Oh, another improvement: Vim Vis can be used as a substitutre for vis of any Art at a 2x1 rate, or 1x1, as you wish.
Cheers,
Xavi
Mainly to avoid rabid fanboy argument on ignoring A&A and going with Newtonian Physics.
snerk
The Berklist is gone, but the rabid fanboys LIVE ON!!!!
Oh, it's still there...
Gets a post now and again.
Direwolf is not around much nowadays.
Cheers,
Xavi
So I ran across a book while I was doing some research. Al-Kindi, "A Manuscript for the Deciphering of Cryptographic Messages". There's no article that directly describes it, but it's mentioned in here: wiki linky.
What would that book be in Ars Magica terms? There's no "Cryptography" skill: would it be a Summa on Guile with a Cryptography specialization?
EDIT: Yes, there's a reason I'm asking. If Stultus is accepted, he's going to either look for it or ask Korvin to put the word out to the Iberian Mercere. He and Korvin are going to need secure communications to make that plan work.
I'll have to see if I can justify Stultus inventing one of my favorite encryption cyphers, the Playfair. 
Guile sounds like an appropriate skill to me. Maybe more a tractatus than a suma, since its area of expertise is limited in scope. really cool, BTW
There is "cryptography" in this age it is just fairly primitive. The Caesar Cypher is a good example. I have done a lot of Identity Management and Cryptography in my normal job. I am playing with some ideas to do a kind of public-private key with magic so we can talk.
FYI: I'm still around. Floored by a fever and a cough. Living on the couch basically. 
I'll be back in soon. I imagine Stultus will eventually tire of talking to himself... or maybe he won't?
What, tire of talking to someone truly intelligent?
Besides, this is Stultus we're discussing. Tire of talking, period? Nah, never happen. He's kind of like his player that way. 
Interesting ideas! There are various types of cryptography: hiding a message in an otherwise innocent-seeming document (steganography), all the way to full-on hard core substitution ciphers. The former type definitely seems like Guile, the latter perhaps more like Artes Liberales of some sort?
Then there are pre-arranged codes where you need a code book to translate; or a scheme where you both agree on a fixed reference book and then send each other messages like "53 8" to indicate the 8th word on page 53 of your shared book....) With magic at one's disposal, one could create a spell (or a pair of enchanted items, E and D) that changes the words on a page to completely different words....
Edited to add: I meant to mention that I looked up the two things from the game that I could think of that were similar to breaking codes - translating someone else's lab texts, and the Trianomae cipher (HoH:TL pages 20-21). The former is a lab activity hence wouldn't generalize. For the latter, the decipherer makes an Int + Latin roll against the encipherer's Com + Intrigue (+6) roll. So - maybe Intrigue should be in the mix as well? Makes sense....
new personality flaw? new invisible friend?
Lots of story potential here.
Actually, thinking about it, encryption might not be the best way to go. Our opponents have Intellego magic at their disposal, which renders any encryption scheme moot. So the best way to go would be to avoid suspicion, so as to give them no reason to use Intellego magic. Which means steganography, rather than encryption.
I'll have to investigate some solutions once I get home. Tytalan ink is an obvious one, but there've got to be some others. 
Given a piece of paper with writing on it, how would intellego magic "decrypt" the words?
My thoughts exactly
And I would point out my favorite. Magi of Hermes pg 29. Message in a bottle. Turns thought into liquid. Drink the liquid and get the message.
Given only a piece of paper, Intellego magic can't do squat. Given the piece of paper and the sender, Intellego magic can pluck the encryption scheme and the key from his mind. That's the case with steganography, too: the advantage to steganography is that an encrypted message looks like an encrypted message, and so arouses suspicion. So the point is to give our opponents no reason to suspect our agents of passing information.
I don't have MoH... how is "Message in a Bottle" built? Charged item with CrMe?
So if a person did not know the encryption scheme it would work. Or if he took that memory out of his head and stored it in a crystal that he could resort later.
Message in a bottle is a charged magic item. CrMe5 Base 4 +1 Touch. The potion is the charged item.
An encrypted message could look like weaving where the knots are the code.