[Cryptography] Brainstorming for encrypted text messaging ideas...with a twist

[Grant Schultz]

I'm trying to brainstorm ideas for uncrackable encrypted text messaging 
over smartphones.  Since any software running on the phone is subject to 
being hacked, the encryption would have to take place on a separate 
device before the message was entered on the phone.

But unless you soldered it together yourself (and maybe not even then), 
you couldn't be sure that an electronic device was not also pre-hacked 
by the NSA or the Chinese, et al.  Or its electromagnetic emanations 
could be intercepted.  So I'm thinking along the lines of a mechanical 
device.  Of course, no pocket-size mechanical cipher device (consider 
the Kryha Liliput for instance) could possibly incorporate enough 
complexity to resist cryptanalysis...unless...

What if the encryption method was the one-time pad?  Naively, you could 
carry a one-time pad on paper, along with a pencil.  You would perform 
the en/decryption manually, and type the message into the phone.  (Of 
course the smartphone with its cameras would be in your pocket during 
en/decryption.)

Besides pencil and paper, is there any sort of mechanism you can 
envision that would aid in en/decryption in conjunction with a one-time 
pad, where (with the plaintext and key hidden) you could then photograph 
the ciphertext (perhaps showing on wheels of some sort), and transmit 
the photograph.  On the other end, the ciphertext could be manually 
entered into the recipient's device and decrypted.  This would bypass 
the tedium of typing random letters into the smartphone.

I realize any such method would be slow, cumbersome, and incriminating.  
But this is just a mental exercise at this point. Thanks for any ideas.

Grant Schultz