I considered XOR before choosing the algorithm and rejected it because XOR only changes those bits that are different = about 50% of them. Although that seems plausibly enough to have the same effect, it seemed easier to block possible security loopholes therein than to have to do days of work analysing them statistically.

And because you then have to choose something like MIME::Base64 to make it printable, this would increase the average lengths of both key and message by (95-64)/64 * 100% = 60.8%, exacerbating your own final point about lengths.

Conversely, the OP is trying to encode a small list of info, I imagine to be something like:-

pwd google p&BBw0RD
pin iphone 8642
etc.

So I chose the solution that best fits what I imagine to be the use case. Yes the user needs to note the key which is issued at encryption time, equal in length to his secret info list and although it might well be 50+ characters in length, the OP is highly suggestive of wanting to reject keys that are too short - hence I stick by my post. I feel I responded as optimally as I could within a reasonable level of background research.

update re having to store the key somewhere - this is already consequent from the boundary conditions of the OP. And its precisely what the OTP and the OP for that matter is about - write it on the back of a business card and close the terminal window used to generate the key, so there is only one physical copy, no electronic copies reducing the weakness to one physical item to keep in your physical wallet.

more update: flipping half the bits with xor is analytically breakable for repeated use on same document where changes are small. modulo solution does not have same weakness.

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