For RSA, there is a certain datum size that you choose, such as (say) 64 bits = 
8 bytes.

If your string is shorter than that, zero-fill the 64-bit chunk before 
encrypting.

If your string is longer than that, just do it 64 bits at a time, and zero-fill 
the last chunk, if it doesn't come out even.

This also requires that you either find or write your own math package for 
large integers, to do operations like T^e mod phi.

I suspect it will run slowly, especially if you want to allow people with older 
Palms to run it, and if you choose a large key size.  Anything reasonably 
secure against attack by a big computer will be painfully slow encrypting and 
decrypting on a Palm.  Or, if all you really want to do is deter casual prying, 
consider just XORing the data with a key.

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