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Glad you got it going.

I suppose, one char at a time.

For testing I'd use a very short String, 1 or two characters, so the results can be easily verified manually.

Please copy the full text of the error message and paste it here. It has important info about the error.

Have you tried a simple test program creating some BIs and using some methods and printing...

Is that what the doc said? Did you write a small program to create two BIs , add them together and print out the results to see how they worked?

The next step will be to convert a character to...

That looks promising. Go read the details for that one.

Try some experiments using Strings of decimal digits: "56" or "65" and see what it does when you use some of the methods to add etc two BIs

What does the BigInteger constructor take to create an instance of BigInteger?
What technique can you use to convert a character to what the constructor takes?

I don't understand why you are using BigInteger if the task does it one character at a time.
What is done with the encrypted character?
How are the encrypted characters saved? In an array?